JavaScriptMed
JavaScript Promises & Async/Await — Complete Visual Guide
Promises are JavaScript's primary abstraction for asynchronous operations. Understanding Promise internals, combinators (all/race/any/allSettled), and error handling patterns is essential for writing robust async code.
8 concepts·6 practice problems·Intermediate level
Key Concepts
- •A Promise represents a value that may be available now, later, or never
- •Promise states: pending → fulfilled OR rejected (immutable once settled)
- •Promise.all() fails fast: rejects on first rejection
- •Promise.race() returns first settled (fulfilled OR rejected)
- •Promise.any() returns first fulfilled (ignores rejections until all fail)
- •Promise.allSettled() waits for all, never short-circuits
- •.then() always returns a new Promise (enabling chaining)
- •Unhandled rejections are dangerous - always add .catch() or try/catch
Topics Covered
Promises Deep DiveAsync operations and combinatorsCreating Promisesnew Promise() and the executor functionConsumsuming Promises: then, catch, finallyHandling Promise results and errorsPromise ChainingSequential async operations in a flat structurePromise.all, race, allSettled, anyWorking with multiple PromisesAsync/Await Syntax BasicsSyntactic sugar for PromisesError Handling with Async/Awaittry/catch with async/awaitParallel Async OperationsRunning async operations in parallel with Promise.all
Practice Problems
Common Mistakes to Avoid
- ×Forgetting to return in .then() chains (breaks the chain)
- ×Not handling Promise rejections (.catch or try/catch)
- ×Using Promise.all() when you need allSettled() behavior
- ×Creating promises in loops without proper batching
- ×Mixing async/await with .then() inconsistently
Interview Tips
- ✓Know the difference between all/race/any/allSettled
- ✓Implement Promise.all() from scratch
- ✓Explain the microtask queue and Promise execution order
- ✓Write a promisify function
- ✓Implement retry with exponential backoff