How does JavaScript's sort (compareFunction) work? [duplicate]

Answer 1

It will sort the item by the subtracted value by moving it that value lower or higher.

Here is some info:

https://developer.mozilla.org/en/docs/Web/JavaScript/Reference/Global_Objects/Array/sort

If compareFunction is supplied, the array elements are sorted according to the return value of the compare function. If a and b are two elements being compared, then:

• If compareFunction(a, b) is less than 0, sort a to a lower index than b, i.e. a comes first.
• If compareFunction(a, b) returns 0, leave a and b unchanged with respect to each other, but sorted with respect to all different
  elements. Note: the ECMAscript standard does not guarantee this
  behaviour, and thus not all browsers (e.g. Mozilla versions dating
  back to at least 2003) respect this.
• If compareFunction(a, b) is greater than 0, sort b to a lower index than a.
• compareFunction(a, b) must always return the same value when given a specific pair of elements a and b as its two arguments. If inconsistent results are returned then the sort order is undefined.

So, the compare function has the following form:

function compare(a, b) {
  if (a is less than b by some ordering criterion) {
    return -1;
  }
  if (a is greater than b by the ordering criterion) {
    return 1;
  }
  // a must be equal to b
  return 0;
}

To compare numbers instead of strings, the compare function can simply subtract b from a. The following function will sort the array ascending (if it doesn't contain Infinity and NaN):

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