Paste a Promise chain above
and click VISUALIZE
// LEGEND
Pending
Resolved
Rejected
Finally
Catch
// TIMELINE
Run animation to see execution order
// draw and trace promise chains visually
Visualize JavaScript Promise chains, async/await flows, and resolution order interactively. Draw, trace, and debug async logic in your browser for free.
Paste a Promise chain above
and click VISUALIZE
Enter your Promise chain, async/await function, or use a preset template to get started quickly.
Click Visualize to parse your code and generate an interactive node graph of the promise flow.
Click Animate to see the resolution order play out step by step, or choose a simulation mode.
The JS Promise Visualizer parses JavaScript Promise chains and async/await code to render an interactive node-graph diagram. Each then(), catch(), and finally() becomes a visual node with animated resolution paths, so you can trace exactly how data flows through your async code.
You can visualize .then() / .catch() / .finally() chains, Promise.all(), Promise.race(), Promise.allSettled(), async/await functions, and nested Promise patterns. The parser handles most common real-world patterns.
No — the visualizer is a static parser only. It reads the structure of your code and renders the flow diagram without executing any JavaScript. This keeps it safe and works with pseudocode too.
"All Resolve" shows the happy path where every promise resolves successfully. "Reject @2" simulates a rejection at the second step so you can see how catch() handlers activate. "Mixed" shows a combination of resolved and rejected states.
Click the "↓ PNG" button after generating a visualization. The canvas will be saved as a PNG image you can embed in documentation, slide decks, or share with teammates.
Yes. The tool recognizes async function declarations and await expressions, converting them into an equivalent Promise-node flow. async/await is syntactic sugar over Promises, so they map 1-to-1 in the visual output.
No — all parsing and rendering happens entirely in your browser using JavaScript and Canvas. No code leaves your machine. This tool works fully offline once loaded.
A JavaScript Promise Visualizer is a browser-based tool that parses your async code and renders it as an interactive node-graph diagram. Instead of mentally tracing through .then() chains or trying to follow nested callbacks in your head, you get a clear visual map of how data flows through each step, which handlers fire on resolve versus reject, and in what order execution proceeds.
Modern JavaScript applications are almost entirely asynchronous. Whether you're fetching data from REST APIs, reading files, querying databases, or chaining multiple async operations, Promises are the backbone of it all. But as chains grow longer and error paths multiply, understanding exactly what happens — and when — becomes genuinely difficult.
💡 Looking for premium JavaScript plugins and scripts? MonsterONE offers unlimited downloads of JS templates, UI components, and developer assets — worth checking out.
A Promise is an object representing the eventual completion or failure of an asynchronous operation. It exists in one of three states:
Once a Promise settles (either resolves or rejects), it stays in that state permanently. You handle the outcome using .then() for fulfillment, .catch() for rejection, and .finally() for cleanup regardless of outcome.
The real power of Promises comes from chaining. Each .then() call returns a new Promise, allowing you to sequence async operations:
fetch('/api/user/1')
.then(res => res.json())
.then(user => fetchProfile(user.id))
.then(profile => renderUI(profile))
.catch(err => showError(err))
.finally(() => hideSpinner())Visually, this is a linear chain — each node receives the resolved value from the previous node and passes its return value to the next. The visualizer renders this as a top-to-bottom flow graph with color-coded nodes for each operation type.
One of the trickiest aspects of Promise chains is understanding how errors propagate. When any Promise in a chain rejects, control jumps to the nearest .catch() handler downstream. All intermediate .then() handlers between the rejection point and the catch are skipped.
The visualizer makes this immediately obvious: when you choose "Reject @2" simulation mode, you'll see the resolve path dim out after node 2, with a red rejection arrow jumping directly to the catch node. This is far clearer than reading the code alone.
Promise.all() takes an array of Promises and returns a new Promise that resolves when all input Promises resolve, or rejects as soon as any one of them rejects. The visualizer renders this as a fan-out pattern: multiple parallel nodes that converge to a single resolution node.
Promise.all([
fetchUser(id),
fetchPermissions(id),
fetchSettings(id)
]).then(([user, permissions, settings]) => {
setupApp(user, permissions, settings)
})Seeing the three branches run in parallel and merge at the .then() makes the concurrent nature obvious — something that's hard to convey in text alone.
async/await is syntactic sugar built on top of Promises. An async function always returns a Promise, and await pauses execution until the awaited Promise settles. Under the hood, the JavaScript engine converts await expressions into Promise chains — which is exactly what the visualizer shows.
async function loadDashboard(userId) {
const user = await fetchUser(userId)
const posts = await fetchPosts(user.id)
return renderDashboard(user, posts)
}Each await becomes a node in the flow graph. This makes it easy to see where your function pauses, what value it's waiting for, and what happens next.
Promise.race() resolves or rejects with the value of the first Promise to settle, regardless of whether that Promise resolved or rejected. Common use case: adding a timeout to any async operation.
Promise.race([
fetchData(),
new Promise((_, reject) => setTimeout(() => reject('timeout'), 5000))
]).then(data => display(data))
.catch(err => handleError(err))The visualizer shows both branches racing, with the winning path highlighted and the losing path dimmed — making the race semantics immediately intuitive.
This tool is useful for JavaScript developers at all levels. Beginners learning async programming for the first time will find the visual representation much easier to understand than reading code. Intermediate developers debugging complex chains can quickly spot where error paths diverge. Senior developers writing architecture documentation can export diagrams directly into technical specs or README files.
It's also invaluable for teaching and code review — instead of walking teammates through async logic verbally, show them the diagram. The animation mode plays out the resolution sequence step by step, making live explanations effortless.
The visualizer can help you spot several common mistakes. The "Promise nesting" anti-pattern — where you create a new Promise inside a .then() callback — shows up as an unexpected indentation in the flow graph. Forgetting .catch() creates a chain that terminates without an error handler, which the visualizer highlights. And using async/await inside a .forEach() — which doesn't await properly — produces a flat parallel structure when you expected sequential execution.
Seeing your code as a graph makes these structural issues far more obvious than reading line-by-line.