Promises in JavaScript: clean chaining and error handling

Master `then`, `catch` and `finally`, compose promises clearly and avoid callback hell when asynchronous flows start to grow.

📘 Theory

Every promise starts pending and eventually settles as fulfilled or rejected.

While a promise is pending, there is still no final value to work with. Once it settles, the result will never change again.

A reliable asynchronous design always accounts for both paths: the useful value and the possible failure.

Each `then()` receives the result of the previous step, so think in a pipeline.

When a `then()` returns a value, that value flows into the next step. When it returns another promise, the chain waits for it before continuing.

This is why a good chain reads like a sequence of small transformations instead of a stack of nested callbacks.

Return either a value or a promise from each `then()`.
Avoid nested callbacks when chaining already expresses the order clearly.
Use one final `catch()` for global failure handling unless a step needs a local recovery strategy.

If two asynchronous tasks do not depend on each other, do not wait for them one by one.

`Promise.all` is the cleanest way to parallelize independent tasks and shorten total waiting time.

The trade-off is fail-fast behavior: if one promise rejects, the whole combined promise rejects too.

Example Guided example: a chain with final cleanup Transform values step by step and close the flow with `finally()` no matter how it ends.

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Challenge Challenge: build a promise-based delay helper Create a promise that resolves after a timeout and then consume it with a clean chain.

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Test

Check your knowledge with a test about JavaScript.

Course resources 62