async function waitAndMaybeReject() {
  await new Promise(r => setTimeout(r, 1000));

  const isHeads = Boolean(Math.round(Math.random()));

  if (isHeads) return 'yay';
  throw Error('Boo!');
}

async function foo() {
  try {
    waitAndMaybeReject();
  }
  catch (e) {
    return 'caught';
  }
}

async function foo() {
  try {
    await waitAndMaybeReject();
  }
  catch (e) {
    return 'caught';
  }
}

async function foo() {
  try {
    return waitAndMaybeReject();
  }
  catch (e) {
    return 'caught';
  }
}

async function foo() {
  try {
    return await waitAndMaybeReject();
  }
  catch (e) {
    return 'caught';
  }
}

class SpecialArray extends Array {}
const specialArray = new SpecialArray();
console.log(specialArray.constructor === Array); // false
console.log(specialArray.constructor === SpecialArray); // true

console.log(specialArray instanceof Array); // true
console.log(specialArray instanceof SpecialArray); // true

<iframe srcdoc="<script>var arr = [];</script>"></iframe>
<script>
  const iframe = document.querySelector('iframe');
  const arr = iframe.contentWindow.arr;
  console.log(arr.constructor === Array); // false
  console.log(arr.constructor instanceof Array); // false
</script>

console.log(Array === iframe.contentWindow.Array); // false

const noProtoArray = [];
Object.setPrototypeOf(noProtoArray, null);
console.log(noProtoArray.map); // undefined
console.log(noProtoArray instanceof Array); // false
console.log(Array.isArray(noProtoArray)); // true

<iframe srcdoc="<script>var arr = [1, 2, 3];</script>"></iframe>
<script>
  const iframe = document.querySelector('iframe');
  const arr = iframe.contentWindow.arr;

  for (const item of arr) {
    console.log(item);
  }
</script>

const iframe = document.querySelector('iframe');
const iframeWindow = iframe.contentWindow;
console.log(Symbol === iframeWindow.Symbol); // false
console.log(Symbol.iterator === iframeWindow.Symbol.iterator); // true

const symbolOne = Symbol('foo');
const symbolTwo = Symbol('foo');
console.log(symbolOne === symbolTwo); // false
const obj = {};
obj[symbolOne] = 'hello';
console.log(obj[symbolTwo]); // undefined
console.log(obj[symbolOne]); // 'hello'

const iframe = document.querySelector('iframe');
const iframeWindow = iframe.contentWindow;
console.log(Symbol.for('foo') === iframeWindow.Symbol.for('foo')); // true

const iframe = document.querySelector('iframe');
const iframeWindow = iframe.contentWindow;
console.log(Symbol.for('foo') === iframeWindow.Symbol.for('foo')); // true

const typeSymbol = Symbol.for('whatever-type-symbol');

class Whatever {
  static isWhatever(obj) {
    return obj && Boolean(obj[typeSymbol]);
  }
  constructor() {
    this[typeSymbol] = true;
  }
}

const whatever = new Whatever();
Whatever.isWhatever(whatever); // true