test.js

function createSlowObject() {
  let obj = {};

  for (let i = 0; i < 100; i++) {
    obj['f' + i] = i;
  }

  return obj;
}

function benchmark(obj) {
  let blackhole = 0;

  let start = performance.now();
  let iterations = 0;
  let took = 0;
  while ((took = (performance.now() - start)) < 2000) {
    for (let i = 0; i < 1e6; i++) {
      blackhole ^= obj.f0;
    }
    iterations++;
  }

  return `computed ${blackhole} @ ${took / iterations} ms/iteration`;
}

function areYouFast(obj) {
  print(`object has ${%HasFastProperties(obj) ? 'fast' : 'slow'} properties`);
}

function measure(obj, f) {
  f(obj);
  print(`${f.displayName || f.name}: ${f(obj)}`);
}

function clone(f) {
  let clone = Function(`return ${f.toString()}`)();
  clone.displayName = `clone of ${f.displayName || f.name}`;
  return clone;
}

let slowObj = createSlowObject();
print('--- Created slowObj with 100 properties');
areYouFast(slowObj);                                 // => slow
measure(slowObj, benchmark);                         // => 8.2 ms/iteration

(function () {}).prototype = slowObj;
print('--- After (function () { }).prototype = F');
areYouFast(slowObj);                                 // => slow
measure(slowObj, benchmark);                         // => 7.0 ms/iteration
areYouFast(slowObj);                                 // => fast
measure(slowObj, benchmark);                         // => 7.0 ms/iteration
measure(slowObj, clone(benchmark));                  // => 0.5 ms/iteration