Async & Await / Promises

Async

Main function

(async () => {
  throw new Error('Hello World');
})().catch((err) => console.error(err));

With Promises

• https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/async_function

async () => {
  await new Promise(...);
  const [
    result1,
    result2,
  ] = await Promise.all([
    new Promise(),
    new Promise(...)],
  );
  return value;
}

Promises

• MDN
• https://github.com/wbinnssmith/awesome-promises
• http://bluebirdjs.com/docs/why-promises.html

When you start chaining a lot of promises and passing variables around, consider usings aysnc instead.

Pay attention to the style, because it's really stupid.

const start = new Promise((resolve, reject) => {
  xhr.get(url)
    .success((result) => resolve(result))
    .failure((error) => reject(error));
});

const afterStart = (startResult) => new Promise((resolve, reject) => {
});

start
  .then(afterStart)
  // Always start on a new line, its easier to catch
  // Always use a Promise or only { Promise.all([]) }, its eaiser to catch
  .then() => new Promise((resolve, reject) => {
  })
  .catch((error) => {
    console.log(error);
  });

• Always have a catch() to ensure you can see all errors.

Promise.all

Gotcha:

• You have to call .catch() on each of the promises

• You should only catch things which are idempotent; for example:

  • Sending emails

await Promise.all([
  ...,
  ...,
].map((promise) => promise.catch(error => console.error(error))));