Get the current index in sort function

Question

What I need to know is a way to get the current index in the compare function of sort method in an array. Suppose this code:

var points = [40, 100, 1, 5, 25, 10];
points.sort(function (a, b) { 
  return a - b; 
  //i need the current index of `a` and `b` here 
});

In the anonymous function, I need the current index of a and b objects. How can I get it?

Answer 1

Make an array of objects with indices...

var arr = [1,5,2,3,8,13, 1];
var arr2 = arr.map(function(o, i) {return {idx: i, obj: o}; }).sort(function(a, b) {
    console.log('a.idx:', a.idx, 'b.idx:', b.idx);
    return a.obj - b.obj;
});

for(var i = 0, j = arr2.length; i < j; i++){
    console.log('i:', i, 'arr2[i].obj:', arr2[i].obj);
}

fiddle: https://jsfiddle.net/k6xdqpb2/

Answer 2

The index of a or b?

var indexOfA = points.indexOf(a);

This will give you the index that a first appears in the array.

Depending on what you are trying to do, another option may be to use a map for sorting as described in more details under the 'Sorting with map' heading here. An example of this approach is given in deostroll's answer.