Why does for(var i in Math) not iterate through Math.* in Javascript?

Question

For some reason for(var i in Math){console.log(i)} doesn\'t show the expected tan, cos, atan2, E, PI in Javascript.

Answer 1

As with most built-in objects in JavaScript, properties and methods of the Math object are defined in the ECMAScript spec (section 15.8.1) as not being enumerable via the (inaccessible to script) DontEnum attribute. In ECMAScript 5, you can mark properties and methods of your own objects as being non-enumerable also:

var o = {};

Object.defineProperty(o, "p", {
    enumerable: false,
    value: 1
});

Object.defineProperty(o, "q", {
    enumerable: true,
    value: 2
});

for (var i in o) {
    console.log(i + "=>" + o[i]);
}
// q=>2