How does async JavaScript run?

Definition:

Asynchronous JavaScript lets a program start work that may finish later without waiting synchronously for the result. A promise represents eventual completion, and async/await provides a readable way to work with promises. The runtime queues continuations and runs them when the current synchronous work has finished and the relevant checkpoints are reached.

Key points:

• Synchronous code runs first; queued promise callbacks run after the current code finishes.

• await pauses that async function, not the whole JavaScript thread.

• Making a CPU-heavy function async does not stop it from blocking.

• An async function always returns a promise, including when it returns a plain value.

• Use try/catch around an awaited operation to handle a rejection; synchronous try/catch cannot catch an unrelated later callback error.

• The executor passed to new Promise(...) runs immediately; .then(...) callbacks run asynchronously.

Comparison:

Example:

async function getTotal() {
  const price = await Promise.resolve(100);
  return price + 20;
}
getTotal().then(console.log); // 120

Interview point:

• Promises coordinate results; they do not create a new JavaScript thread or automatically make heavy work non-blocking.