Free Online JavaScript Preview
JavaScript Preview runs your JavaScript and HTML code in a live sandbox directly in your browser, showing the rendered output alongside your source code in a split-pane workspace. We built six preview and analysis tools into one workspace so you can execute JavaScript with live rendering, preview complete HTML pages at multiple viewport sizes, inspect the resulting DOM tree, capture console output, view syntax-highlighted code snapshots, and benchmark execution time. Paste your code into the editor, upload a file from your computer, or drag and drop it onto the workspace. Your code stays on your device because all processing happens locally in a sandboxed iframe.
JS Preview
Runs JavaScript in a sandboxed iframe and renders DOM output, canvas drawings, and styled elements in real time with auto-run support.
HTML Preview
Renders complete HTML documents with CSS and JavaScript at desktop, tablet, and mobile viewport widths for responsive testing.
DOM Inspector
Parses your HTML and displays the resulting element tree with tag names, attributes, and nesting structure in a formatted view.
Console Output
Captures console.log, warn, error, and info messages from your code with color-coded type badges and runtime error detection.
Code Snapshot
Renders your JavaScript with Dracula-themed syntax highlighting showing keywords, strings, built-ins, and comments in a numbered view.
Script Timer
Benchmarks your JavaScript over 100 iterations and reports average, minimum, and maximum execution times with sub-millisecond precision.
Online JavaScript Preview with Live Rendering and Viewport Testing
JavaScript Preview gives you an instant feedback loop between writing code and seeing results. Instead of switching between a text editor and a browser tab, you write JavaScript or HTML on the left and see the rendered output on the right in the same workspace. The viewport bar between the toolbar and the code area shows a pulsing status indicator, an auto-run toggle, execution time, and file size. For the JS Preview and HTML Preview tools, viewport pills let you switch between full width, desktop (1024px), tablet (768px), and mobile (375px) so you can test responsive layouts without resizing your browser window.
Upload JavaScript and HTML Files
You can load code into the workspace three ways: paste it into the code editor on the left, drag and drop a file anywhere onto the workspace, or click the Upload File button in the toolbar. We accept .js, .jsx, .ts, .tsx, .mjs, .cjs, .html, and .htm files. Because everything runs in your browser inside a sandboxed iframe, there is no upload size limit imposed by a server and your files never leave your machine.
Auto-Run for a Live Coding Experience
Enable the auto-run checkbox in the viewport bar and the preview updates automatically 800 milliseconds after you stop typing. This turns the workspace into a live coding environment where you see changes reflected instantly as you write. For longer scripts or code that modifies the DOM heavily, keep auto-run off and click the green Run button manually when you are ready to execute.
Inspect DOM, Capture Console, and Benchmark Performance
Beyond live preview, you often need to debug output or measure how fast your code runs. The Console Output tool captures every console.log, console.warn, console.error, and console.info call from your script and displays them in a styled log panel with color-coded type badges. Runtime errors caught by window.onerror also appear here with line numbers when available, so you can diagnose problems without opening your browser DevTools.
Inspect the Rendered DOM Structure
The DOM Inspector parses your HTML through the browser DOMParser API and displays the resulting element tree in a formatted, indented view. Each element shows its tag name and all attributes. This is useful for verifying that your markup produces the expected hierarchy, especially when working with complex nested structures or dynamically generated HTML.
Benchmark JavaScript Execution Time
The Script Timer runs your JavaScript code 100 times using performance.now() and calculates the average, minimum, and maximum execution times. Results are displayed in a dashboard card with sub-millisecond precision. Use this to benchmark algorithms, compare sorting implementations, or identify which approach is fastest before committing to a solution in your production codebase.
Your JavaScript Runs Safely in a Sandboxed Iframe
We execute your code inside a sandboxed iframe with only the allow-scripts permission enabled. No network access, no same-origin access, no form submissions. Console methods are overridden to capture output via postMessage, but the original console methods still work so messages also appear in your browser DevTools. No server requests, no cookies, no account required. Close the browser tab and your code is gone.
Related JavaScript and Code Tools
If you work with JavaScript regularly, these tools complement what you can do here:
- JS Beautifier -- format, beautify, minify, and validate JavaScript code
- JavaScript Compress -- compress and strip JavaScript for production
- JavaScript Obfuscator -- protect JavaScript with obfuscation
- JS Validator -- validate syntax, lint errors, and analyze code health
- JavaScript Validator -- validate, format, minify, and convert JavaScript
- JSON Formatter -- format, minify, validate, and transform JSON data
- HTML Formatter -- format and beautify HTML documents