isA<T extends JSAny?> method

Whether this Object? is an instance of the JavaScript type that is declared by T.

The result of this function tells you if you can safely cast to T. Because of runtime type differences across platforms, do not expect that that this function returns the same value for the same type on every platform e.g. 0.isA().

Since the type-check this function emits is determined at compile-time, T needs to be an interop extension type that can also be determined at compile-time. In particular, isA can't be provided a generic type variable as a type argument.

This method uses a combination of null, typeof, and instanceof checks and intrinsic functions in order to do this check. Use this instead of is checks for platform-independent type checks.

If T is a primitive JS type like JSString, this uses a typeof check that corresponds to that primitive type like typeofEquals('string').

If T is a non-primitive JS type like JSArray or an interop extension type on one, this uses an instanceof check using the name or the @JS rename of the given type like instanceOfString('Array'). Note that if you rename the library using the @JS annotation, this uses the rename in the instanceof check like instanceOfString('library1.JSClass').

To determine the JavaScript constructor to use as the second operand in the instanceof check, this function uses the JavaScript name associated with the extension type, which is either the argument given to the @JS annotation or the Dart declaration name. So, if you had an interop extension type JSClass that wraps JSArray without a rename, this does an instanceOfString('JSClass') check and not an instanceOfString('Array') check.

There are a few values for T that are exceptions to this rule:

• JSTypedArray: As TypedArray does not exist as a class in JavaScript, this does some prototype checking to make isA<JSTypedArray> do the right thing.
• JSBoxedDartObject: isA<JSBoxedDartObject> will check if the value is a result of a previous ObjectToJSBoxedDartObject.toJSBox call.
• JSAny: isA<JSAny> will call an intrinsic function to check that the value is a JS value.
• JSObject: isA<JSObject> will call an intrinsic function to check that the value is a JS object (instanceof Object is insufficient for some objects).
• JSExportedDartFunction: isA<JSExportedDartFunction> will check if the value is a result of a previous FunctionToJSExportedDartFunction.toJS or FunctionToJSExportedDartFunction.toJSCaptureThis call.
• User interop types whose representation types are JS primitive types: This will result in an error to avoid confusion on whether the user interop type is used in the type-check. Use the primitive JS type as the value for T instead.
• User interop types that have an object literal constructor: This will result in an error as you likely want to use JSObject instead.