Why can't I use .call() in a replace function?

Why is this vali开发者_StackOverflow社区d:

function func(a,b,c) {
    console.log(this, a,b,c);
    return '';
}
'testing'.replace(/e/, func);

but this isn't:

function func(a,b,c) {
    console.log(this, a,b,c);
    return '';
}
'testing'.replace(/e/, func.call);

if func is a function reference, and call is a function reference shouldn't they both work?

Here's a fiddle of this

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Because when you pass the call function, you take it out of the context of func, so inside call the this keyword will refer to window instead of func.

window is not a function, but call expects this to be a function, so it breaks.

For comparison.

var AnObject = {
    call: function () { console.log("this.location is: ", this.location); },
    location: "This string is the location property of AnObject"
};

AnObject.call();
setTimeout(AnObject.call, 500);

---

Because .call() itself is a method, but not one that is useful to replace().

In other words, while your intention is to pass func, you're actually passing a completely different function (call) that serves a purpose not useful as an argument to .replace().