How to define default JavaScript sort callback function?

Question

I'm attempting to create my own sort function (question Async version of sort function in JavaScript). I've taken merge sort function from Rosetta Code and make it async:

// based on: https://rosettacode.org/wiki/Sorting_algorithms/Merge_sort#JavaScript
async function mergeSort(fn, array) {
    if (array.length <= 1) {
        return array;
    }
    const mid = Math.floor(array.length / 2),
          left = array.slice(0, mid), right = array.slice(mid);
    await mergeSort(fn, left)
    await mergeSort(fn, right)
    let ia = 0, il = 0, ir = 0;
    while (il < left.length && ir < right.length) {
        array[ia++] = (await fn(left[il], right[ir]) <= 0) ? left[il++] : right[ir++];
    }
    while (il < left.length) {
        array[ia++] = left[il++];
    }
    while (ir < right.length) {
        array[ia++] = right[ir++];
    }
    return array;
}

But I'm not sure how I can define default function fn to work the same as in JavaScript.

console.log([1, 2, 3, 10, 11, 100, 20].sort());

What should be the default sorting function to match those in the JavaScript engine? Should I convert numbers to strings and compare those? What is the proper implementation?

Answer 1

Updated Answer

The default sort method as defined in core.js looks like so

var getSortCompare = function (comparefn) {
  return function (x, y) {
    if (y === undefined) return -1;
    if (x === undefined) return 1;
    if (comparefn !== undefined) return +comparefn(x, y) || 0;
    return toString(x) > toString(y) ? 1 : -1;
  };

Taken from this repo: https://github.com/zloirock/core-js/blob/master/packages/core-js/modules/es.array.sort.js

Answer 2

Based on @jonrsharpe comments I was able to implement proper default function:

function defaultSortFn(a, b) {
    if (typeof a !== 'string') {
        a = String(a);
    }
    if (typeof b !== 'string') {
        b = String(b);
    }
    if (a < b) {
        return -1;
    }
    if (a > b) {
        return 1;
    }
    return 0;
}

that can be used in my sort:

Array.prototype.sort = function(fn = defaultSortFn) {
   return mergeSort(fn, this);
};

Answer 3

The ECMAScript specification for Arra.prototype.sort mentions that the comparison (in absence of a comparator) involves:

e. Let xString be ? ToString(x).
f. Let yString be ? ToString(y).
g. Let xSmaller be ! IsLessThan(xString, yString, true).
h. If xSmaller is true, return -1_?.
i. Let ySmaller be ! IsLessThan(yString, xString, true).
j. If ySmaller is true, return 1_?.
k. Return +0_?.

Given that the IsLessThan procedure is also executed when two strings are compared with the < operator, we can faithfully replicate the default callback function as follows:

function (x, y) {
    let xString = String(x);
    let yString = String(y);
    return xString < yString ? -1 : yString < xString ? 1 : 0;
}