Async/Await

• An async function always returns a promise.
• If we return a value from an async function, it wraps that value in a promise and returns that promise.
• await can only be used inside an async function.
• When we use async/await, the JavaScript engine waits (not actually—the function execution is suspended) for the promise to resolve. With .then, the engine does not wait.
• The async function is suspended and removed from the call stack, but the rest of the program keeps running. The function resumes when the promise resolves.

async function fetchData() {
    console.log("1")
    await somePromise() // function suspended here
    console.log("3") // resumes after promise resolves
}

fetchData()
console.log("2") // this runs while fetchData is suspended

// prints: 1, 2, 3

• await does not start timers; it only pauses execution until a promise resolves.
• Promises start running immediately when you create them.
• For sequential delays, declare promises inside the async function after the previous await.
• async/await is syntactic sugar.

// Sequential — slow (waits one by one)
const a = await fetch(url1) // waits
const b = await fetch(url2) // waits

// Parallel — fast (both start together)
const [a, b] = await Promise.all([fetch(url1), fetch(url2)])

Promise Phases

A promise has two distinct phases:

• Creation: The asynchronous operation starts immediately.

  const p = fetch("/data"); // request starts here

• Consumption: You decide what to do when it finishes.

  await p; // wait for it
  // or
  p.then(...); // attach a callback