timePromiseEx.js

void async function() {
  "use strict";

  // extending Promise
  class PromiseEx extends Promise {
  }

  const iterations = 500_000;
  const cached = Promise.resolve(); 

  // testing new Promise directly
  console.time("new Promise");
  for (let n = 0; n < iterations; n++) {
    let resolve;
    const p = new Promise(r => resolve = r);
    cached.then(resolve);
    await p;
  }
  console.timeEnd("new Promise");

  // testing new PromiseEx
  console.time("new PromiseEx");
  for (let n = 0; n < iterations; n++) {
    let resolve;
    const p = new PromiseEx(r => resolve = r);
    cached.then(resolve);
    await p;
  }
  console.timeEnd("new PromiseEx");
    
}().catch(console.error);