What a promise is in JavaScript

A pending value that settles once, and never changes after.

const promise = new Promise((resolve, reject) => {
  console.log("executor runs now");
  setTimeout(() => resolve("settled"), 10);
  setTimeout(() => resolve("ignored"), 20);
});

console.log("after the constructor");

promise.then((value) => console.log("then:", value));

Promise.resolve("already done").then((value) => console.log(value));
Promise.reject(new Error("nope")).catch((error) => console.log(error.message));

How it works

1. The executor runs immediately, synchronously.
2. resolve or reject settles it; later calls do nothing.
3. then always runs asynchronously, after the current task.

Keywords and builtins used here

• Promise
• catch
• const

The run, in numbers

Lines

12

Characters to type

416

Tokens

122

Three-star pace

95 tpm

At the three-star pace of 95 tokens a minute, this run takes about 77 seconds.

Type this snippet

Step 2 of 5 in Promises, step 2 of 19 in Promises & async.

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