Question

If I have an object:

A = {a:true}

Why do I have to use:

Object.keys(A)

and not:

A.keys()

If keys is a method of Object, and everything inherits from Object, shouldn’t A be able to call keys?

Answer 1

Object.keys is a so-to-say “static” method attached strictly to the Object function, not to its instances.

For it to be inherited, it would need to be defined as Object.prototype.keys.

You can certainly add it yourself if you so desire:

Object.prototype.keys = function () {
    return Object.keys(this);
};

Just note, as Rocket mentioned in the comments, “own” properties take precedence over prototype properties:

var foo = {};
foo.keys(); // Array of enumerable key names, if any

var bar = { keys: true };
bar.keys(); // TypeError: not a function