One-liner with Strange Output -- What's going on with strings as 'this'?

Question

I found interesting output when I set this to a string using apply and then console.log'd it. What's up?

In Chrome's Javascript console,

(function(){ return this }).apply("hello");

Outputs to:

String {0: "h", 1: "e", 2: "l", 3: "l", 4: "o"}

Why isn't it "hello" like I would have expected?

Interestingly, checking this output with typeof:

typeof (function(){ return this }).apply("hello");

Gives me "object", instead of "string".

I'm guessing that is some wizardry with apply that I don't understand?

Answer 1

When the argument for this is passed in non-strict mode, it is converted to an object, so it's returning a string object, which is different from a string value. Each index in a string object corresponds to the characters of a string value, in order. To convert it back to a "normal" string, just call toString() on it - that makes it a string value like you're used to.

This does not occur in ES5 strict mode (when you insert 'use strict' at the beginning of your program or function), as in that mode arguments are not coerced to objects, but given directly.

// if you're not passing any arguments, it doesn't matter whether you use apply or call
(function () { return this; }).call("see"); // { 0: "s", 1: "e", 2: "e" }, plus some other special properties
(function () { return this.toString(); }).call("see"); // "see"
(function () { 'use strict'; return this; }).call("see"); // "see", where strict mode is supported

Reference: http://www.ecma-international.org/ecma-262/5.1/#sec-10.4.3 (note that the ThisBinding refers to the value of the this keyword inside the function).