fn instanceof RegExp

Question

I was staring at the angular source code (as I do from time to time) and I was surprised to see the following check:

if (isFunction(fn) && !(fn instanceof RegExp)) {

} else {
  // in IE, native methods are not functions so they cannot be bound (note: they don't need to be)
}

It's part of angular.bind implementation. The full snippet can be found here.

This is the implementation for isFunction (in case you were wondering):

function isFunction(value) {return typeof value === 'function';}

Now, let me add this, just in case:

I know what instanceof operator does. It checks to see if a function's prototype (in this case RegExp.prototype) can be found on the prototype chain starting from a particular object.

Now, my question is:

Can you give me an example of a scenario where the above code will follow the else clause? I'm interested in the second part of the expression, I know you can make that condition fail by not providing a function.

That comment warns me about a weird behaviour in IE, but I cannot reproduce it.

I have a vague memory that (perhaps only in IE) a RegExp instance is callable as a function. I can't find anything about that with a few googles however. — Pointy, Sep 08 '15 at 14:32

score 1 · Answer 1 · edited Sep 08 '15 at 15:48

1

In IE7:

typeof window.item // => 'string'

However (in IE7):

window.item(0) // => [object] { onbeforeunload: null, ...}

In IE9:

typeof window.item // => 'function'

In IE11:

typeof window.item // => 'string'

edited Sep 08 '15 at 15:48

sdgluck

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answered Sep 08 '15 at 15:34

Petr Averyanov

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`typeof window.item` yields "function" on IE11 (on my machine, at least). Also none of these evaluates to true when it comes to the expression `fn instanceof RegExp` – Sep 08 '15 at 17:14

score 0 · Answer 2 · edited May 23 '17 at 11:58

In Netscape's original implementation, RegExp objects were actually functions. re(string) and re.exec(string) were supposed to mean the same thing. In the Netscape and Mozilla browsers, it was supported up through Firefox 4 (released on March 22, 2011). However, typeof stopped returning "function" for RegExp objects even earlier.

It's not the first time a legacy of the Dark Ages has been noticed in the code of Angular. Here is another example. Angular 1.0.0 was released on June 13, 2012.

fn instanceof RegExp

2 Answers2