Question

Try and do the following:

for (var key in String.prototype)
    console.log(key);

It gives you…nothing (well, unless you defined some foreign stuff yourself.) However, you still have String.prototype.split for example. I tried it on every other native object (Number, Array, Object) for the same result.

The following also “doesn’t work”:

for (var key in Array)
    console.log(key);

While there’s Array.isArray for example.

Object.keys(Array.prototype) gives an empty array, and so does Object.keys(Array). However, Object.keys(jQuery) for example provides a giant array, as expected.

So, why can’t we iterate over natives provided by the browser, yet still access them?

Answer 1

From the MDC page for for..in:

A for...in loop does not iterate over built-in properties

The reason is that properties in Javascript are either enumerable or non-enumerable; “enumerable” means you can access the property in a for..in loop. All built-in properties are non-enumerable.

Modern browsers support the Object.getOwnPropertyNames method:

console.log(Object.getOwnPropertyNames(Array.prototype));
// ["length", "constructor", "concat", "map", "sort", "join", "indexOf", "filter", "some", "toString", "reduceRight", "splice", "forEach", "shift", "unshift", "toLocaleString", "lastIndexOf", "reverse", "reduce", "pop", "push", "every", "slice"]