description

# bool.hasEnvironment constructor Null safety

const bool.hasEnvironment(

1. String name

)

Whether there is an environment declaration `name`.

Returns true iff there is an environment declaration with the name `name` If there is then the value of that declaration can be accessed using `const String.fromEnvironment(name)`. Otherwise, `String.fromEnvironment(name, defaultValue: someString)` will evaluate to the given `defaultValue`.

This constructor can be used to handle an absent declaration specifically, in ways that cannot be represented by providing a default value to the `C.fromEnvironment` constructor where `C` is String, int, or bool.

Example:

    const loggingIsDeclared = const bool.hasEnvironment("logging");

    const String? logger = loggingIsDeclared
        ? String.fromEnvironment("logging")
        : null;

## Implementation

    // The .hasEnvironment() constructor is 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 bool.hasEnvironment(String name);