What is the in-place alternative to Array.prototype.filter()

Is there an in-place alternative to filter

No, but it’s not hard to write your own. Here is an approach which squeezes out all the values which fail a condition.

function filterInPlace(a, condition) {
  let i = 0, j = 0;

  while (i < a.length) {
    const val = a[i];
    if (condition(val, i, a)) a[j++] = val;
    i++;
  }

  a.length = j;
  return a;
}

condition is designed to have the same signature as the callback passed to Array#filter, namely (value, index, array). For complete compatibility with Array#filter, you could also accept a fourth thisArg parameter.

Using forEach

Using forEach has the minor advantage that it will skip empty slots. This version:

• Compacts arrays with empty slots
• Implements thisArg
• Skipps the assignment, if we have not yet encountered a failing element

function filterInPlace(a, condition, thisArg) {
  let j = 0;

  a.forEach((e, i) => { 
    if (condition.call(thisArg, e, i, a)) {
      if (i!==j) a[j] = e; 
      j++;
    }
  });

  a.length = j;
  return a;
}

a = [ 1,, 3 ];
document.write('<br>[',a,']');

filterInPlace(a, x=>true);
document.write('<br>[',a,'] compaction when nothing changed');

b = [ 1,,3,,5 ];
document.write('<br>[',b,']');

filterInPlace(b, x=>x!==5);
document.write('<br>[',b,'] with 5 removed');