withResolvers and timers in JavaScript

The newer helpers: a promise you settle from outside, and awaitable sleep.

function deferred() {
  const { promise, resolve, reject } = Promise.withResolvers();
  return { promise, resolve, reject };
}

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

async function main() {
  const gate = deferred();
  setTimeout(() => gate.resolve("opened"), 5);
  console.log(await gate.promise);

  const started = Date.now();
  await sleep(10);
  console.log("slept:", Date.now() - started >= 9);

  const failing = deferred();
  failing.reject(new Error("closed"));
  console.log(await failing.promise.catch((e) => e.message));
}

main();

How it works

1. Promise.withResolvers hands back the promise and its two functions.
2. That replaces the let-declared resolve trick.
3. setTimeout wrapped in a promise is the sleep JavaScript lacks.

Keywords and builtins used here

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

The run, in numbers

Lines

22

Characters to type

557

Tokens

166

Three-star pace

105 tpm

At the three-star pace of 105 tokens a minute, this run takes about 95 seconds.

Type this snippet

Step 2 of 3 in Scheduling, step 13 of 19 in Promises & async.

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