Question

var array = [3,9,23,76,1,54,21,12,0,9,2];
var shuffled = array.sort(function() {return 0.5 - Math.random()});
console.log(shuffled);  

I’m aware of the results, and also satified with them.

The code above returns a shuffled order of the elements of my array.

I’m puzzled by why it results in that output.

What’s the point of the function inside of the .sort and how does it contribute to the output?

Answer 1

The function actually takes two arguments, which are 2 items from the array. The purpose is to compare these elements and return a number.
If the number is positive, the second item should be before the first item.
If the number is 0 or negative, the second item should be after the first item.

[0,1].sort(function(a,b){return  1;}); // [1, 0], reverses order
[0,1].sort(function(a,b){return  0;}); // [0, 1], does nothing
[0,1].sort(function(a,b){return -1;}); // [0, 1], does nothing 

In each case of the example above, a === 0 and b === 1.

---

Edit for step by step output

To see step-by-step what is happening with an ascending sort on [1,3,2,4,4,0], one can write a function which logs exactly what happens each step

arr = [1,3,2,4,4,0];
arr.sort(function(a,b){ // ascending order sort
    var result = a-b,
    str = '';
    if(result > 0) str = 'so swapping';
    else if(result === 0) str = 'so ignoring'
    else str = 'so continuing';
    console.log('with [ '+arr.join(', ')+' ]','comparing',a,'to',b,'resulting in',result, str);
    return result;
});
console.log('resulting in [ '+arr.join(', ')+' ]');

which outputs

with [ 1, 3, 2, 4, 4, 0 ] comparing 1 to 3 resulting in -2 so continuing
with [ 1, 3, 2, 4, 4, 0 ] comparing 3 to 2 resulting in 1 so swapping
with [ 1, 3, 3, 4, 4, 0 ] comparing 1 to 2 resulting in -1 so continuing
with [ 1, 2, 3, 4, 4, 0 ] comparing 3 to 4 resulting in -1 so continuing
with [ 1, 2, 3, 4, 4, 0 ] comparing 4 to 4 resulting in 0 so ignoring
with [ 1, 2, 3, 4, 4, 0 ] comparing 4 to 0 resulting in 4 so swapping
with [ 1, 2, 3, 4, 4, 4 ] comparing 4 to 0 resulting in 4 so swapping
with [ 1, 2, 3, 4, 4, 4 ] comparing 3 to 0 resulting in 3 so swapping
with [ 1, 2, 3, 3, 4, 4 ] comparing 2 to 0 resulting in 2 so swapping
with [ 1, 2, 2, 3, 4, 4 ] comparing 1 to 0 resulting in 1 so swapping
resulting in [ 0, 1, 2, 3, 4, 4 ]

---

For completeness, probability table for shuffle algorithm in original question (estimates, based on 500,000 trials for each index), X is starting index

 x     0     1     2     3     4     5     6     7     8     9    10
 0 |  8.0,  8.0,  6.2,  6.6,  9.2, 10.8,  9.3,  6.6,  6.2,  9.7, 18.8
 1 |  4.5,  4.6,  7.8, 12.2, 16.9, 12.9, 11.4, 10.7,  8.7,  6.1,  3.6
 2 | 15.5, 15.5, 10.3,  5.9,  3.7,  3.8,  5.7,  8.3, 10.6, 11.7,  8.5
 3 | 10.4, 10.3, 13.4, 10.2,  7.0,  6.5,  7.8,  9.4,  9.7,  8.8,  6.0
 4 |  6.4,  6.3, 10.7, 15.4, 11.4,  9.5,  9.6,  9.9,  8.9,  6.9,  4.4
 5 | 16.1, 16.1, 10.9,  7.7,  7.4,  7.6,  6.2,  4.4,  4.1,  6.5, 12.5
 6 |  4.7,  4.7,  7.1,  9.7, 12.6, 16.3, 13.6, 11.9,  9.2,  6.1,  3.6
 7 |  6.0,  6.0,  7.7,  8.9,  9.4, 10.9, 14.0, 13.6, 11.2,  7.4,  4.3
 8 |  8.4,  8.3,  9.1,  8.6,  7.3,  6.7,  8.2, 11.6, 13.8, 10.8,  6.7
 9 | 11.5, 11.4, 10.1,  7.6,  5.0,  3.7,  4.2,  6.7, 10.9, 15.8, 12.6
10 |  8.0,  8.1,  6.2,  6.6,  9.2, 10.9,  9.2,  6.6,  6.1,  9.8, 18.8