isA<T extends JSAny?> method

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

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 in order to do this check. Use this instead of is 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 two exceptions to this rule. The first exception is JSTypedArray. As TypedArray does not exist as a property in JavaScript, this does some prototype checking to make isA<JSTypedArray> do the right thing. The other exception is JSAny. If you do a isA<JSAny> check, it will only do a null check.

Using this method with a T that has an object literal constructor will result in an error as you likely want to use JSObject instead.

Using this method with a T that wraps a primitive JS type will result in an error telling you to use the primitive JS type instead.