Following up on discussion at @munichjs how to create packed arrays pre-initialized in V8

mathias.js

"use strict";

// These are examples how to created PACKED_*_ELEMENTS arrays preinitialized
// in V8, following up on offline discussion at the last MunichJS meetup.

function createPackedViaArrayFrom(length, value) {
  return Array.from.call(null, Array.prototype.map.call({length}, _ => value));
}

function createPackedViaGenerator(length, value) {
  return [...(function* gen() { while (--length >= 0) yield value; })()];
}

function createPackedViaPush(length, value) {
  let result = [];
  while (--length >= 0) result.push(value);
  return result;
}

// Uncomment and verify via d8 --allow-natives-syntax
//
// if (!%HasFastPackedElements(createPackedViaPush(10, null)) ||
//     !%HasFastPackedElements(createPackedViaGenerator(10, null)) ||
//     !%HasFastPackedElements(createPackedViaArrayFrom(10, null))) {
//   throw new Error("Oooops...");
// }

const TESTS = [
    createPackedViaArrayFrom,
    createPackedViaGenerator,
    createPackedViaPush
];
const n = 1e6;
const length = 1024;
const value = null;

function test(fn) {
  var result;
  for (var i = 0; i < n; ++i) result = fn();
  return result;
}

for (var j = 0; j < TESTS.length; ++j) {
  test(TESTS[j]);
}

for (var j = 0; j < TESTS.length; ++j) {
  var startTime = Date.now();
  test(TESTS[j]);
  console.log(TESTS[j].name + ':', (Date.now() - startTime), 'ms.');
}