All 15 Async Patterns questions
More in Async Patterns
How do you convert a Node.js callback-based function into a Promise-based one manually, without using util.promisify()? What is the purpose of the 'node:timers/promises' module, and how does it simplify working with delays in async code? What is the difference between Promise.all(), Promise.allSettled(), Promise.race(), and Promise.any()? How does async/await relate to Promises under the hood, and what does 'await' actually do to program execution? How do you implement a retry-with-exponential-backoff pattern for a flaky async operation in Node.js? What is util.promisify(), and when would you still need it in modern Node.js? What is the difference between micro tasks and macro tasks in Node.js's event loop, and how does this affect Promise callback ordering? What is an async generator, and how does 'for await...of' let you consume asynchronously produced data? What is the difference between an unhandled promise rejection and an uncaught exception in Node.js, and how does Node handle each by default? What is a common pitfall when using async/await inside Array.prototype.forEach(), and how do you fix it?
Advertisement
Back to results for “Async”
Node.js › Async Patterns

What is the difference between running async operations sequentially with multiple awaits versus concurrently with Promise.all()?

Intermediate

Answer

Writing 'const a = await taskA(); const b = await taskB();' runs the two tasks sequentially -- taskB doesn't even start until taskA fully completes, wasting time when the tasks are independent. Starting both promises first (without awaiting immediately) and then awaiting them together via Promise.all() lets both operations run concurrently, since they're both already in flight by the time either await actually pauses execution, meaningfully reducing total wait time for independent I/O-bound operations.
// Sequential: total time = taskA time + taskB time
const a = await taskA();
const b = await taskB();

// Concurrent: total time = max(taskA time, taskB time)
const [a2, b2] = await Promise.all([taskA(), taskB()]);
Real-world example A page-load handler fetching a user's profile, recent orders, and notification count from three independent database queries switches from three sequential awaits to a single Promise.all(), cutting the endpoint's response time roughly to the duration of the single slowest query instead of the sum of all three.

Common follow-ups: In what specific scenario is sequential awaiting actually the correct choice rather than a mistake?;How does this concurrency differ from true parallelism, given Node.js still runs on a single main thread?

Async Patterns;Performance Optimization & Profiling
All 15 Async Patterns questions