Question

The answers from this question says to use this to check if function is defined:

typeof yourFunction === 'function'

But I’ve tried this on a non-standard function link(). And actually this returned false. The function is available on every browser I’ve tried – IE, Chrome, Opera, FireFox.

typeof String.link === 'function' // false
typeof String.link() === 'function' // Uncaught error ...

Then somewhere I find:

typeof String.prototype.link === 'function' //true

which actually returns true. What is the difference and why the first one fails?

Answer 1

String is a constructor function, and functions are also objects. You can append properties to it.

For example:

function foo(){
    alert('from foo');
}

foo.bar = function(){
    alert('bar on foo');
}

foo();     //from foo
foo.bar(); //bar on foo

It’s the same reason how jQuery’s $ acts like an object (eg. $.each()) and like a function as well (eg. $(selector)).

And so:

• using String.link is accessing a property of the constructor function itself – which does not exist.

• using String.prototype.link accesses the link() function that comes with every string – which does exist (and which you should use)