ECMA-262 quote of the day

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: 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)
ECMA-262 3rd edition or ECMA-262 5th edition, Array.prototype.sort (comparefn)