Type guard from Function | RegExp to Function fails

[dsherret]

Is this a bug?

var a: Function | RegExp;

if (typeof a === "function") {
    a(100); // compile error, still a Function | RegExp
}

...since the typeof a RegExp will be an object and RegExp is a type that lacks a call signature.

It works fine when a user-defined type guard is used, which seems inconsistent:

function isFunction(obj: any) : obj is Function {
    return typeof obj === "function";
}

var a: Function | RegExp;

if (isFunction(a)) {
    a(10); // no compile error
}

Version: TypeScript 1.6
Discovered here.

[danquirk]

Today the === pattern for type guards only supports the literal types. The supported pattern for type guarding function types is to use instanceof. So this does work:

var a: Function | RegExp;

if (a instanceof Function) {
    a(100); // ok
}

This is usually the general recommendation for doing actual type tests on functions in JavaScript anyway.

[RyanCavanaugh]

Logged #4868