What JavaScript to TypeScript Converter helps you do

Turn plain JavaScript into a conservative TypeScript starter with a handful of regex-based rewrites. It adds parameter types, converts var to let, and maps CommonJS exports to ES module syntax. It is a starting point for a manual migration, not a compiler and not a type inference engine.

Conversion runs in your browser. Your source code is not uploaded to Haivix.

How it works

These tools transform text locally: they parse, format, encode or render a string in your browser. Nothing you paste is uploaded to Haivix, but some tools load a library from a public CDN, so the page needs a network connection and your browser makes that request. The transformations operate on syntax, not meaning: a formatter can make invalid code look tidy, an encoder can hide a value without protecting it, and a preview can render markup that was never meant to execute. Outputs are exact strings, which means line endings, byte order marks and character encoding all matter when you paste the result somewhere else.

How to use JavaScript to TypeScript Converter

Paste the JavaScript to convert.
Generate the TypeScript version and read the change count.
Review every unknown parameter type by hand.
Copy the output and run your project type-checker.

When to use it

Start migrating a JavaScript file to TypeScript.
Add rough type annotations to a snippet before refining them.
Produce a typed starting point for review.
Convert a small CommonJS utility to ES module imports.
Show a teammate the shape of a typed version.
Bootstrap a file before adding interfaces and return types.

When not to use it

Do not treat the output as a finished migration.
Do not expect const declarations to receive type annotations.
Do not use it on JSX, decorators or complex class hierarchies without manual review.

Example

A function with untyped parameters gets unknown for each parameter, or a literal type when a default value is present. A var declaration becomes let with an inferred literal type. A const declaration is left untouched. A require call becomes an import, and module.exports becomes export default.

What you can do with JavaScript to TypeScript Converter

Convert var declarations to let.
Add a type to function parameters.
Infer a literal type from a default value.
Turn module.exports into export default.
Turn require calls into namespace imports.
Report how many changes were applied.

Example: test with a real sample before trusting the output

Keep the original text. These tools produce a new value rather than editing in place, so a bad transform is reversible only if you still have the source.

Test with a representative sample, including the awkward case: a quote inside a string, a tag inside an attribute, a newline in the middle of a value.

Check line endings and encoding when moving between Windows, Unix and a browser form. A trailing carriage return can break a signature or a hash.

Treat pasted HTML and Markdown as untrusted. A preview that renders raw markup can execute event handlers even when it looks harmless.

Verify the result with the real parser, compiler or linter before shipping. A browser-side formatter is a convenience, not a substitute for the toolchain.

Prefer exact values over screenshots or copied rendered text. Rendering can normalize quotes, dashes and spaces in ways that break the next tool in the chain.

What to know before you start

It is a best-effort conversion, not the TypeScript compiler, so some types are left as unknown.
Const declarations are not annotated, and return types are not generated.
Complex patterns such as destructuring, classes and JSX need manual typing.
The output must be checked by a real type-checker before it is committed.

Common mistakes

Expecting const declarations to be typed.
Shipping the output without replacing unknown with real types.
Assuming return types are generated.
Converting JSX or decorators that the regex does not understand.

Troubleshooting

If a parameter stays untyped, it may already contain a colon or be part of a destructuring pattern.

If the output will not compile, an unknown type may need narrowing or a real interface.

If an import looks wrong, the require pattern may not match the regex.

If nothing changed, the source may already be typed or use unsupported syntax.

JavaScript to TypeScript Converter