Is it ok to define a prototype function on Object in Javascript? [duplicate]

Question

Object.prototype.doSomething = function(p) {
    this.innerHTML = "<em>bar</em>";
    this.style.color = "#f00";
    alert(p);
};

document.getElementById("foo").doSomething("Hello World");

<div id="foo"><strong>foo</strong></div>

The code above works fine.

But I remember that I saw this somewhere: Do not mess with native Object. well, something like that.

So is it ok to define a prototype function on Object? Are there any reasons that I should not do this?

Answer 1

The only safe way to add to Object.prototype is to use the ES5 method Object.defineProperty to create a non-enumerable property:

Object.defineProperty(Object.prototype, 'doSomething', {
    value: function(p) { ... },
    enumerable: false,  // actually the default
});

This ensures that the new property doesn't appear if you do for (key in object).

Note that this function cannot be reliably shimmed because non-enumerable properties didn't exist in previous versions of ECMAScript / JavaScript.

In any event, if your method only applies to HTML page elements, you would be better adding this to Element.prototype instead of to Object.prototype, although older (IE) browsers may complain if you do this.