Table of Contents

• Buffer

  • Buffers and Character Encodings
  • Buffers and TypedArrays
  • Buffers and iteration

  • Class: Buffer

    • Class Method: Buffer.alloc(size[, fill[, encoding]])
    • Class Method: Buffer.allocUnsafe(size)
    • Class Method: Buffer.allocUnsafeSlow(size)
    • Class Method: Buffer.byteLength(string[, encoding])
    • Class Method: Buffer.compare(buf1, buf2)
    • Class Method: Buffer.concat(list[, totalLength])
    • Class Method: Buffer.from(array)
    • Class Method: Buffer.from(arrayBuffer[, byteOffset[, length]])
    • Class Method: Buffer.from(buffer)
    • Class Method: Buffer.from(object[, offsetOrEncoding[, length]])
    • Class Method: Buffer.from(string[, encoding])
    • Class Method: Buffer.isBuffer(obj)
    • Class Method: Buffer.isEncoding(encoding)
    • Class Property: Buffer.poolSize
    • buf[index]
    • buf.buffer
    • buf.byteOffset
    • buf.compare(target[, targetStart[, targetEnd[, sourceStart[, sourceEnd]]]])
    • buf.copy(target[, targetStart[, sourceStart[, sourceEnd]]])
    • buf.entries()
    • buf.equals(otherBuffer)
    • buf.fill(value[, offset[, end]][, encoding])
    • buf.includes(value[, byteOffset][, encoding])
    • buf.indexOf(value[, byteOffset][, encoding])
    • buf.keys()
    • buf.lastIndexOf(value[, byteOffset][, encoding])
    • buf.length
    • buf.parent
    • buf.readBigInt64BE([offset])
    • buf.readBigInt64LE([offset])
    • buf.readBigUInt64BE([offset])
    • buf.readBigUInt64LE([offset])
    • buf.readDoubleBE([offset])
    • buf.readDoubleLE([offset])
    • buf.readFloatBE([offset])
    • buf.readFloatLE([offset])
    • buf.readInt8([offset])
    • buf.readInt16BE([offset])
    • buf.readInt16LE([offset])
    • buf.readInt32BE([offset])
    • buf.readInt32LE([offset])
    • buf.readIntBE(offset, byteLength)
    • buf.readIntLE(offset, byteLength)
    • buf.readUInt8([offset])
    • buf.readUInt16BE([offset])
    • buf.readUInt16LE([offset])
    • buf.readUInt32BE([offset])
    • buf.readUInt32LE([offset])
    • buf.readUIntBE(offset, byteLength)
    • buf.readUIntLE(offset, byteLength)
    • buf.subarray([start[, end]])
    • buf.slice([start[, end]])
    • buf.swap16()
    • buf.swap32()
    • buf.swap64()
    • buf.toJSON()
    • buf.toString([encoding[, start[, end]]])
    • buf.values()
    • buf.write(string[, offset[, length]][, encoding])
    • buf.writeBigInt64BE(value[, offset])
    • buf.writeBigInt64LE(value[, offset])
    • buf.writeBigUInt64BE(value[, offset])
    • buf.writeBigUInt64LE(value[, offset])
    • buf.writeDoubleBE(value[, offset])
    • buf.writeDoubleLE(value[, offset])
    • buf.writeFloatBE(value[, offset])
    • buf.writeFloatLE(value[, offset])
    • buf.writeInt8(value[, offset])
    • buf.writeInt16BE(value[, offset])
    • buf.writeInt16LE(value[, offset])
    • buf.writeInt32BE(value[, offset])
    • buf.writeInt32LE(value[, offset])
    • buf.writeIntBE(value, offset, byteLength)
    • buf.writeIntLE(value, offset, byteLength)
    • buf.writeUInt8(value[, offset])
    • buf.writeUInt16BE(value[, offset])
    • buf.writeUInt16LE(value[, offset])
    • buf.writeUInt32BE(value[, offset])
    • buf.writeUInt32LE(value[, offset])
    • buf.writeUIntBE(value, offset, byteLength)
    • buf.writeUIntLE(value, offset, byteLength)
    • new Buffer(array)
    • new Buffer(arrayBuffer[, byteOffset[, length]])
    • new Buffer(buffer)
    • new Buffer(size)
    • new Buffer(string[, encoding])
  • buffer.INSPECT_MAX_BYTES
  • buffer.kMaxLength
  • buffer.transcode(source, fromEnc, toEnc)

  • Class: SlowBuffer

    • new SlowBuffer(size)

  • Buffer Constants

    • buffer.constants.MAX_LENGTH
    • buffer.constants.MAX_STRING_LENGTH

  • Buffer.from(), Buffer.alloc(), and Buffer.allocUnsafe()

    • The --zero-fill-buffers command line option
    • What makes Buffer.allocUnsafe() and Buffer.allocUnsafeSlow() "unsafe"?

Buffer#

In Node.js, Buffer objects are used to represent binary data in the form of a sequence of bytes. Many Node.js APIs, for example streams and file system operations, support Buffers, as interactions with the operating system or other processes generally always happen in terms of binary data.

The Buffer class is a subclass of the Uint8Array class that is built into the JavaScript language. A number of additional methods are supported that cover additional use cases. Node.js APIs accept plain Uint8Arrays wherever Buffers are supported as well.

Instances of Buffer, and instances of Uint8Array in general, are similar to arrays of integers from 0 to 255, but correspond to fixed-sized blocks of memory and cannot contain any other values. The size of a Buffer is established when it is created and cannot be changed.

The Buffer class is within the global scope, making it unlikely that one would need to ever use require('buffer').Buffer.

// Creates a zero-filled Buffer of length 10.
const buf1 = Buffer.alloc(10);

// Creates a Buffer of length 10,
// filled with bytes which all have the value `1`.
const buf2 = Buffer.alloc(10, 1);

// Creates an uninitialized buffer of length 10.
// This is faster than calling Buffer.alloc() but the returned
// Buffer instance might contain old data that needs to be
// overwritten using fill(), write(), or other functions that fill the Buffer's
// contents.
const buf3 = Buffer.allocUnsafe(10);

// Creates a Buffer containing the bytes [1, 2, 3].
const buf4 = Buffer.from([1, 2, 3]);

// Creates a Buffer containing the bytes [1, 1, 1, 1] – the entries
// are all truncated using `(value & 255)` to fit into the range 0–255.
const buf5 = Buffer.from([257, 257.5, -255, '1']);

// Creates a Buffer containing the UTF-8-encoded bytes for the string 'tést':
// [0x74, 0xc3, 0xa9, 0x73, 0x74] (in hexadecimal notation)
// [116, 195, 169, 115, 116] (in decimal notation)
const buf6 = Buffer.from('tést');

// Creates a Buffer containing the Latin-1 bytes [0x74, 0xe9, 0x73, 0x74].
const buf7 = Buffer.from('tést', 'latin1');

Buffers and Character Encodings#

When converting between Buffers and strings, a character encoding may be specified. If no character encoding is specified, UTF-8 will be used as the default.

const buf = Buffer.from('hello world', 'utf8');

console.log(buf.toString('hex'));
// Prints: 68656c6c6f20776f726c64
console.log(buf.toString('base64'));
// Prints: aGVsbG8gd29ybGQ=

console.log(Buffer.from('fhqwhgads', 'utf8'));
// Prints: <Buffer 66 68 71 77 68 67 61 64 73>
console.log(Buffer.from('fhqwhgads', 'utf16le'));
// Prints: <Buffer 66 00 68 00 71 00 77 00 68 00 67 00 61 00 64 00 73 00>

The character encodings currently supported by Node.js are the following:

• 'utf8': Multi-byte encoded Unicode characters. Many web pages and other document formats use UTF-8. This is the default character encoding. When decoding a Buffer into a string that does not exclusively contain valid UTF-8 data, the Unicode replacement character U+FFFD � will be used to represent those errors.

• 'utf16le': Multi-byte encoded Unicode characters. Unlike 'utf8', each character in the string will be encoded using either 2 or 4 bytes. Node.js only supports the little-endian variant of UTF-16.

• 'latin1': Latin-1 stands for ISO-8859-1. This character encoding only supports the Unicode characters from U+0000 to U+00FF. Each character is encoded using a single byte. Characters that do not fit into that range are truncated and will be mapped to characters in that range.

Converting a Buffer into a string using one of the above is referred to as decoding, and converting a string into a Buffer is referred to as encoding.

Node.js also supports the following two binary-to-text encodings. For binary-to-text encodings, the naming convention is reversed: Converting a Buffer into a string is typically referred to as encoding, and converting a string into a Buffer as decoding.

• 'base64': Base64 encoding. When creating a Buffer from a string, this encoding will also correctly accept "URL and Filename Safe Alphabet" as specified in RFC 4648, Section 5.

• 'hex': Encode each byte as two hexadecimal characters. Data truncation may occur when decoding string that do exclusively contain valid hexadecimal characters. See below for an example.

The following legacy character encodings are also supported:

• 'ascii': For 7-bit ASCII data only. When encoding a string into a Buffer, this is equivalent to using 'latin1'. When decoding a Buffer into a string, using encoding this will additionally unset the highest bit of each byte before decoding as 'latin1'. Generally, there should be no reason to use this encoding, as 'utf8' (or, if the data is known to always be ASCII-only, 'latin1') will be a better choice when encoding or decoding ASCII-only text. It is only provided for legacy compatibility.

• 'binary': Alias for 'latin1'. See binary strings for more background on this topic. The name of this encoding can be very misleading, as all of the encodings listed here convert between strings and binary data. For converting between strings and Buffers, typically 'utf-8' is the right choice.

• 'ucs2': Alias of 'utf16le'. UCS-2 used to refer to a variant of UTF-16 that did not support characters that had code points larger than U+FFFF. In Node.js, these code points are always supported.

Buffer.from('1ag', 'hex');
// Prints <Buffer 1a>, data truncated when first non-hexadecimal value
// ('g') encountered.

Buffer.from('1a7g', 'hex');
// Prints <Buffer 1a>, data truncated when data ends in single digit ('7').

Buffer.from('1634', 'hex');
// Prints <Buffer 16 34>, all data represented.

Modern Web browsers follow the WHATWG Encoding Standard which aliases both 'latin1' and 'ISO-8859-1' to 'win-1252'. This means that while doing something like http.get(), if the returned charset is one of those listed in the WHATWG specification it is possible that the server actually returned 'win-1252'-encoded data, and using 'latin1' encoding may incorrectly decode the characters.

Buffers and TypedArrays#

Buffer instances are also Uint8Array instances, which is the language’s built-in class for working with binary data. Uint8Array in turn is a subclass of TypedArray. Therefore, all TypedArray methods are also available on Buffers. However, there are subtle incompatibilities between the Buffer API and the TypedArray API.

In particular:

• While TypedArray#slice() creates a copy of part of the TypedArray, Buffer#slice() creates a view over the existing Buffer without copying. This behavior can be surprising, and only exists for legacy compatibility. TypedArray#subarray() can be used to achieve the behavior of Buffer#slice() on both Buffers and other TypedArrays.
• buf.toString() is incompatible with its TypedArray equivalent.
• A number of methods, e.g. buf.indexOf(), support additional arguments.

There are two ways to create new TypedArray instances from a Buffer.

When passing a Buffer to a TypedArray constructor, the Buffer’s elements will be copied, interpreted as an array of integers, and not as a byte array of the target type. For example, new Uint32Array(Buffer.from([1, 2, 3, 4])) creates a 4-element Uint32Array with elements [1, 2, 3, 4], rather than a Uint32Array with a single element [0x1020304] or [0x4030201].

In order to create a TypedArray that shares its memory with the Buffer, the underlying ArrayBuffer can be passed to the TypedArray constructor instead:

const buf = Buffer.from('hello', 'utf16le');
const uint16arr = new Uint16Array(
  buf.buffer, buf.byteOffset, buf.length / Uint16Array.BYTES_PER_ELEMENT);

It is also possible to create a new Buffer that shares the same allocated memory as a TypedArray instance by using the TypedArray object’s .buffer property in the same way. Buffer.from() behaves like new Uint8Array() in this context.

const arr = new Uint16Array(2);

arr[0] = 5000;
arr[1] = 4000;

// Copies the contents of `arr`.
const buf1 = Buffer.from(arr);
// Shares memory with `arr`.
const buf2 = Buffer.from(arr.buffer);

console.log(buf1);
// Prints: <Buffer 88 a0>
console.log(buf2);
// Prints: <Buffer 88 13 a0 0f>

arr[1] = 6000;

console.log(buf1);
// Prints: <Buffer 88 a0>
console.log(buf2);
// Prints: <Buffer 88 13 70 17>

When creating a Buffer using a TypedArray's .buffer, it is possible to use only a portion of the underlying ArrayBuffer by passing in byteOffset and length parameters.

const arr = new Uint16Array(20);
const buf = Buffer.from(arr.buffer, 0, 16);

console.log(buf.length);
// Prints: 16

The Buffer.from() and TypedArray.from() have different signatures and implementations. Specifically, the TypedArray variants accept a second argument that is a mapping function that is invoked on every element of the typed array:

• TypedArray.from(source[, mapFn[, thisArg]])

The Buffer.from() method, however, does not support the use of a mapping function:

• Buffer.from(array)
• Buffer.from(buffer)
• Buffer.from(arrayBuffer[, byteOffset[, length]])
• Buffer.from(string[, encoding])

Buffers and iteration#

Buffer instances can be iterated over using for..of syntax:

const buf = Buffer.from([1, 2, 3]);

for (const b of buf) {
  console.log(b);
}
// Prints:
//   1
//   2
//   3

Additionally, the buf.values(), buf.keys(), and buf.entries() methods can be used to create iterators.

Class: Buffer#

The Buffer class is a global type for dealing with binary data directly. It can be constructed in a variety of ways.

Class Method: Buffer.alloc(size[, fill[, encoding]])#

• size <integer> The desired length of the new Buffer.
• fill <string> | <Buffer> | <Uint8Array> | <integer> A value to pre-fill the new Buffer with. Default: 0.
• encoding <string> If fill is a string, this is its encoding. Default: 'utf8'.

Allocates a new Buffer of size bytes. If fill is undefined, the Buffer will be zero-filled.

const buf = Buffer.alloc(5);

console.log(buf);
// Prints: <Buffer 00 00 00 00 00>

If size is larger than buffer.constants.MAX_LENGTH or smaller than 0, ERR_INVALID_OPT_VALUE is thrown.

If fill is specified, the allocated Buffer will be initialized by calling buf.fill(fill).

const buf = Buffer.alloc(5, 'a');

console.log(buf);
// Prints: <Buffer 61 61 61 61 61>

If both fill and encoding are specified, the allocated Buffer will be initialized by calling buf.fill(fill, encoding).

const buf = Buffer.alloc(11, 'aGVsbG8gd29ybGQ=', 'base64');

console.log(buf);
// Prints: <Buffer 68 65 6c 6c 6f 20 77 6f 72 6c 64>

Calling Buffer.alloc() can be measurably slower than the alternative Buffer.allocUnsafe() but ensures that the newly created Buffer instance contents will never contain sensitive data from previous allocations, including data that might not have been allocated for Buffers.

A TypeError will be thrown if size is not a number.

Class Method: Buffer.allocUnsafe(size)#

• size <integer> The desired length of the new Buffer.

Allocates a new Buffer of size bytes. If size is larger than buffer.constants.MAX_LENGTH or smaller than 0, ERR_INVALID_OPT_VALUE is thrown.

The underlying memory for Buffer instances created in this way is not initialized. The contents of the newly created Buffer are unknown and may contain sensitive data. Use Buffer.alloc() instead to initialize Buffer instances with zeroes.

const buf = Buffer.allocUnsafe(10);

console.log(buf);
// Prints (contents may vary): <Buffer a0 8b 28 3f 01 00 00 00 50 32>

buf.fill(0);

console.log(buf);
// Prints: <Buffer 00 00 00 00 00 00 00 00 00 00>

A TypeError will be thrown if size is not a number.

The Buffer module pre-allocates an internal Buffer instance of size Buffer.poolSize that is used as a pool for the fast allocation of new Buffer instances created using Buffer.allocUnsafe(), Buffer.from(array), and the deprecated new Buffer(size) constructor only when size is less than or equal to Buffer.poolSize >> 1 (floor of Buffer.poolSize divided by two).

Use of this pre-allocated internal memory pool is a key difference between calling Buffer.alloc(size, fill) vs. Buffer.allocUnsafe(size).fill(fill). Specifically, Buffer.alloc(size, fill) will never use the internal Buffer pool, while Buffer.allocUnsafe(size).fill(fill) will use the internal Buffer pool if size is less than or equal to half Buffer.poolSize. The difference is subtle but can be important when an application requires the additional performance that Buffer.allocUnsafe() provides.

Class Method: Buffer.allocUnsafeSlow(size)#

• size <integer> The desired length of the new Buffer.

Allocates a new Buffer of size bytes. If size is larger than buffer.constants.MAX_LENGTH or smaller than 0, ERR_INVALID_OPT_VALUE is thrown. A zero-length Buffer is created if size is 0.

The underlying memory for Buffer instances created in this way is not initialized. The contents of the newly created Buffer are unknown and may contain sensitive data. Use buf.fill(0) to initialize such Buffer instances with zeroes.

When using Buffer.allocUnsafe() to allocate new Buffer instances, allocations under 4KB are sliced from a single pre-allocated Buffer. This allows applications to avoid the garbage collection overhead of creating many individually allocated Buffer instances. This approach improves both performance and memory usage by eliminating the need to track and clean up as many individual ArrayBuffer objects.

However, in the case where a developer may need to retain a small chunk of memory from a pool for an indeterminate amount of time, it may be appropriate to create an un-pooled Buffer instance using Buffer.allocUnsafeSlow() and then copying out the relevant bits.

// Need to keep around a few small chunks of memory.
const store = [];

socket.on('readable', () => {
  let data;
  while (null !== (data = readable.read())) {
    // Allocate for retained data.
    const sb = Buffer.allocUnsafeSlow(10);

    // Copy the data into the new allocation.
    data.copy(sb, 0, 0, 10);

    store.push(sb);
  }
});

A TypeError will be thrown if size is not a number.

Class Method: Buffer.byteLength(string[, encoding])#

• string <string> | <Buffer> | <TypedArray> | <DataView> | <ArrayBuffer> | <SharedArrayBuffer> A value to calculate the length of.
• encoding <string> If string is a string, this is its encoding. Default: 'utf8'.
• Returns: <integer> The number of bytes contained within string.

Returns the byte length of a string when encoded using encoding. This is not the same as String.prototype.length, which does not account for the encoding that is used to convert the string into bytes.

For 'base64' and 'hex', this function assumes valid input. For strings that contain non-base64/hex-encoded data (e.g. whitespace), the return value might be greater than the length of a Buffer created from the string.

const str = '\u00bd + \u00bc = \u00be';

console.log(`${str}: ${str.length} characters, ` +
            `${Buffer.byteLength(str, 'utf8')} bytes`);
// Prints: ½ + ¼ = ¾: 9 characters, 12 bytes

When string is a Buffer/DataView/TypedArray/ArrayBuffer/ SharedArrayBuffer, the byte length as reported by .byteLength is returned.

Class Method: Buffer.compare(buf1, buf2)#

• buf1 <Buffer> | <Uint8Array>
• buf2 <Buffer> | <Uint8Array>
• Returns: <integer> Either -1, 0, or 1, depending on the result of the comparison. See buf.compare() for details.

Compares buf1 to buf2, typically for the purpose of sorting arrays of Buffer instances. This is equivalent to calling buf1.compare(buf2).

const buf1 = Buffer.from('1234');
const buf2 = Buffer.from('0123');
const arr = [buf1, buf2];

console.log(arr.sort(Buffer.compare));
// Prints: [ <Buffer 30 31 32 33>, <Buffer 31 32 33 34> ]
// (This result is equal to: [buf2, buf1].)

Class Method: Buffer.concat(list[, totalLength])#

• list <Buffer[]> | <Uint8Array[]> List of Buffer or Uint8Array instances to concatenate.
• totalLength <integer> Total length of the Buffer instances in list when concatenated.
• Returns: <Buffer>

Returns a new Buffer which is the result of concatenating all the Buffer instances in the list together.

If the list has no items, or if the totalLength is 0, then a new zero-length Buffer is returned.

If totalLength is not provided, it is calculated from the Buffer instances in list by adding their lengths.

If totalLength is provided, it is coerced to an unsigned integer. If the combined length of the Buffers in list exceeds totalLength, the result is truncated to totalLength.

// Create a single `Buffer` from a list of three `Buffer` instances.

const buf1 = Buffer.alloc(10);
const buf2 = Buffer.alloc(14);
const buf3 = Buffer.alloc(18);
const totalLength = buf1.length + buf2.length + buf3.length;

console.log(totalLength);
// Prints: 42

const bufA = Buffer.concat([buf1, buf2, buf3], totalLength);

console.log(bufA);
// Prints: <Buffer 00 00 00 00 ...>
console.log(bufA.length);
// Prints: 42

Class Method: Buffer.from(array)#

• array <integer[]>

Allocates a new Buffer using an array of bytes in the range 0 – 255. Array entries outside that range will be truncated to fit into it.

// Creates a new Buffer containing the UTF-8 bytes of the string 'buffer'.
const buf = Buffer.from([0x62, 0x75, 0x66, 0x66, 0x65, 0x72]);

A TypeError will be thrown if array is not an Array or another type appropriate for Buffer.from() variants.

Buffer.from(array) and Buffer.from(string) may also use the internal Buffer pool like Buffer.allocUnsafe() does.

Class Method: Buffer.from(arrayBuffer[, byteOffset[, length]])#

• arrayBuffer <ArrayBuffer> | <SharedArrayBuffer> An ArrayBuffer, SharedArrayBuffer, for example the .buffer property of a TypedArray.
• byteOffset <integer> Index of first byte to expose. Default: 0.
• length <integer> Number of bytes to expose. Default: arrayBuffer.byteLength - byteOffset.

This creates a view of the ArrayBuffer without copying the underlying memory. For example, when passed a reference to the .buffer property of a TypedArray instance, the newly created Buffer will share the same allocated memory as the TypedArray.

const arr = new Uint16Array(2);

arr[0] = 5000;
arr[1] = 4000;

// Shares memory with `arr`.
const buf = Buffer.from(arr.buffer);

console.log(buf);
// Prints: <Buffer 88 13 a0 0f>

// Changing the original Uint16Array changes the Buffer also.
arr[1] = 6000;

console.log(buf);
// Prints: <Buffer 88 13 70 17>

The optional byteOffset and length arguments specify a memory range within the arrayBuffer that will be shared by the Buffer.

const ab = new ArrayBuffer(10);
const buf = Buffer.from(ab, 0, 2);

console.log(buf.length);
// Prints: 2

A TypeError will be thrown if arrayBuffer is not an ArrayBuffer or a SharedArrayBuffer or another type appropriate for Buffer.from() variants.

Class Method: Buffer.from(buffer)#

• buffer <Buffer> | <Uint8Array> An existing Buffer or Uint8Array from which to copy data.

Copies the passed buffer data onto a new Buffer instance.

const buf1 = Buffer.from('buffer');
const buf2 = Buffer.from(buf1);

buf1[0] = 0x61;

console.log(buf1.toString());
// Prints: auffer
console.log(buf2.toString());
// Prints: buffer

A TypeError will be thrown if buffer is not a Buffer or another type appropriate for Buffer.from() variants.

Class Method: Buffer.from(object[, offsetOrEncoding[, length]])#

• object <Object> An object supporting Symbol.toPrimitive or valueOf().
• offsetOrEncoding <integer> | <string> A byte-offset or encoding.
• length <integer> A length.

For objects whose valueOf() function returns a value not strictly equal to object, returns Buffer.from(object.valueOf(), offsetOrEncoding, length).

const buf = Buffer.from(new String('this is a test'));
// Prints: <Buffer 74 68 69 73 20 69 73 20 61 20 74 65 73 74>

For objects that support Symbol.toPrimitive, returns Buffer.from(object[Symbol.toPrimitive]('string'), offsetOrEncoding).

class Foo {
  [Symbol.toPrimitive]() {
    return 'this is a test';
  }
}

const buf = Buffer.from(new Foo(), 'utf8');
// Prints: <Buffer 74 68 69 73 20 69 73 20 61 20 74 65 73 74>

A TypeError will be thrown if object does not have the mentioned methods or is not of another type appropriate for Buffer.from() variants.

Class Method: Buffer.from(string[, encoding])#

• string <string> A string to encode.
• encoding <string> The encoding of string. Default: 'utf8'.

Creates a new Buffer containing string. The encoding parameter identifies the character encoding to be used when converting string into bytes.

const buf1 = Buffer.from('this is a tést');
const buf2 = Buffer.from('7468697320697320612074c3a97374', 'hex');

console.log(buf1.toString());
// Prints: this is a tést
console.log(buf2.toString());
// Prints: this is a tést
console.log(buf1.toString('latin1'));
// Prints: this is a tést

A TypeError will be thrown if string is not a string or another type appropriate for Buffer.from() variants.

Class Method: Buffer.isBuffer(obj)#

• obj <Object>
• Returns: <boolean>

Returns true if obj is a Buffer, false otherwise.

Class Method: Buffer.isEncoding(encoding)#

• encoding <string> A character encoding name to check.
• Returns: <boolean>

Returns true if encoding is the name of a supported character encoding, or false otherwise.

console.log(Buffer.isEncoding('utf-8'));
// Prints: true

console.log(Buffer.isEncoding('hex'));
// Prints: true

console.log(Buffer.isEncoding('utf/8'));
// Prints: false

console.log(Buffer.isEncoding(''));
// Prints: false

Class Property: Buffer.poolSize#

• <integer> Default: 8192

This is the size (in bytes) of pre-allocated internal Buffer instances used for pooling. This value may be modified.

buf[index]#

• index <integer>

The index operator [index] can be used to get and set the octet at position index in buf. The values refer to individual bytes, so the legal value range is between 0x00 and 0xFF (hex) or 0 and 255 (decimal).

This operator is inherited from Uint8Array, so its behavior on out-of-bounds access is the same as Uint8Array. In other words, buf[index] returns undefined when index is negative or >= buf.length, and buf[index] = value does not modify the buffer if index is negative or >= buf.length.

// Copy an ASCII string into a `Buffer` one byte at a time.
// (This only works for ASCII-only strings. In general, one should use
// `Buffer.from()` to perform this conversion.)

const str = 'Node.js';
const buf = Buffer.allocUnsafe(str.length);

for (let i = 0; i < str.length; i++) {
  buf[i] = str.charCodeAt(i);
}

console.log(buf.toString('utf8'));
// Prints: Node.js

buf.buffer#

• <ArrayBuffer> The underlying ArrayBuffer object based on which this Buffer object is created.

This ArrayBuffer is not guaranteed to correspond exactly to the original Buffer. See the notes on buf.byteOffset for details.

const arrayBuffer = new ArrayBuffer(16);
const buffer = Buffer.from(arrayBuffer);

console.log(buffer.buffer === arrayBuffer);
// Prints: true

buf.byteOffset#

• <integer> The byteOffset on the underlying ArrayBuffer object based on which this Buffer object is created.

When setting byteOffset in Buffer.from(ArrayBuffer, byteOffset, length), or sometimes when allocating a buffer smaller than Buffer.poolSize, the buffer doesn't start from a zero offset on the underlying ArrayBuffer.

This can cause problems when accessing the underlying ArrayBuffer directly using buf.buffer, as other parts of the ArrayBuffer may be unrelated to the buf object itself.

A common issue when creating a TypedArray object that shares its memory with a Buffer is that in this case one needs to specify the byteOffset correctly:

// Create a buffer smaller than `Buffer.poolSize`.
const nodeBuffer = new Buffer.from([0, 1, 2, 3, 4, 5, 6, 7, 8, 9]);

// When casting the Node.js Buffer to an Int8Array, use the byteOffset
// to refer only to the part of `nodeBuffer.buffer` that contains the memory
// for `nodeBuffer`.
new Int8Array(nodeBuffer.buffer, nodeBuffer.byteOffset, nodeBuffer.length);

buf.compare(target[, targetStart[, targetEnd[, sourceStart[, sourceEnd]]]])#

• target <Buffer> | <Uint8Array> A Buffer or Uint8Array with which to compare buf.
• targetStart <integer> The offset within target at which to begin comparison. Default: 0.
• targetEnd <integer> The offset within target at which to end comparison (not inclusive). Default: target.length.
• sourceStart <integer> The offset within buf at which to begin comparison. Default: 0.
• sourceEnd <integer> The offset within buf at which to end comparison (not inclusive). Default: buf.length.
• Returns: <integer>

Compares buf with target and returns a number indicating whether buf comes before, after, or is the same as target in sort order. Comparison is based on the actual sequence of bytes in each Buffer.

• 0 is returned if target is the same as buf
• 1 is returned if target should come before buf when sorted.
• -1 is returned if target should come after buf when sorted.

const buf1 = Buffer.from('ABC');
const buf2 = Buffer.from('BCD');
const buf3 = Buffer.from('ABCD');

console.log(buf1.compare(buf1));
// Prints: 0
console.log(buf1.compare(buf2));
// Prints: -1
console.log(buf1.compare(buf3));
// Prints: -1
console.log(buf2.compare(buf1));
// Prints: 1
console.log(buf2.compare(buf3));
// Prints: 1
console.log([buf1, buf2, buf3].sort(Buffer.compare));
// Prints: [ <Buffer 41 42 43>, <Buffer 41 42 43 44>, <Buffer 42 43 44> ]
// (This result is equal to: [buf1, buf3, buf2].)

The optional targetStart, targetEnd, sourceStart, and sourceEnd arguments can be used to limit the comparison to specific ranges within target and buf respectively.

const buf1 = Buffer.from([1, 2, 3, 4, 5, 6, 7, 8, 9]);
const buf2 = Buffer.from([5, 6, 7, 8, 9, 1, 2, 3, 4]);

console.log(buf1.compare(buf2, 5, 9, 0, 4));
// Prints: 0
console.log(buf1.compare(buf2, 0, 6, 4));
// Prints: -1
console.log(buf1.compare(buf2, 5, 6, 5));
// Prints: 1

ERR_OUT_OF_RANGE is thrown if targetStart < 0, sourceStart < 0, targetEnd > target.byteLength, or sourceEnd > source.byteLength.

buf.copy(target[, targetStart[, sourceStart[, sourceEnd]]])#

• target <Buffer> | <Uint8Array> A Buffer or Uint8Array to copy into.
• targetStart <integer> The offset within target at which to begin writing. Default: 0.
• sourceStart <integer> The offset within buf from which to begin copying. Default: 0.
• sourceEnd <integer> The offset within buf at which to stop copying (not inclusive). Default: buf.length.
• Returns: <integer> The number of bytes copied.

Copies data from a region of buf to a region in target, even if the target memory region overlaps with buf.

TypedArray#set() performs the same operation, and is available for all TypedArrays, including Node.js Buffers, although it takes different function arguments.

// Create two `Buffer` instances.
const buf1 = Buffer.allocUnsafe(26);
const buf2 = Buffer.allocUnsafe(26).fill('!');

for (let i = 0; i < 26; i++) {
  // 97 is the decimal ASCII value for 'a'.
  buf1[i] = i + 97;
}

// Copy `buf1` bytes 16 through 19 into `buf2` starting at byte 8 of `buf2`.
buf1.copy(buf2, 8, 16, 20);
// This is equivalent to:
// buf2.set(buf1.subarray(16, 20), 8);

console.log(buf2.toString('ascii', 0, 25));
// Prints: !!!!!!!!qrst!!!!!!!!!!!!!

// Create a `Buffer` and copy data from one region to an overlapping region
// within the same `Buffer`.

const buf = Buffer.allocUnsafe(26);

for (let i = 0; i < 26; i++) {
  // 97 is the decimal ASCII value for 'a'.
  buf[i] = i + 97;
}

buf.copy(buf, 0, 4, 10);

console.log(buf.toString());
// Prints: efghijghijklmnopqrstuvwxyz

buf.entries()#

• Returns: <Iterator>

Creates and returns an iterator of [index, byte] pairs from the contents of buf.

// Log the entire contents of a `Buffer`.

const buf = Buffer.from('buffer');

for (const pair of buf.entries()) {
  console.log(pair);
}
// Prints:
//   [0, 98]
//   [1, 117]
//   [2, 102]
//   [3, 102]
//   [4, 101]
//   [5, 114]

buf.equals(otherBuffer)#

• otherBuffer <Buffer> | <Uint8Array> A Buffer or Uint8Array with which to compare buf.
• Returns: <boolean>

Returns true if both buf and otherBuffer have exactly the same bytes, false otherwise. Equivalent to buf.compare(otherBuffer) === 0.

const buf1 = Buffer.from('ABC');
const buf2 = Buffer.from('414243', 'hex');
const buf3 = Buffer.from('ABCD');

console.log(buf1.equals(buf2));
// Prints: true
console.log(buf1.equals(buf3));
// Prints: false

buf.fill(value[, offset[, end]][, encoding])#

• value <string> | <Buffer> | <Uint8Array> | <integer> The value with which to fill buf.
• offset <integer> Number of bytes to skip before starting to fill buf. Default: 0.
• end <integer> Where to stop filling buf (not inclusive). Default: buf.length.
• encoding <string> The encoding for value if value is a string. Default: 'utf8'.
• Returns: <Buffer> A reference to buf.

Fills buf with the specified value. If the offset and end are not given, the entire buf will be filled:

// Fill a `Buffer` with the ASCII character 'h'.

const b = Buffer.allocUnsafe(50).fill('h');

console.log(b.toString());
// Prints: hhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhh

value is coerced to a uint32 value if it is not a string, Buffer, or integer. If the resulting integer is greater than 255 (decimal), buf will be filled with value & 255.

If the final write of a fill() operation falls on a multi-byte character, then only the bytes of that character that fit into buf are written:

// Fill a `Buffer` with character that takes up two bytes in UTF-8.

console.log(Buffer.allocUnsafe(5).fill('\u0222'));
// Prints: <Buffer c8 a2 c8 a2 c8>

If value contains invalid characters, it is truncated; if no valid fill data remains, an exception is thrown:

const buf = Buffer.allocUnsafe(5);

console.log(buf.fill('a'));
// Prints: <Buffer 61 61 61 61 61>
console.log(buf.fill('aazz', 'hex'));
// Prints: <Buffer aa aa aa aa aa>
console.log(buf.fill('zz', 'hex'));
// Throws an exception.

buf.includes(value[, byteOffset][, encoding])#

• value <string> | <Buffer> | <Uint8Array> | <integer> What to search for.
• byteOffset <integer> Where to begin searching in buf. If negative, then offset is calculated from the end of buf. Default: 0.
• encoding <string> If value is a string, this is its encoding. Default: 'utf8'.
• Returns: <boolean> true if value was found in buf, false otherwise.

Equivalent to buf.indexOf() !== -1.

const buf = Buffer.from('this is a buffer');

console.log(buf.includes('this'));
// Prints: true
console.log(buf.includes('is'));
// Prints: true
console.log(buf.includes(Buffer.from('a buffer')));
// Prints: true
console.log(buf.includes(97));
// Prints: true (97 is the decimal ASCII value for 'a')
console.log(buf.includes(Buffer.from('a buffer example')));
// Prints: false
console.log(buf.includes(Buffer.from('a buffer example').slice(0, 8)));
// Prints: true
console.log(buf.includes('this', 4));
// Prints: false

buf.indexOf(value[, byteOffset][, encoding])#

• value <string> | <Buffer> | <Uint8Array> | <integer> What to search for.
• byteOffset <integer> Where to begin searching in buf. If negative, then offset is calculated from the end of buf. Default: 0.
• encoding <string> If value is a string, this is the encoding used to determine the binary representation of the string that will be searched for in buf. Default: 'utf8'.
• Returns: <integer> The index of the first occurrence of value in buf, or -1 if buf does not contain value.

If value is:

• a string, value is interpreted according to the character encoding in encoding.
• a Buffer or Uint8Array, value will be used in its entirety. To compare a partial Buffer, use buf.slice().
• a number, value will be interpreted as an unsigned 8-bit integer value between 0 and 255.

const buf = Buffer.from('this is a buffer');

console.log(buf.indexOf('this'));
// Prints: 0
console.log(buf.indexOf('is'));
// Prints: 2
console.log(buf.indexOf(Buffer.from('a buffer')));
// Prints: 8
console.log(buf.indexOf(97));
// Prints: 8 (97 is the decimal ASCII value for 'a')
console.log(buf.indexOf(Buffer.from('a buffer example')));
// Prints: -1
console.log(buf.indexOf(Buffer.from('a buffer example').slice(0, 8)));
// Prints: 8

const utf16Buffer = Buffer.from('\u039a\u0391\u03a3\u03a3\u0395', 'utf16le');

console.log(utf16Buffer.indexOf('\u03a3', 0, 'utf16le'));
// Prints: 4
console.log(utf16Buffer.indexOf('\u03a3', -4, 'utf16le'));
// Prints: 6

If value is not a string, number, or Buffer, this method will throw a TypeError. If value is a number, it will be coerced to a valid byte value, an integer between 0 and 255.

If byteOffset is not a number, it will be coerced to a number. If the result of coercion is NaN or 0, then the entire buffer will be searched. This behavior matches String#indexOf().

const b = Buffer.from('abcdef');

// Passing a value that's a number, but not a valid byte.
// Prints: 2, equivalent to searching for 99 or 'c'.
console.log(b.indexOf(99.9));
console.log(b.indexOf(256 + 99));

// Passing a byteOffset that coerces to NaN or 0.
// Prints: 1, searching the whole buffer.
console.log(b.indexOf('b', undefined));
console.log(b.indexOf('b', {}));
console.log(b.indexOf('b', null));
console.log(b.indexOf('b', []));

If value is an empty string or empty Buffer and byteOffset is less than buf.length, byteOffset will be returned. If value is empty and byteOffset is at least buf.length, buf.length will be returned.

buf.keys()#

• Returns: <Iterator>

Creates and returns an iterator of buf keys (indices).

const buf = Buffer.from('buffer');

for (const key of buf.keys()) {
  console.log(key);
}
// Prints:
//   0
//   1
//   2
//   3
//   4
//   5

buf.lastIndexOf(value[, byteOffset][, encoding])#

• value <string> | <Buffer> | <Uint8Array> | <integer> What to search for.
• byteOffset <integer> Where to begin searching in buf. If negative, then offset is calculated from the end of buf. Default: buf.length - 1.
• encoding <string> If value is a string, this is the encoding used to determine the binary representation of the string that will be searched for in buf. Default: 'utf8'.
• Returns: <integer> The index of the last occurrence of value in buf, or -1 if buf does not contain value.

Identical to buf.indexOf(), except the last occurrence of value is found rather than the first occurrence.

const buf = Buffer.from('this buffer is a buffer');

console.log(buf.lastIndexOf('this'));
// Prints: 0
console.log(buf.lastIndexOf('buffer'));
// Prints: 17
console.log(buf.lastIndexOf(Buffer.from('buffer')));
// Prints: 17
console.log(buf.lastIndexOf(97));
// Prints: 15 (97 is the decimal ASCII value for 'a')
console.log(buf.lastIndexOf(Buffer.from('yolo')));
// Prints: -1
console.log(buf.lastIndexOf('buffer', 5));
// Prints: 5
console.log(buf.lastIndexOf('buffer', 4));
// Prints: -1

const utf16Buffer = Buffer.from('\u039a\u0391\u03a3\u03a3\u0395', 'utf16le');

console.log(utf16Buffer.lastIndexOf('\u03a3', undefined, 'utf16le'));
// Prints: 6
console.log(utf16Buffer.lastIndexOf('\u03a3', -5, 'utf16le'));
// Prints: 4

If value is not a string, number, or Buffer, this method will throw a TypeError. If value is a number, it will be coerced to a valid byte value, an integer between 0 and 255.

If byteOffset is not a number, it will be coerced to a number. Any arguments that coerce to NaN, like {} or undefined, will search the whole buffer. This behavior matches String#lastIndexOf().

const b = Buffer.from('abcdef');

// Passing a value that's a number, but not a valid byte.
// Prints: 2, equivalent to searching for 99 or 'c'.
console.log(b.lastIndexOf(99.9));
console.log(b.lastIndexOf(256 + 99));

// Passing a byteOffset that coerces to NaN.
// Prints: 1, searching the whole buffer.
console.log(b.lastIndexOf('b', undefined));
console.log(b.lastIndexOf('b', {}));

// Passing a byteOffset that coerces to 0.
// Prints: -1, equivalent to passing 0.
console.log(b.lastIndexOf('b', null));
console.log(b.lastIndexOf('b', []));

If value is an empty string or empty Buffer, byteOffset will be returned.

buf.length#

• <integer>

Returns the number of bytes in buf.

// Create a `Buffer` and write a shorter string to it using UTF-8.

const buf = Buffer.alloc(1234);

console.log(buf.length);
// Prints: 1234

buf.write('some string', 0, 'utf8');

console.log(buf.length);
// Prints: 1234

buf.parent#

The buf.parent property is a deprecated alias for buf.buffer.

buf.readBigInt64BE([offset])#

buf.readBigInt64LE([offset])#

• offset <integer> Number of bytes to skip before starting to read. Must satisfy: 0 <= offset <= buf.length - 8. Default: 0.
• Returns: <bigint>

Reads a signed 64-bit integer from buf at the specified offset with the specified endianness (readBigInt64BE() reads as big endian, readBigInt64LE() reads as little endian).

Integers read from a Buffer are interpreted as two's complement signed values.

buf.readBigUInt64BE([offset])#

buf.readBigUInt64LE([offset])#

• offset <integer> Number of bytes to skip before starting to read. Must satisfy: 0 <= offset <= buf.length - 8. Default: 0.
• Returns: <bigint>

Reads an unsigned 64-bit integer from buf at the specified offset with the specified endianness (readBigUInt64BE() reads as big endian, readBigUInt64LE() reads as little endian).

const buf = Buffer.from([0x00, 0x00, 0x00, 0x00, 0xff, 0xff, 0xff, 0xff]);

console.log(buf.readBigUInt64BE(0));
// Prints: 4294967295n

console.log(buf.readBigUInt64LE(0));
// Prints: 18446744069414584320n

buf.readDoubleBE([offset])#

buf.readDoubleLE([offset])#

• offset <integer> Number of bytes to skip before starting to read. Must satisfy 0 <= offset <= buf.length - 8. Default: 0.
• Returns: <number>

Reads a 64-bit double from buf at the specified offset with the specified endianness (readDoubleBE() reads as big endian, readDoubleLE() reads as little endian).

const buf = Buffer.from([1, 2, 3, 4, 5, 6, 7, 8]);

console.log(buf.readDoubleBE(0));
// Prints: 8.20788039913184e-304
console.log(buf.readDoubleLE(0));
// Prints: 5.447603722011605e-270
console.log(buf.readDoubleLE(1));
// Throws ERR_OUT_OF_RANGE.

buf.readFloatBE([offset])#

buf.readFloatLE([offset])#

• offset <integer> Number of bytes to skip before starting to read. Must satisfy 0 <= offset <= buf.length - 4. Default: 0.
• Returns: <number>

Reads a 32-bit float from buf at the specified offset with the specified endianness (readFloatBE() reads as big endian, readFloatLE() reads as little endian).

const buf = Buffer.from([1, 2, 3, 4]);

console.log(buf.readFloatBE(0));
// Prints: 2.387939260590663e-38
console.log(buf.readFloatLE(0));
// Prints: 1.539989614439558e-36
console.log(buf.readFloatLE(1));
// Throws ERR_OUT_OF_RANGE.

buf.readInt8([offset])#

• offset <integer> Number of bytes to skip before starting to read. Must satisfy 0 <= offset <= buf.length - 1. Default: 0.
• Returns: <integer>

Reads a signed 8-bit integer from buf at the specified offset.

Integers read from a Buffer are interpreted as two's complement signed values.

const buf = Buffer.from([-1, 5]);

console.log(buf.readInt8(0));
// Prints: -1
console.log(buf.readInt8(1));
// Prints: 5
console.log(buf.readInt8(2));
// Throws ERR_OUT_OF_RANGE.

buf.readInt16BE([offset])#

buf.readInt16LE([offset])#

• offset <integer> Number of bytes to skip before starting to read. Must satisfy 0 <= offset <= buf.length - 2. Default: 0.
• Returns: <integer>

Reads a signed 16-bit integer from buf at the specified offset with the specified endianness (readInt16BE() reads as big endian, readInt16LE() reads as little endian).

Integers read from a Buffer are interpreted as two's complement signed values.

const buf = Buffer.from([0, 5]);

console.log(buf.readInt16BE(0));
// Prints: 5
console.log(buf.readInt16LE(0));
// Prints: 1280
console.log(buf.readInt16LE(1));
// Throws ERR_OUT_OF_RANGE.

buf.readInt32BE([offset])#

buf.readInt32LE([offset])#

• offset <integer> Number of bytes to skip before starting to read. Must satisfy 0 <= offset <= buf.length - 4. Default: 0.
• Returns: <integer>

Reads a signed 32-bit integer from buf at the specified offset with the specified endianness (readInt32BE() reads as big endian, readInt32LE() reads as little endian).

Integers read from a Buffer are interpreted as two's complement signed values.

const buf = Buffer.from([0, 0, 0, 5]);

console.log(buf.readInt32BE(0));
// Prints: 5
console.log(buf.readInt32LE(0));
// Prints: 83886080
console.log(buf.readInt32LE(1));
// Throws ERR_OUT_OF_RANGE.

buf.readIntBE(offset, byteLength)#

buf.readIntLE(offset, byteLength)#

• offset <integer> Number of bytes to skip before starting to read. Must satisfy 0 <= offset <= buf.length - byteLength.
• byteLength <integer> Number of bytes to read. Must satisfy 0 < byteLength <= 6.
• Returns: <integer>

Reads byteLength number of bytes from buf at the specified offset and interprets the result as a two's complement signed value. Supports up to 48 bits of accuracy.

const buf = Buffer.from([0x12, 0x34, 0x56, 0x78, 0x90, 0xab]);

console.log(buf.readIntLE(0, 6).toString(16));
// Prints: -546f87a9cbee
console.log(buf.readIntBE(0, 6).toString(16));
// Prints: 1234567890ab
console.log(buf.readIntBE(1, 6).toString(16));
// Throws ERR_OUT_OF_RANGE.
console.log(buf.readIntBE(1, 0).toString(16));
// Throws ERR_OUT_OF_RANGE.

buf.readUInt8([offset])#

• offset <integer> Number of bytes to skip before starting to read. Must satisfy 0 <= offset <= buf.length - 1. Default: 0.
• Returns: <integer>

Reads an unsigned 8-bit integer from buf at the specified offset.

const buf = Buffer.from([1, -2]);

console.log(buf.readUInt8(0));
// Prints: 1
console.log(buf.readUInt8(1));
// Prints: 254
console.log(buf.readUInt8(2));
// Throws ERR_OUT_OF_RANGE.

buf.readUInt16BE([offset])#

buf.readUInt16LE([offset])#

• offset <integer> Number of bytes to skip before starting to read. Must satisfy 0 <= offset <= buf.length - 2. Default: 0.
• Returns: <integer>

Reads an unsigned 16-bit integer from buf at the specified offset with the specified endianness (readUInt16BE() reads as big endian, readUInt16LE() reads as little endian).

const buf = Buffer.from([0x12, 0x34, 0x56]);

console.log(buf.readUInt16BE(0).toString(16));
// Prints: 1234
console.log(buf.readUInt16LE(0).toString(16));
// Prints: 3412
console.log(buf.readUInt16BE(1).toString(16));
// Prints: 3456
console.log(buf.readUInt16LE(1).toString(16));
// Prints: 5634
console.log(buf.readUInt16LE(2).toString(16));
// Throws ERR_OUT_OF_RANGE.

buf.readUInt32BE([offset])#

buf.readUInt32LE([offset])#

• offset <integer> Number of bytes to skip before starting to read. Must satisfy 0 <= offset <= buf.length - 4. Default: 0.
• Returns: <integer>

Reads an unsigned 32-bit integer from buf at the specified offset with the specified endianness (readUInt32BE() reads as big endian, readUInt32LE() reads as little endian).

const buf = Buffer.from([0x12, 0x34, 0x56, 0x78]);

console.log(buf.readUInt32BE(0).toString(16));
// Prints: 12345678
console.log(buf.readUInt32LE(0).toString(16));
// Prints: 78563412
console.log(buf.readUInt32LE(1).toString(16));
// Throws ERR_OUT_OF_RANGE.

buf.readUIntBE(offset, byteLength)#

buf.readUIntLE(offset, byteLength)#

• offset <integer> Number of bytes to skip before starting to read. Must satisfy 0 <= offset <= buf.length - byteLength.
• byteLength <integer> Number of bytes to read. Must satisfy 0 < byteLength <= 6.
• Returns: <integer>

Reads byteLength number of bytes from buf at the specified offset and interprets the result as an unsigned integer. Supports up to 48 bits of accuracy.

const buf = Buffer.from([0x12, 0x34, 0x56, 0x78, 0x90, 0xab]);

console.log(buf.readUIntBE(0, 6).toString(16));
// Prints: 1234567890ab
console.log(buf.readUIntLE(0, 6).toString(16));
// Prints: ab9078563412
console.log(buf.readUIntBE(1, 6).toString(16));
// Throws ERR_OUT_OF_RANGE.

buf.subarray([start[, end]])#

• start <integer> Where the new Buffer will start. Default: 0.
• end <integer> Where the new Buffer will end (not inclusive). Default: buf.length.
• Returns: <Buffer>

Returns a new Buffer that references the same memory as the original, but offset and cropped by the start and end indices.

Specifying end greater than buf.length will return the same result as that of end equal to buf.length.

This method is inherited from TypedArray#subarray().

Modifying the new Buffer slice will modify the memory in the original Buffer because the allocated memory of the two objects overlap.

// Create a `Buffer` with the ASCII alphabet, take a slice, and modify one byte
// from the original `Buffer`.

const buf1 = Buffer.allocUnsafe(26);

for (let i = 0; i < 26; i++) {
  // 97 is the decimal ASCII value for 'a'.
  buf1[i] = i + 97;
}

const buf2 = buf1.subarray(0, 3);

console.log(buf2.toString('ascii', 0, buf2.length));
// Prints: abc

buf1[0] = 33;

console.log(buf2.toString('ascii', 0, buf2.length));
// Prints: !bc

Specifying negative indexes causes the slice to be generated relative to the end of buf rather than the beginning.

const buf = Buffer.from('buffer');

console.log(buf.subarray(-6, -1).toString());
// Prints: buffe
// (Equivalent to buf.subarray(0, 5).)

console.log(buf.subarray(-6, -2).toString());
// Prints: buff
// (Equivalent to buf.subarray(0, 4).)

console.log(buf.subarray(-5, -2).toString());
// Prints: uff
// (Equivalent to buf.subarray(1, 4).)

buf.slice([start[, end]])#

• start <integer> Where the new Buffer will start. Default: 0.
• end <integer> Where the new Buffer will end (not inclusive). Default: buf.length.
• Returns: <Buffer>

Returns a new Buffer that references the same memory as the original, but offset and cropped by the start and end indices.

This is the same behavior as buf.subarray().

This method is not compatible with the Uint8Array.prototype.slice(), which is a superclass of Buffer. To copy the slice, use Uint8Array.prototype.slice().

const buf = Buffer.from('buffer');

const copiedBuf = Uint8Array.prototype.slice.call(buf);
copiedBuf[0]++;
console.log(copiedBuf.toString());
// Prints: cuffer

console.log(buf.toString());
// Prints: buffer

buf.swap16()#

• Returns: <Buffer> A reference to buf.

Interprets buf as an array of unsigned 16-bit integers and swaps the byte order in-place. Throws ERR_INVALID_BUFFER_SIZE if buf.length is not a multiple of 2.

const buf1 = Buffer.from([0x1, 0x2, 0x3, 0x4, 0x5, 0x6, 0x7, 0x8]);

console.log(buf1);
// Prints: <Buffer 01 02 03 04 05 06 07 08>

buf1.swap16();

console.log(buf1);
// Prints: <Buffer 02 01 04 03 06 05 08 07>

const buf2 = Buffer.from([0x1, 0x2, 0x3]);

buf2.swap16();
// Throws ERR_INVALID_BUFFER_SIZE.

One convenient use of buf.swap16() is to perform a fast in-place conversion between UTF-16 little-endian and UTF-16 big-endian:

const buf = Buffer.from('This is little-endian UTF-16', 'utf16le');
buf.swap16(); // Convert to big-endian UTF-16 text.

buf.swap32()#

• Returns: <Buffer> A reference to buf.

Interprets buf as an array of unsigned 32-bit integers and swaps the byte order in-place. Throws ERR_INVALID_BUFFER_SIZE if buf.length is not a multiple of 4.

const buf1 = Buffer.from([0x1, 0x2, 0x3, 0x4, 0x5, 0x6, 0x7, 0x8]);

console.log(buf1);
// Prints: <Buffer 01 02 03 04 05 06 07 08>

buf1.swap32();

console.log(buf1);
// Prints: <Buffer 04 03 02 01 08 07 06 05>

const buf2 = Buffer.from([0x1, 0x2, 0x3]);

buf2.swap32();
// Throws ERR_INVALID_BUFFER_SIZE.

buf.swap64()#

• Returns: <Buffer> A reference to buf.

Interprets buf as an array of 64-bit numbers and swaps byte order in-place. Throws ERR_INVALID_BUFFER_SIZE if buf.length is not a multiple of 8.

const buf1 = Buffer.from([0x1, 0x2, 0x3, 0x4, 0x5, 0x6, 0x7, 0x8]);

console.log(buf1);
// Prints: <Buffer 01 02 03 04 05 06 07 08>

buf1.swap64();

console.log(buf1);
// Prints: <Buffer 08 07 06 05 04 03 02 01>

const buf2 = Buffer.from([0x1, 0x2, 0x3]);

buf2.swap64();
// Throws ERR_INVALID_BUFFER_SIZE.

buf.toJSON()#

• Returns: <Object>

Returns a JSON representation of buf. JSON.stringify() implicitly calls this function when stringifying a Buffer instance.

Buffer.from() accepts objects in the format returned from this method. In particular, Buffer.from(buf.toJSON()) works like Buffer.from(buf).

const buf = Buffer.from([0x1, 0x2, 0x3, 0x4, 0x5]);
const json = JSON.stringify(buf);

console.log(json);
// Prints: {"type":"Buffer","data":[1,2,3,4,5]}

const copy = JSON.parse(json, (key, value) => {
  return value && value.type === 'Buffer' ?
    Buffer.from(value) :
    value;
});

console.log(copy);
// Prints: <Buffer 01 02 03 04 05>

buf.toString([encoding[, start[, end]]])#

• encoding <string> The character encoding to use. Default: 'utf8'.
• start <integer> The byte offset to start decoding at. Default: 0.
• end <integer> The byte offset to stop decoding at (not inclusive). Default: buf.length.
• Returns: <string>

Decodes buf to a string according to the specified character encoding in encoding. start and end may be passed to decode only a subset of buf.

If encoding is 'utf8' and a byte sequence in the input is not valid UTF-8, then each invalid byte is replaced with the replacement character U+FFFD.

The maximum length of a string instance (in UTF-16 code units) is available as buffer.constants.MAX_STRING_LENGTH.

const buf1 = Buffer.allocUnsafe(26);

for (let i = 0; i < 26; i++) {
  // 97 is the decimal ASCII value for 'a'.
  buf1[i] = i + 97;
}

console.log(buf1.toString('utf8'));
// Prints: abcdefghijklmnopqrstuvwxyz
console.log(buf1.toString('utf8', 0, 5));
// Prints: abcde

const buf2 = Buffer.from('tést');

console.log(buf2.toString('hex'));
// Prints: 74c3a97374
console.log(buf2.toString('utf8', 0, 3));
// Prints: té
console.log(buf2.toString(undefined, 0, 3));
// Prints: té

buf.values()#

• Returns: <Iterator>

Creates and returns an iterator for buf values (bytes). This function is called automatically when a Buffer is used in a for..of statement.

const buf = Buffer.from('buffer');

for (const value of buf.values()) {
  console.log(value);
}
// Prints:
//   98
//   117
//   102
//   102
//   101
//   114

for (const value of buf) {
  console.log(value);
}
// Prints:
//   98
//   117
//   102
//   102
//   101
//   114

buf.write(string[, offset[, length]][, encoding])#

• string <string> String to write to buf.
• offset <integer> Number of bytes to skip before starting to write string. Default: 0.
• length <integer> Maximum number of bytes to write (written bytes will not exceed buf.length - offset). Default: buf.length - offset.
• encoding <string> The character encoding of string. Default: 'utf8'.
• Returns: <integer> Number of bytes written.

Writes string to buf at offset according to the character encoding in encoding. The length parameter is the number of bytes to write. If buf did not contain enough space to fit the entire string, only part of string will be written. However, partially encoded characters will not be written.

const buf = Buffer.alloc(256);

const len = buf.write('\u00bd + \u00bc = \u00be', 0);

console.log(`${len} bytes: ${buf.toString('utf8', 0, len)}`);
// Prints: 12 bytes: ½ + ¼ = ¾

const buffer = Buffer.alloc(10);

const length = buffer.write('abcd', 8);

console.log(`${length} bytes: ${buffer.toString('utf8', 8, 10)}`);
// Prints: 2 bytes : ab

buf.writeBigInt64BE(value[, offset])#

buf.writeBigInt64LE(value[, offset])#

• value <bigint> Number to be written to buf.
• offset <integer> Number of bytes to skip before starting to write. Must satisfy: 0 <= offset <= buf.length - 8. Default: 0.
• Returns: <integer> offset plus the number of bytes written.

Writes value to buf at the specified offset with the specified endianness (writeBigInt64BE() writes as big endian, writeBigInt64LE() writes as little endian).

value is interpreted and written as a two's complement signed integer.

const buf = Buffer.allocUnsafe(8);

buf.writeBigInt64BE(0x0102030405060708n, 0);

console.log(buf);
// Prints: <Buffer 01 02 03 04 05 06 07 08>

buf.writeBigUInt64BE(value[, offset])#

buf.writeBigUInt64LE(value[, offset])#

• value <bigint> Number to be written to buf.
• offset <integer> Number of bytes to skip before starting to write. Must satisfy: 0 <= offset <= buf.length - 8. Default: 0.
• Returns: <integer> offset plus the number of bytes written.

Writes value to buf at the specified offset with specified endianness (writeBigUInt64BE() writes as big endian, writeBigUInt64LE() writes as little endian).

const buf = Buffer.allocUnsafe(8);

buf.writeBigUInt64LE(0xdecafafecacefaden, 0);

console.log(buf);
// Prints: <Buffer de fa ce ca fe fa ca de>

buf.writeDoubleBE(value[, offset])#

buf.writeDoubleLE(value[, offset])#

• value <number> Number to be written to buf.
• offset <integer> Number of bytes to skip before starting to write. Must satisfy 0 <= offset <= buf.length - 8. Default: 0.
• Returns: <integer> offset plus the number of bytes written.

Writes value to buf at the specified offset with the specified endianness (writeDoubleBE() writes as big endian, writeDoubleLE() writes as little endian). value must be a JavaScript number. Behavior is undefined when value is anything other than a JavaScript number.

const buf = Buffer.allocUnsafe(8);

buf.writeDoubleBE(123.456, 0);

console.log(buf);
// Prints: <Buffer 40 5e dd 2f 1a 9f be 77>

buf.writeDoubleLE(123.456, 0);

console.log(buf);
// Prints: <Buffer 77 be 9f 1a 2f dd 5e 40>

buf.writeFloatBE(value[, offset])#

buf.writeFloatLE(value[, offset])#

• value <number> Number to be written to buf.
• offset <integer> Number of bytes to skip before starting to write. Must satisfy 0 <= offset <= buf.length - 4. Default: 0.
• Returns: <integer> offset plus the number of bytes written.

Writes value to buf at the specified offset with specified endianness (writeFloatBE() writes as big endian, writeFloatLE() writes as little endian). value must be a JavaScript number. Behavior is undefined when value is anything other than a JavaScript number.

const buf = Buffer.allocUnsafe(4);

buf.writeFloatBE(0xcafebabe, 0);

console.log(buf);
// Prints: <Buffer 4f 4a fe bb>

buf.writeFloatLE(0xcafebabe, 0);

console.log(buf);
// Prints: <Buffer bb fe 4a 4f>

buf.writeInt8(value[, offset])#

• value <integer> Number to be written to buf.
• offset <integer> Number of bytes to skip before starting to write. Must satisfy 0 <= offset <= buf.length - 1. Default: 0.
• Returns: <integer> offset plus the number of bytes written.

Writes value to buf at the specified offset. value must be a valid signed 8-bit integer. Behavior is undefined when value is anything other than a signed 8-bit integer.

value is interpreted and written as a two's complement signed integer.

const buf = Buffer.allocUnsafe(2);

buf.writeInt8(2, 0);
buf.writeInt8(-2, 1);

console.log(buf);
// Prints: <Buffer 02 fe>

buf.writeInt16BE(value[, offset])#

buf.writeInt16LE(value[, offset])#

• value <integer> Number to be written to buf.
• offset <integer> Number of bytes to skip before starting to write. Must satisfy 0 <= offset <= buf.length - 2. Default: 0.
• Returns: <integer> offset plus the number of bytes written.

Writes value to buf at the specified offset with the specified endianness (writeInt16BE() writes as big endian, writeInt16LE() writes as little endian). value must be a valid signed 16-bit integer. Behavior is undefined when value is anything other than a signed 16-bit integer.

value is interpreted and written as a two's complement signed integer.

const buf = Buffer.allocUnsafe(4);

buf.writeInt16BE(0x0102, 0);
buf.writeInt16LE(0x0304, 2);

console.log(buf);
// Prints: <Buffer 01 02 04 03>

buf.writeInt32BE(value[, offset])#

buf.writeInt32LE(value[, offset])#

• value <integer> Number to be written to buf.
• offset <integer> Number of bytes to skip before starting to write. Must satisfy 0 <= offset <= buf.length - 4. Default: 0.
• Returns: <integer> offset plus the number of bytes written.

Writes value to buf at the specified offset with the specified endianness (writeInt32BE() writes aS big endian, writeInt32LE() writes as little endian). value must be a valid signed 32-bit integer. Behavior is undefined when value is anything other than a signed 32-bit integer.

value is interpreted and written as a two's complement signed integer.

const buf = Buffer.allocUnsafe(8);

buf.writeInt32BE(0x01020304, 0);
buf.writeInt32LE(0x05060708, 4);

console.log(buf);
// Prints: <Buffer 01 02 03 04 08 07 06 05>

buf.writeIntBE(value, offset, byteLength)#

buf.writeIntLE(value, offset, byteLength)#

• value <integer> Number to be written to buf.
• offset <integer> Number of bytes to skip before starting to write. Must satisfy 0 <= offset <= buf.length - byteLength.
• byteLength <integer> Number of bytes to write. Must satisfy 0 < byteLength <= 6.
• Returns: <integer> offset plus the number of bytes written.

Writes byteLength bytes of value to buf at the specified offset. Supports up to 48 bits of accuracy. Behavior is undefined when value is anything other than a signed integer.

const buf = Buffer.allocUnsafe(6);

buf.writeIntBE(0x1234567890ab, 0, 6);

console.log(buf);
// Prints: <Buffer 12 34 56 78 90 ab>

buf.writeIntLE(0x1234567890ab, 0, 6);

console.log(buf);
// Prints: <Buffer ab 90 78 56 34 12>

buf.writeUInt8(value[, offset])#

• value <integer> Number to be written to buf.
• offset <integer> Number of bytes to skip before starting to write. Must satisfy 0 <= offset <= buf.length - 1. Default: 0.
• Returns: <integer> offset plus the number of bytes written.

Writes value to buf at the specified offset. value must be a valid unsigned 8-bit integer. Behavior is undefined when value is anything other than an unsigned 8-bit integer.

const buf = Buffer.allocUnsafe(4);

buf.writeUInt8(0x3, 0);
buf.writeUInt8(0x4, 1);
buf.writeUInt8(0x23, 2);
buf.writeUInt8(0x42, 3);

console.log(buf);
// Prints: <Buffer 03 04 23 42>

buf.writeUInt16BE(value[, offset])#

buf.writeUInt16LE(value[, offset])#

• value <integer> Number to be written to buf.
• offset <integer> Number of bytes to skip before starting to write. Must satisfy 0 <= offset <= buf.length - 2. Default: 0.
• Returns: <integer> offset plus the number of bytes written.

Writes value to buf at the specified offset with the specified endianness (writeUInt16BE() writes as big endian, writeUInt16LE() writes as little endian). value must be a valid unsigned 16-bit integer. Behavior is undefined when value is anything other than an unsigned 16-bit integer.

const buf = Buffer.allocUnsafe(4);

buf.writeUInt16BE(0xdead, 0);
buf.writeUInt16BE(0xbeef, 2);

console.log(buf);
// Prints: <Buffer de ad be ef>

buf.writeUInt16LE(0xdead, 0);
buf.writeUInt16LE(0xbeef, 2);

console.log(buf);
// Prints: <Buffer ad de ef be>

buf.writeUInt32BE(value[, offset])#

buf.writeUInt32LE(value[, offset])#

• value <integer> Number to be written to buf.
• offset <integer> Number of bytes to skip before starting to write. Must satisfy 0 <= offset <= buf.length - 4. Default: 0.
• Returns: <integer> offset plus the number of bytes written.

Writes value to buf at the specified offset with the specified endianness (writeUInt32BE() writes as big endian, writeUInt32LE() writes as little endian). value must be a valid unsigned 32-bit integer. Behavior is undefined when value is anything other than an unsigned 32-bit integer.

const buf = Buffer.allocUnsafe(4);

buf.writeUInt32BE(0xfeedface, 0);

console.log(buf);
// Prints: <Buffer fe ed fa ce>

buf.writeUInt32LE(0xfeedface, 0);

console.log(buf);
// Prints: <Buffer ce fa ed fe>

buf.writeUIntBE(value, offset, byteLength)#

buf.writeUIntLE(value, offset, byteLength)#

• value <integer> Number to be written to buf.
• offset <integer> Number of bytes to skip before starting to write. Must satisfy 0 <= offset <= buf.length - byteLength.
• byteLength <integer> Number of bytes to write. Must satisfy 0 < byteLength <= 6.
• Returns: <integer> offset plus the number of bytes written.

Writes byteLength bytes of value to buf at the specified offset. Supports up to 48 bits of accuracy. Behavior is undefined when value is anything other than an unsigned integer.

const buf = Buffer.allocUnsafe(6);

buf.writeUIntBE(0x1234567890ab, 0, 6);

console.log(buf);
// Prints: <Buffer 12 34 56 78 90 ab>

buf.writeUIntLE(0x1234567890ab, 0, 6);

console.log(buf);
// Prints: <Buffer ab 90 78 56 34 12>

new Buffer(array)#

• array <integer[]> An array of bytes to copy from.

See Buffer.from(array).

new Buffer(arrayBuffer[, byteOffset[, length]])#

• arrayBuffer <ArrayBuffer> | <SharedArrayBuffer> An ArrayBuffer, SharedArrayBuffer or the .buffer property of a TypedArray.
• byteOffset <integer> Index of first byte to expose. Default: 0.
• length <integer> Number of bytes to expose. Default: arrayBuffer.byteLength - byteOffset.

See Buffer.from(arrayBuffer[, byteOffset[, length]]).

new Buffer(buffer)#

• buffer <Buffer> | <Uint8Array> An existing Buffer or Uint8Array from which to copy data.

See Buffer.from(buffer).

new Buffer(size)#

• size <integer> The desired length of the new Buffer.

See Buffer.alloc() and Buffer.allocUnsafe(). This variant of the constructor is equivalent to Buffer.alloc().

new Buffer(string[, encoding])#

• string <string> String to encode.
• encoding <string> The encoding of string. Default: 'utf8'.

See Buffer.from(string[, encoding]).

buffer.INSPECT_MAX_BYTES#

• <integer> Default: 50

Returns the maximum number of bytes that will be returned when buf.inspect() is called. This can be overridden by user modules. See util.inspect() for more details on buf.inspect() behavior.

This is a property on the buffer module returned by require('buffer'), not on the Buffer global or a Buffer instance.

buffer.kMaxLength#

• <integer> The largest size allowed for a single Buffer instance.

An alias for buffer.constants.MAX_LENGTH.

This is a property on the buffer module returned by require('buffer'), not on the Buffer global or a Buffer instance.

buffer.transcode(source, fromEnc, toEnc)#

• source <Buffer> | <Uint8Array> A Buffer or Uint8Array instance.
• fromEnc <string> The current encoding.
• toEnc <string> To target encoding.
• Returns: <Buffer>

Re-encodes the given Buffer or Uint8Array instance from one character encoding to another. Returns a new Buffer instance.

Throws if the fromEnc or toEnc specify invalid character encodings or if conversion from fromEnc to toEnc is not permitted.

Encodings supported by buffer.transcode() are: 'ascii', 'utf8', 'utf16le', 'ucs2', 'latin1', and 'binary'.

The transcoding process will use substitution characters if a given byte sequence cannot be adequately represented in the target encoding. For instance:

const buffer = require('buffer');

const newBuf = buffer.transcode(Buffer.from('€'), 'utf8', 'ascii');
console.log(newBuf.toString('ascii'));
// Prints: '?'

Because the Euro (€) sign is not representable in US-ASCII, it is replaced with ? in the transcoded Buffer.

This is a property on the buffer module returned by require('buffer'), not on the Buffer global or a Buffer instance.

Class: SlowBuffer#

See Buffer.allocUnsafeSlow(). This was never a class in the sense that the constructor always returned a Buffer instance, rather than a SlowBuffer instance.

new SlowBuffer(size)#

• size <integer> The desired length of the new SlowBuffer.

See Buffer.allocUnsafeSlow().

Buffer Constants#

buffer.constants is a property on the buffer module returned by require('buffer'), not on the Buffer global or a Buffer instance.

buffer.constants.MAX_LENGTH#

• <integer> The largest size allowed for a single Buffer instance.

On 32-bit architectures, this value currently is (2^30)-1 (~1GB). On 64-bit architectures, this value currently is (2^31)-1 (~2GB).

This value is also available as buffer.kMaxLength.

buffer.constants.MAX_STRING_LENGTH#

• <integer> The largest length allowed for a single string instance.

Represents the largest length that a string primitive can have, counted in UTF-16 code units.

This value may depend on the JS engine that is being used.

Buffer.from(), Buffer.alloc(), and Buffer.allocUnsafe()#

In versions of Node.js prior to 6.0.0, Buffer instances were created using the Buffer constructor function, which allocates the returned Buffer differently based on what arguments are provided:

• Passing a number as the first argument to Buffer() (e.g. new Buffer(10)) allocates a new Buffer object of the specified size. Prior to Node.js 8.0.0, the memory allocated for such Buffer instances is not initialized and can contain sensitive data. Such Buffer instances must be subsequently initialized by using either buf.fill(0) or by writing to the entire Buffer before reading data from the Buffer. While this behavior is intentional to improve performance, development experience has demonstrated that a more explicit distinction is required between creating a fast-but-uninitialized Buffer versus creating a slower-but-safer Buffer. Since Node.js 8.0.0, Buffer(num) and new Buffer(num) return a Buffer with initialized memory.
• Passing a string, array, or Buffer as the first argument copies the passed object's data into the Buffer.
• Passing an ArrayBuffer or a SharedArrayBuffer returns a Buffer that shares allocated memory with the given array buffer.

Because the behavior of new Buffer() is different depending on the type of the first argument, security and reliability issues can be inadvertently introduced into applications when argument validation or Buffer initialization is not performed.

For example, if an attacker can cause an application to receive a number where a string is expected, the application may call new Buffer(100) instead of new Buffer("100"), leading it to allocate a 100 byte buffer instead of allocating a 3 byte buffer with content "100". This is commonly possible using JSON API calls. Since JSON distinguishes between numeric and string types, it allows injection of numbers where a naively written application that does not validate its input sufficiently might expect to always receive a string. Before Node.js 8.0.0, the 100 byte buffer might contain arbitrary pre-existing in-memory data, so may be used to expose in-memory secrets to a remote attacker. Since Node.js 8.0.0, exposure of memory cannot occur because the data is zero-filled. However, other attacks are still possible, such as causing very large buffers to be allocated by the server, leading to performance degradation or crashing on memory exhaustion.

To make the creation of Buffer instances more reliable and less error-prone, the various forms of the new Buffer() constructor have been deprecated and replaced by separate Buffer.from(), Buffer.alloc(), and Buffer.allocUnsafe() methods.

Developers should migrate all existing uses of the new Buffer() constructors to one of these new APIs.

• Buffer.from(array) returns a new Buffer that contains a copy of the provided octets.
• Buffer.from(arrayBuffer[, byteOffset[, length]]) returns a new Buffer that shares the same allocated memory as the given ArrayBuffer.
• Buffer.from(buffer) returns a new Buffer that contains a copy of the contents of the given Buffer.
• Buffer.from(string[, encoding]) returns a new Buffer that contains a copy of the provided string.
• Buffer.alloc(size[, fill[, encoding]]) returns a new initialized Buffer of the specified size. This method is slower than Buffer.allocUnsafe(size) but guarantees that newly created Buffer instances never contain old data that is potentially sensitive. A TypeError will be thrown if size is not a number.
• Buffer.allocUnsafe(size) and Buffer.allocUnsafeSlow(size) each return a new uninitialized Buffer of the specified size. Because the Buffer is uninitialized, the allocated segment of memory might contain old data that is potentially sensitive.

Buffer instances returned by Buffer.allocUnsafe() and Buffer.from(array) may be allocated off a shared internal memory pool if size is less than or equal to half Buffer.poolSize. Instances returned by Buffer.allocUnsafeSlow() never use the shared internal memory pool.

The --zero-fill-buffers command line option#

Node.js can be started using the --zero-fill-buffers command line option to cause all newly-allocated Buffer instances to be zero-filled upon creation by default. Without the option, buffers created with Buffer.allocUnsafe(), Buffer.allocUnsafeSlow(), and new SlowBuffer(size) are not zero-filled. Use of this flag can have a measurable negative impact on performance. Use the --zero-fill-buffers option only when necessary to enforce that newly allocated Buffer instances cannot contain old data that is potentially sensitive.

$ node --zero-fill-buffers
> Buffer.allocUnsafe(5);
<Buffer 00 00 00 00 00>

What makes Buffer.allocUnsafe() and Buffer.allocUnsafeSlow() "unsafe"?#

When calling Buffer.allocUnsafe() and Buffer.allocUnsafeSlow(), the segment of allocated memory is uninitialized (it is not zeroed-out). While this design makes the allocation of memory quite fast, the allocated segment of memory might contain old data that is potentially sensitive. Using a Buffer created by Buffer.allocUnsafe() without completely overwriting the memory can allow this old data to be leaked when the Buffer memory is read.

While there are clear performance advantages to using Buffer.allocUnsafe(), extra care must be taken in order to avoid introducing security vulnerabilities into an application.