Promises

In short: a Promise is a claim ticket for a value that isn't ready yet. It starts pending, and settles exactly once — either fulfilled, with a value (.then handles this), or rejected, with a reason (.catch handles this). .finally runs afterwards no matter which happened, the same way finally always runs after try/catch. Once settled, a Promise's outcome is locked in forever; it can never change or settle a second time.

Waiting for stock to be checked

Wrapping an async operation in a Promise gives it a clean, single result to react to, instead of a callback:

checkStock returns a Promise immediately — pending. Once the simulated check finishes, it calls resolve with the stock message, which settles the Promise as fulfilled. await pauses here until that happens, then gives you the resolved value directly.

then, catch, and finally, all in one chain

The three handlers cover fulfilled, rejected, and "either way":

checkStock("sold-out-item") rejects, so .then is skipped entirely — only .catch runs, receiving the Error. .finally runs after either outcome, exactly like a finally block: the right place for cleanup that must happen whether the operation succeeded or failed.

Running several checks at once

Promise.all waits for every Promise in an array to fulfill, and gives you their results in the same order — a clean way to run independent async work in parallel instead of one after another:

All three checks run at the same time, not one after another — the total wait is roughly one check's worth of time, not three. If any one of them had rejected, Promise.all itself would reject immediately with that reason, skipping the rest.

Quick practice