Async JavaScript

async/await

async functions always return Promises, and await pauses the current async function until a Promise settles while allowing the event loop to continue running other work.

async wraps return values in Promises
await unwraps fulfillment or throws rejection
Await pauses the function, not the whole runtime
Sequential awaits serialize work
Top-level await changes module loading

Sequential vs concurrent awaits
Pattern	Behavior	Use when
await a(); await b();	b starts after a finishes	b depends on a
const pa = a(); const pb = b(); await Promise.all([pa, pb])	a and b run concurrently	independent work
for await (...)	Consumes async sequence	Stream/incremental data

Avoid accidental serialization

// Slower if independent:
const user = await loadUser(id)
const projects = await loadProjects(id)

// Faster if independent:
const [user2, projects2] = await Promise.all([
  loadUser(id),
  loadProjects(id),
])

The best async code says which operations depend on which.