Paste your original JavaScript or TypeScript file in the left panel and the updated version in the right panel, then click Find Differences. Added lines appear in green, removed in red. Upload .js, .ts, .jsx, or .tsx files directly. Everything runs in your browser - no code is sent to a server.
- All code is processed entirely in your browser - nothing is ever sent to a server.
- Supports .js, .ts, .jsx, .tsx, .mjs, and .cjs - TypeScript is compared line-by-line exactly like JavaScript.
- Prettier or ESLint reformats can change dozens of lines without touching logic - run the same formatter on both files before diffing to isolate real changes.
- Minified code is typically a single line - always compare unminified source files for a useful diff.
- For Python use Compare Python; for CSS use Compare CSS; for any language use Compare Code.
How to use this JavaScript diff checker
Paste or upload your original JavaScript or TypeScript on the left and the changed version on the right. Click "Find Differences." The diff shows every added and removed line with line numbers. Toggle Unified or Side-by-Side view. The language is pre-selected as JavaScript, so uploads filter to .js, .ts, .jsx, .tsx, .mjs, and .cjs files.
JS and TS diff check explained to a beginner
JavaScript and TypeScript are close relatives - TypeScript adds type annotations like : string or : number[] on top of JavaScript. When you compare a JS file to its TypeScript equivalent, those annotation lines show as added, but the underlying logic is usually unchanged.
The most confusing thing about diffing JS is that code formatters (Prettier, ESLint) can change dozens of lines at once without changing any logic. Switching from single quotes to double quotes, adding trailing commas, or inserting semicolons all affect many lines simultaneously. A diff showing 200 changed lines in a 300-line file is often just a formatter running - not a real code change.
Think of it like comparing two copies of a memo where one person used Oxford commas and the other didn't. The sentences are identical, but almost every line looks different. The fix: run the same formatter on both files before diffing. The real changes then collapse to a handful of lines - those are the ones worth reviewing.
What to watch for when you compare JavaScript files
Formatter noise: Prettier and ESLint reformats
Prettier or ESLint reformatting changes many lines at once - semicolons, quote style (single vs double), trailing commas, and bracket spacing. A formatter-only diff shows many changed lines with identical logic. To isolate real changes, run the same formatter on both files before diffing.
When I get a diff back from a collaborator showing 200 changed lines in a 300-line file, the first thing I do is check whether it's a formatting diff. If most changes are quote style, semicolons, and trailing commas, I run Prettier on both files and diff again. Usually the real changes collapse to 10-15 lines - that's where I focus the review.
Arrow functions vs function declarations in JavaScript
Converting function foo() to const foo = () => changes multiple lines. The diff shows the function declaration as removed and the arrow function as added. This is a real structural change, not formatting noise - it affects this binding and the function's prototype property.
TypeScript type annotations added to JavaScript
Adding TypeScript types to a JavaScript file shows each annotated line as changed. A migration from plain JS to TypeScript produces a large diff even if the runtime logic is unchanged - every function signature and variable declaration gains new annotation lines.
When to use a JavaScript or TypeScript diff checker
Reviewing AI-generated JavaScript rewrites
When an AI assistant rewrites a component or refactors a function, paste the original and the new version side by side to verify only the requested changes were made. Switch to Side-by-Side view to track the logic block by block without jumping between files.
Bundle output comparison
Compare two build outputs to verify that a dependency update, webpack config change, or code split adjustment changed only what was expected in the final bundle.
Config file auditing
Compare two versions of tsconfig.json, .eslintrc, or package.json to catch unintended changes between environments or after a dependency upgrade.
Framework migration
During a migration (class components to hooks, CommonJS to ESM), compare before and after versions of each module to confirm the migration is complete and correct.
Side-by-side view is most useful for framework migrations. Comparing class component syntax to hook-based syntax is hard to follow in unified view because the structure changes so much. Side by side, you can track the logic block by block - the constructor becomes state initialization, componentDidMount becomes useEffect - and verify each equivalent in parallel.
Common mistakes when comparing JavaScript files
Comparing before and after a formatter run
If one file was saved with Prettier auto-format and the other was not, the diff shows every formatting difference - quote style, semicolons, trailing commas, indentation. Run the same formatter on both files before diffing to see only logical changes.
Comparing minified vs unminified code
A minified file collapses to one or a few very long lines. Diffing minified against unminified shows the entire file as changed and gives no useful information. Always compare files at the same minification level - or better, compare source files only.
Comparing transpiled output vs source TypeScript
The compiled ES5 output of a TypeScript file looks nothing like the source - async functions become state machines, decorators expand into dozens of lines. If you want to compare TypeScript logic, compare the .ts source files, not the compiled output.
Line ending mismatches (CRLF vs LF)
A file saved on Windows (CRLF line endings) and the same file on a Unix-based system (LF) will show every single line as changed. If you're seeing unexpected diffs across all lines, check whether the files come from different operating systems and normalize line endings first.
FAQs about comparing JS and TS code
Can I compare TypeScript files?
Yes. TypeScript .ts files are plain text. The file upload accepts .js, .ts, .jsx, .tsx, .mjs, and .cjs.
Will this compare minified JavaScript?
Yes, but minified code is typically one line, so the entire minified output will appear as one changed line. For a useful diff, compare unminified source files.
Is my code private?
Yes. All processing runs in your browser. No code is sent to any server. Your code is 100% private.
Does this work with JSX and TSX files?
Yes. JSX and TSX files are plain text - the tool compares them line by line like any other file. The JSX syntax (<Component />, className=, etc.) is treated as text, so any JSX-level change shows up as a changed line.
Can I compare JavaScript against TypeScript?
You can paste any two files regardless of extension. Comparing a .js file against its .ts equivalent will show every line where a type annotation was added, plus any lines where the TS version differs in logic. Expect a large diff if the JS file was fully annotated into TypeScript.
What if both files have different indentation (tabs vs spaces)?
The tool compares exact strings. A file indented with tabs and the same file indented with spaces will show every indented line as changed. Run both files through the same formatter (Prettier with a shared config) to normalize indentation before diffing.