javascript sort sparse array keep indexes

Question

What is the best method to sort a sparse array and keep the elements on the same indexes? For example:

a[0] = 3, 
a[1] = 2, 
a[2] = 6,
a[7] = 4,
a[8] = 5,

I would like after the sort to have

a[0] = 2, 
a[1] = 3, 
a[2] = 4, 
a[7] = 5, 
a[8] = 6.

Solution 1

Here's one approach. It copies the defined array elements to a new array and saves their indexes. It sorts the new array and then puts the sorted results back into the indexes that were previously used.

var a = [];
a[0] = 3;
a[1] = 2; 
a[2] = 6; 
a[7] = 4; 
a[8] = 5;


// sortFn is optional array sort callback function, 
// defaults to numeric sort if not passed
function sortSparseArray(arr, sortFn) {
    var tempArr = [], indexes = [];
    for (var i = 0; i < arr.length; i++) {
        // find all array elements that are not undefined
        if (arr[i] !== undefined) {
            tempArr.push(arr[i]);    // save value
            indexes.push(i);         // save index
        }
    }
    // sort values (numeric sort by default)
    if (!sortFn) {
        sortFn = function(a,b) {
            return(a - b);
        }
    }
    tempArr.sort(sortFn);
    // put sorted values back into the indexes in the original array that were used
    for (var i = 0; i < indexes.length; i++) {
        arr[indexes[i]] = tempArr[i];
    }
    return(arr);
}

Working demo: http://jsfiddle.net/jfriend00/3ank4/

Solution 2

You can

1. Use filter or Object.values to obtain an array with the values of your sparse array.
2. Then sort that array, from largest to smaller. Be aware it's not stable, which may be specially problematic if some values are not numeric. You can use your own sorting implementation.
3. Use map and pop to obtain the desired array. Assign it to a.

var b = a.filter(function(x) {
    return true;
}).sort(function(x,y) {
    return y - x;
});
a = a.map([].pop, b);

Or, in ECMAScript 2017,

a = a.map([].pop, Object.values(a).sort((x,y) => y-x));