# String.fromEnvironment constructor

const String.fromEnvironment(String name, { String defaultValue })

Returns the string value of the environment declaration `name`.

Environment declarations are provided by the surrounding system compiling or running the Dart program. Declarations map a string key to a string value.

If `name` is not declared in the environment, the result is instead `defaultValue`.

Example of getting a value:

    const String.fromEnvironment("defaultFloo", defaultValue: "no floo")

Example of checking whether a declaration is there at all:

    var isDeclared = const String.fromEnvironment("maybeDeclared") != null;

## Implementation

    // The .fromEnvironment() constructors are special in that we do not want
    // users to call them using "new". We prohibit that by giving them bodies
    // that throw, even though const constructors are not allowed to have bodies.
    // Disable those static errors.
    //ignore: const_constructor_with_body
    //ignore: const_factory
    external const factory String.fromEnvironment(String name,
        {String defaultValue});