Run results for: Array concat vs spread operator vs push for array flat implementation

Run details:

User agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:84.0) Gecko/20100101 Firefox/84.0

Browser: Firefox 84

Operating system: Mac OS X 10.15

Device Platform: Desktop

Date tested: 3 years ago

Test name	Executions per second
native flat	1516899.0 Ops/sec
flatPush	677630.2 Ops/sec
flatConcat	328789.9 Ops/sec
flatSpread	177297.9 Ops/sec

Script Preparation code:

var isArray = Array.isArray;
var sourceArray = [1, [1, 2, [3, 4, [5, 6]]], 2, [3, 4, [1, 2, [3, [1, 2, [3, 4]], 4]], [5, 6, [1, 2, [3, 4, [5, 6]]], [7, 8, [9, 10]]]]];

function flatPush(arr, depth) {
  if (depth === undefined) { depth = 100; } else if (depth <= 0) { return arr; }
  var result = [];
  for (var len = arr.length, i = 0, val; i < len; ++i) {
    if (isArray((val = arr[i]))) {
      val = flatPush(val, depth - 1);
      for (var jen = val.length, j = 0; j < jen; ++j) { result.push(val[j]); }
    } else {
      result.push(val);
    }
  }
  return result;
}
function flatConcat(arr, depth) {
  if (depth === undefined) { depth = 100; } else if (depth <= 0) { return arr; }
  var result = [];
  for (var len = arr.length, i = 0, val; i < len; ++i) {
    if (isArray((val = arr[i]))) {
      result = result.concat(flatConcat(val, depth - 1));
    } else {
      result.push(val);
    }
  }
  return result;
}
function flatSpread(arr, depth) {
  if (depth === undefined) { depth = 100; } else if (depth <= 0) { return arr; }
  var result = [];
  for (var len = arr.length, i = 0, val; i < len; ++i) {
    if (isArray((val = arr[i]))) {
      result.push(...flatSpread(val, depth - 1));
    } else {
      result.push(val);
    }
  }
  return result;
}

Tests:

native flat
return sourceArray.flat(100);
flatPush
return flatPush(sourceArray);
flatConcat
return flatConcat(sourceArray);
flatSpread
return flatSpread(sourceArray);

Run results for: Array concat vs spread operator vs push for array flat implementation

Compare the new ES6 spread operator with the traditional concat() method and push

native flat

flatPush

flatConcat

flatSpread