Why can't JavaScript sort [5, 10, 1]? [duplicate]

Question

This seems like a simple sort, yet JavaScript is giving an incorrect result.

Am I doing something wrong or is this a language quirk?

[5, 10, 1].sort();

[ 1, 10, 5 ]

Answer 1

Javascript sorts alphabetically. This means that "10" is lower than "5", because "1" is lower than "5".

To sort numerical values you need to pass in numerical comparator like this:

function sorter(a, b) {
  if (a < b) return -1;  // any negative number works
  if (a > b) return 1;   // any positive number works
  return 0; // equal values MUST yield zero
}

[1,10, 5].sort(sorter);

Or you can cheat by passing simpler function:

function sorter(a, b){
  return a - b;
}

[1, 10, 5].sort(sorter);

Logic behind this shorter function is that comparator must return x>0 if a > b, x<0 if a < b and zero if a is equal to b. So in case you have

a=1 b=5
a-b will yield negative(-4) number meaning b is larger than a

a=5 b=1
a-b will yield positive number(4) meaning a is larger than b

a=3 b=3
a-b will yield 0 meaning they are equal