Learn JavaScript Promises interactively. Step through async code execution, watch promise states animate in real time, and understand how async/await really works.
Try the Visualizer →Pick an example, edit the code, then step through execution to see how promises resolve.
Select an example and click Next to step through the execution.
Master async JavaScript with visual, step-by-step learning.
Watch promises animate through pending → fulfilled/rejected states with color-coded bars that show real timing and ordering of async operations.
Advance one await at a time. See exactly where execution pauses, what resolves, and how control flows through your async functions — no guessing.
Six curated examples covering basic await, Promise.all, Promise.race, error handling, sequential vs parallel, and chaining. Edit the code and re-visualize.
Every step comes with a plain-English explanation of what's happening. Understand the event loop, microtask queue, and promise resolution in context.
Common questions about async/await and this visualizer.
Async/await is syntactic sugar over Promises. An async function always returns a Promise, and await pauses execution inside that function until the awaited Promise settles. It makes asynchronous code look and behave like synchronous code, improving readability and error handling with try/catch.
Promise.all waits for all input promises to fulfill and returns an array of results — if any rejects, it immediately rejects with that reason. Promise.race settles as soon as the first promise settles, adopting its state (fulfilled or rejected). Use .all when you need every result; use .race for timeouts or first-response-wins patterns.
Yes! The code editor is fully editable. After modifying the code, click Reset and then Next to step through the new execution. The visualizer parses your code to detect await expressions, Promise.all, Promise.race, and try/catch blocks, then generates an animated timeline showing how each operation resolves.
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