A stableSort function that uses plain .sort if stable

stable-sort.js

// 11 and 513 come from here: https://mathiasbynens.be/demo/sort-stability

function checkStable(length) {
  return Array.apply(null, { length: length })
    .map(function (_, i) {
      return i;
    })
    .sort(function () {
      return 0;
    })
    .every(function (n, i, a) {
      return i === 0 || a[i - 1] === n - 1;
    });
}

var stable11 = undefined;
var stable513 = undefined;

function stableSort(a, f) {
  if (a.length < 11) {
    a.sort(f);
  } else if (a.length < 513) {
    if (stable11 === undefined) {
      stable11 = checkStable(11);
    }
    if (stable11) {
      a.sort(f);
    } else {
      whateverStableSortImplementation(a, f);
    }
  } else {
    if (stable513 === undefined) {
      stable513 = checkStable(513);
    }
    if (stable513) {
      a.sort(f);
    } else {
      whateverStableSortImplementation(a, f);
    }
  }
}

function whateverStableSortImplementation(a, f) {}