Asynchronous

Call Stack

A stack data structure that keeps track of function execution.

Event Loop

A mechanism that checks the call stack and pushes tasks from queues when the stack is empty.

Managing asynchronous operations (like timers, API calls, user clicks) without blocking the main program flow

Macro-task queue

Queue for tasks like setTimeout, setInterval, DOM events.

Microtask Queue

Queue for Promise callbacks (.then, .catch, .finally) and queueMicrotask.

Priority: microtasks vs macrotasks

Microtasks are executed before macrotasks.

Execution order

Call stack → Microtasks → Macrotasks.

Promise

An object representing the eventual completion or failure of an asynchronous operation.

async function

A function that always returns a Promise.

await

Pauses execution of an async function until a Promise resolves or rejects.

async/await vs .then() –

async/await is syntactic sugar over Promises and makes async code easier to read.

Promise states

Pending, Fulfilled, Rejected.

Promise.all

Resolves when all Promises resolve; rejects if any Promise rejects.

Promise.allSettled

Resolves when all Promises settle, returning their outcomes.

Are Promises async by default?

Promise executor runs synchronously, but .then callbacks are async.