How's JavaScript's native sort function implemented?

Question

Let's see an example first.

var everything = [4,'Red', '$200', 'white', 7.4, 12, true, 0.3, false];
console.log(everything.sort(function(a, b) {return a - b;})); 
// [4, "Red", "$200", "white", false, 0.3, true, 7.4, 12]

I think this is weird, even when I know Number("Red"), Number("$200") and Number("white") all give NaN when comparing.

Why is 4 at the first of the result? I guess it has something to do with the implementation of Array.prototype.sort, so how can I see its implementation?

Answer 1

Specification

According to ECMAScript 5 - Section 15.4.4.11,

If comparefn is not undefined and is not a consistent comparison function for the elements of this array (see below), the behaviour of sort is implementation-defined.

[...]

A function comparefn is a consistent comparison function for a set of values S if all of the requirements below are met for all values a, b, and c (possibly the same value) in the set S [...]

• Calling comparefn(a,b) always returns the same value v when given a specific pair of values a and b as its two arguments. Furthermore, Type(v) is Number, and v is not NaN. [...]

That is to say, if the return value of the comparison function is NaN, then the behaviour of the call to sort is 'implementation defined'.

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In practice

Google Chrome and NodeJS (V8 Engine), as well as Opera (Futhark/Carakan) return:

[4, "Red", "$200", "white", false, 0.3, true, 7.4, 12]

Firefox (SpiderMonkey) returns:

[false, 0.3, true, 4, "Red", "$200", "white", 7.4, 12]

Internet Explorer returns:

["Red", "$200", false, 0.3, true, 4, "white", 7.4, 12]