TOPIC OVERVIEW

javascript/Promises

A structured way to represent a value that isn't ready yet.

10 QuestionsAdvancedHigh Priority ~12 min read

A Promise represents an eventual result — pending until it either resolves with a value or rejects with a reason, and once it settles, it can never change state again. That last part matters more than it sounds: because a settled promise is immutable, chaining .then() handlers is safe and predictable, unlike passing callbacks around, where nothing guarantees a callback only fires once.

The real value of Promises is in composition. Promise.all() waits for every promise to resolve and fails fast on the first rejection; Promise.allSettled() waits for all of them regardless of outcome and gives you each result or error; Promise.race() and Promise.any() resolve based on whichever promise finishes first, with subtly different rules on rejection. async/await is built entirely on top of this same Promise machinery — it's syntax that makes chained .then() calls read like synchronous code, not a separate concurrency model.

What you'll walk away knowing

The three states of a Promise and why 'settled' is a one-way transition

The real difference between Promise.all, allSettled, race, and any — and when to use each

Why async/await is syntactic sugar over Promises, not a different mechanism

How an unhandled promise rejection differs from a thrown synchronous error, and why it's easy to miss

1Explain the concept of 'callback hell' and how to mitigate it.ExpertHigh 2What is a Promise in JavaScript?ExpertHigh 3Explain the three states of a Promise.ExpertHigh 4How do you create a new Promise in JavaScript?ExpertHigh 5What is the purpose of `Promise.resolve()` and `Promise.reject()`?ExpertHigh 6How do you handle the result of a Promise?ExpertHigh 7What is Promise chaining?ExpertHigh 8How do you handle errors in a chain of Promises?ExpertHigh 9What is the purpose of `Promise.all()`, `Promise.race()`, `Promise.any()`, `Promise.allSettled()`?ExpertHigh 10How can you handle multiple Promises concurrently without using `Promise.all()`?ExpertHigh 11What is the purpose of the `async` and `await` keyword in JavaScript??ExpertHigh 12How do you handle errors when using `async/await`?ExpertHigh 13Do all `async` functions return Promises? Why or why not?ExpertHigh 14How does `Promise.finally()` work?ExpertHigh 15How do you convert a callback-based function into a Promise-based function?ExpertHigh 16What is the purpose of `Promise.promisify()`?ExpertHigh 17How can you create a custom Promise with timeout functionality?ExpertHigh

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