Promise

                new Promise((resolve, reject) => {}) / Promise.all([...]) / Promise.resolve(value)
              

Purpose

A Promise represents a value that isn’t available yet. It starts pending, and settles exactly once — either fulfilled with a value, or rejected with a reason (usually an Error). fetch, timers wrapped in a promise, and most modern async APIs return one.

Syntax

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

fetch('/api/data')
  .then((response) => response.json())
  .then((data) => console.log(data))
  .catch((error) => console.error(error));

const [a, b] = await Promise.all([fetchA(), fetchB()]); // run concurrently

async/await (see async/await) is the more common way to work with promises today; .then()/.catch() is the underlying mechanism it’s built on and still shows up in older code and library documentation.

Behavior notes

• A promise settles exactly once — it cannot go from fulfilled back to pending, or from rejected to fulfilled.
• Promise.all(promises) resolves once every promise resolves, with an array of their results in order; it rejects immediately if any one of them rejects.
• Promise.allSettled(promises) (Intermediate tier) resolves once every promise has settled, regardless of success or failure, and reports each outcome individually — useful when partial success should be handled rather than treated as a total failure.
• Promise.resolve(value)/Promise.reject(reason) create an already-settled promise, useful for matching a function’s return type when a value is already known synchronously.

Common mistakes

• Forgetting that Promise.all rejects as a whole if any single promise in the group rejects — which can be surprising when the group includes both critical and optional requests.
• Mixing .then() chains with async/await in the same function, making control flow harder to follow than picking one consistently.

Runtime notes

Promise is core ECMAScript, identical across every current browser and Node version.