JavaScript to TypeScript Converter
Convert JavaScript files to TypeScript with type annotations instantly.
JavaScript Input
TypeScript Output
What Is JavaScript to TypeScript?
You've got a working JavaScript project, but the team has decided to move to TypeScript — and now you're staring at hundreds of files that need type annotations. The TypeScript language adds static types to JavaScript, catching entire classes of bugs at compile time instead of runtime. The TypeScript handbook is the definitive guide, and Prettier handles formatting once your types are in place. This converter gives you a head start by inferring types, adding annotations, and producing .ts-ready output — all in your browser, no install needed. Paste your .js code and see the typed version instantly. It's not a replacement for a full migration audit, but it turns a week of groundwork into a few minutes.
Give it a JavaScript file and it hands back a TypeScript version with annotations added, all worked out by script in this tab rather than on a server. To tidy plain JS first, use the JavaScript Formatter. For validation, use JavaScript Validator. For running code, use JavaScript Console.
How to Use This Tool
Paste or Upload
Drop the module you want to migrate into the input pane, or load a .js or .txt file with Upload. Sample fills in a small order-total function, and Clear starts you over.
View TypeScript Output
The right panel shows TypeScript with type annotations. Types may be inferred or added as any where unclear.
Copy or Download
Once you have the .ts result, Copy it or save it with Download. Then tighten any loose types by hand and let tsc have the final word.
JavaScript to TypeScript Examples
Here is an example of converting JavaScript to TypeScript with type annotations.
Example: Subscriber plan lookup
JavaScript input:
TypeScript output:
When JavaScript to TypeScript Helps
When migrating a JS project to TypeScript, this tool gives a starting point. You will need to refine types, fix any issues, and run the TypeScript compiler (tsc). Build tools like webpack, Vite, and Angular integrate TypeScript. For JSX/React, use JSX Formatter for formatting.
Frequently Asked Questions
Is my JavaScript code safe to paste here?
Yes. The type inference is done by script loaded in this tab, and your source is never posted anywhere, so production modules, tokens and business rules stay on your machine.
How accurate are the inferred types?
Simple patterns (primitives, return values, object shapes) convert well. Complex generics or dynamic patterns may fall back to any. Always review the output and run tsc to catch remaining issues.
Will it work for a large JS codebase?
This tool handles individual files or small snippets well. For large migrations, use the TypeScript compiler (tsc) with allowJs to incrementally add types. For JSX, use JSX Formatter to format first.
How do I convert TypeScript back to JavaScript?
Use the TypeScript compiler (tsc) with --declaration false or Babel. This tool only converts JS → TS.
What about TypeScript strict mode?
The output is a starting point, not strict-clean code. Enable "strict": true in your tsconfig.json, fix the errors the compiler surfaces, and you'll be in good shape.
Related Tools
TypeScript. ECMAScript. MDN JavaScript. webpack. Babel. Prettier.