JavaScript to TypeScript Converter (Best-Effort)
Paste JavaScript source to get back the same code with obvious type annotations added to untyped parameters and literal-initialized declarations. Free online developer tool.
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TypeScript (best-effort)
Annotated TypeScript output will appear here...
Free JavaScript to TypeScript Converter (Best-Effort)
Paste JavaScript source to get back the same code with obvious type annotations added — untyped function parameters, and const/let declarations initialized with a literal value. This is a best-effort syntactic pass, not full type inference.
How to Use It
Paste any JavaScript source into the input box, or click "Load Example." The tool parses your code with the TypeScript compiler and walks its syntax tree: function parameters with a literal default value (like a = 0) get typed from that default (a: number = 0), parameters with no default get typed : any, and const/let declarations initialized with a number, string, or boolean literal get an explicit type annotation. The result is reprinted from the modified syntax tree, so formatting stays close to a normal TypeScript printer's output rather than your original spacing.
Example
Given function add(a = 0, b = 0) { return a + b; } and const name = 'Ann';, the tool emits function add(a: number = 0, b: number = 0) { return a + b; } and const name: string = 'Ann';.
Common Use Cases
- Getting a head start on a manual JavaScript-to-TypeScript migration.
- Quickly seeing which function parameters and variables have no obvious inferable type.
- Adding the easy, mechanical annotations before doing the harder type work by hand.
FAQs
Does this do real, full type inference?
No — this is explicitly a best-effort tool, not real control-flow type inference. It only adds obvious, syntactic annotations: any for untyped parameters, primitive types inferred from literal parameter defaults, and primitive/array types inferred from literal variable initializers. Complex logic, conditional types, object literal shapes, and any type that depends on how a value is actually used elsewhere in your code are left untouched and will need manual typing afterward.
Why did some declarations get a type and others didn't?
Only declarations initialized with a literal value the tool recognizes — numbers, strings, booleans, and array literals for variables, plus literal defaults for parameters — get an inferred annotation. Object literals, function calls, and anything else are left as-is rather than risk an incorrect guess.
Is my JavaScript uploaded anywhere?
No — parsing and annotation happen entirely client-side in your browser, using the TypeScript compiler loaded on demand. Nothing you paste is sent to a server.