JavaScript Array.sort doesn't work on some arrays of numbers

Question

I'm trying to use JavaScript's sort function on arrays of numbers and sometimes it doesn't do anything:

var a = [200,20].sort(); // [20,200]
var b = [200,21].sort(); // [200,21]

jsfiddle

When a method doesn't work the way you'd expect, step 1 is to read [documentation](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/sort) — cookie monster, Feb 24 '14 at 14:05
I didn't even suspect that js would sort everything alphanumerically. I was certain that I was doing something wrong. — nima, Feb 24 '14 at 14:13

score 5 · Accepted Answer · answered Feb 24 '14 at 13:49

5

Javascript sorts everything as strings (=alphabetically) by default. The string "200" is less than the string "21". To sort as numbers you have to tell it so:

[200,21].sort(function(a,b) { return  a-b })

answered Feb 24 '14 at 13:49

georg

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This code can be shortened using an [Arrow Function](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions/Arrow_functions). `numberArray.sort((a, b) => (a - b));` Yay! **Note: check if your JS engine supports Arrow Functions.** – Константин Ван Dec 28 '15 at 09:54

score 0 · Answer 2 · answered Feb 24 '14 at 13:53

Yep, this is the standard behaviour for "sort" because it perform "string" reordering. If you want to sort by number value you must pass a "compare" function to sort, like this:

[200,21].sort(function (a, b) {
  return a-b; 
})
// [21, 200]

The function must return 0 for identical value, n < 0 if a < b and n > 0 if a > b. For that reason, the difference is enough to provide sorting (assuming that you're not using huge numbers)

JavaScript Array.sort doesn't work on some arrays of numbers

2 Answers2

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