Sequential or parallel in JavaScript

Two awaits in a row wait twice; starting both first does not.

const work = (name, ms) =>
  new Promise((resolve) => setTimeout(() => resolve(name), ms));

async function sequential() {
  const started = Date.now();
  await work("a", 20);
  await work("b", 20);
  return Date.now() - started;
}

async function parallel() {
  const started = Date.now();
  const [first, second] = await Promise.all([work("a", 20), work("b", 20)]);
  return [Date.now() - started, first, second];
}

async function main() {
  console.log("sequential ms >= 40:", (await sequential()) >= 39);
  const [elapsed] = await parallel();
  console.log("parallel ms < 40:", elapsed < 39);
}

main();

How it works

1. await a(); await b(); is sequential, and often a bug.
2. Start the promises, then await them together.
3. The timing difference is the whole point of Promise.all.

Keywords and builtins used here

• Date
• Promise
• async
• await
• const
• function
• return

The run, in numbers

Lines

23

Characters to type

586

Tokens

179

Three-star pace

100 tpm

At the three-star pace of 100 tokens a minute, this run takes about 107 seconds.

Type this snippet

Step 3 of 4 in async and await, step 8 of 19 in Promises & async.

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