JavaScript IE 9: Custom sort function

Question

In IE 9, if I type this in the console:

[1, 4, 2, 3].sort(function (a, b) { return a < b; });

The resulting array is: [1, 4, 2, 3] .

If I do this in FF/Chrome, I get it, reverse-sorted: [4, 3, 2, 1] .

How come this doesn't work in IE?

EDIT: Is there a function out there that abstracts these browser differences? In other words, is there a function I can pass function(a, b) { return a < b; } in and get the same result in all browsers? Any open-source stuff?

Answer 1

Maybe because the function is supposed to return -1 if a is smaller, 0 if they are equal, and 1 if a is larger (or vice versa if you want reverse order). Edit: In fact, it must be zero, a positive or negative number (as @pimvdb pointed out, and that's what I make use of in the example below). Your function will never return -1 though and that might create problems.

Consider 1 and 3 . Your function returns 1 for 1 < 3 , which is ok, but returns 0 for 3 < 1 . In one case the number are different, in the other you are saying that they are equal.

That FF/Chrome sort it in reverse order might be due to the sorting algorithm they are using.

Try:

[1, 4, 2, 3].sort(function (a, b) { return b - a; });

Update: To substantiate this, we can have a look at the specification, Section 15.4.4.11 Array.prototype.sort(comparefn ), where the properties are given which have to be fulfilled by a comparison function:

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 : The notation a < _CF b means comparefn(a,b) < 0; a = _CF b means comparefn(a,b) = 0 (of either sign); and a > _CF b means comparefn(a,b) > 0.

• 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 . Note that this implies that exactly one of a < _CF b , a = _CF b , and a > _CF b will be true for a given pair of a and b .
• Calling comparefn(a,b) does not modify the this object.
• a = _CF a (reflexivity)
• If a = _CF b , then b = _CF a (symmetry)
• If a = _CF b and b = _CF c , then a = _CF c (transitivity of = _CF )
• If a < _CF b and b < _CF c , then a < _CF c (transitivity of < _CF )
• If a > _CF b and b > _CF c , then a > _CF c (transitivity of > _CF )

NOTE The above conditions are necessary and sufficient to ensure that comparefn divides the set S into equivalence classes and that these equivalence classes are totally ordered.