In Array.sort()’s compareFn, the order of a and b doesn't matter

MDN documentation on Array.prototype.sort() says of the the optional CompareFn,

A function that determines the order of the elements. The function is called with the following arguments:

• a The first element for comparison. Will never be undefined.
• b The second element for comparison. Will never be undefined.

It should return a number where:

• A negative value indicates that a should come before b.
• A positive value indicates that a should come after b.
• Zero or NaN indicates that a and b are considered equal.

To memorize this, remember that (a, b) => a - b sorts numbers in ascending order.

But Stephen Girder¹ observes that, if we console.log within the comapreFn on Chrome, a actually comes after b E.g.

const compareFn = (a, b) => {
console.log("a:", a)
console.log("b:", b)
console.log('a - b', a - b )
return a-b
}

const data = [4, 3, 2, 1]

data.sort(compareFn)

Logs:

a: 3
b: 4
a - b: -1
a: 2
b: 3
a - b: -1
a: 1
b: 2
a - b: -1

After some googling (and finding this article), I realised that the order of a and b doesn’t matter.

If a is the first element and b the second element, everything just flips around. The programme should instead log something like:

a: 4
b: 3
a - b: 1
a: 4
b: 2
a - b: 2
a: 4
b: 1
a - b: 3

In any case, conceptually the comparator function has nothing to do with this kind of step-by-step comparison (which is part of the procedure for bubble sort). The comparator function just needs to specify the direction of the sort: it doesn’t matter how the sorting is done.