JavaScript to TypeScript Converter Online Free | JS to TS

Input

var count = 3;
const user = { name: "Prasad", active: true };

function greet(name) {
  return "Hello " + name;
}

const isEnabled = true;

Input chars: 138Input lines: 8Regex + heuristic conversion

Output

interface User {
  name: string;
  active: boolean;
}

let count: number = 3;
const user: User = { name: "Prasad", active: true };

function greet(name: any): string {
  return "Hello " + name;
}

const isEnabled: boolean = true;

Output chars: 230Output lines: 14Starter typings generated

About This Tool

JavaScript to TypeScript Converter helps you migrate JS code to TypeScript in seconds, directly in your browser for free. As teams adopt TypeScript for better code quality and IDE support, manually converting every file is slow and error-prone. This tool adds type annotations to variable declarations, infers function parameter and return types, and scaffolds interfaces from plain object literals — giving you a clean starting point for strict TypeScript without writing boilerplate. Because everything runs client-side, your source code never leaves your machine.

How to Convert JavaScript to TypeScript

Paste your JavaScript code into the input editor on the left.
Review the converted TypeScript output on the right panel.
Adjust inferred `any` types based on your actual data shapes.
Copy the output or download it as a .ts file.
Repeat for each file or function during your migration.

What This Converter Does

Converts var declarations into let with inferred primitive types.
Adds string, number, boolean annotations from literal values.
Adds parameter type hints and return type placeholders to function declarations.
Attempts interface generation for simple inline object literals.

Common Use Cases

Migrate legacy JS codebases to TypeScript gradually
Convert JavaScript utility functions to typed TS modules
Scaffold TypeScript types for Node.js backend modules
Speed up React component migration to Typed TSX
Provide a typed base before strict mode TypeScript checks

Frequently Asked Questions

How do I convert JavaScript to TypeScript online for free?

Paste your JavaScript code into the input editor on the left. The tool instantly produces TypeScript output with inferred type annotations on the right. Copy or download the result for free with no sign-up required.

Can this tool convert JavaScript functions to TypeScript?

Yes. It adds inferred parameter types and return type annotations to function declarations. For complex cases, the tool uses `any` which you can then narrow to your specific types.

Does this converter handle JavaScript classes?

Yes. Class property types are inferred from constructor assignments and method signatures are annotated with basic return types.

Can I convert a JS file to TypeScript using this tool?

Yes. Paste the JS file content into the input panel. The TypeScript output can be downloaded as a file with the .ts extension using the download button.

What is the difference between JavaScript and TypeScript?

TypeScript is a superset of JavaScript that adds static type annotations. This means every valid JavaScript file is also valid TypeScript. TypeScript compiles down to JavaScript and provides better IDE support, early error detection, and safer refactoring.

Does this tool convert Node.js code to TypeScript?

Yes. Node.js code is plain JavaScript, so the converter handles Node.js modules and utility functions just like any other JS. You may need to add Node.js type definitions (e.g., @types/node) separately.

Is my JavaScript code safe to paste here?

Yes. The conversion runs entirely in your browser. Your code is never sent to any server or stored anywhere.

How accurate is the JS to TS conversion?

The converter uses lightweight heuristics for common patterns. It covers variable declarations, function parameters, return types, and simple object interfaces. Complex generics, type predicates, and advanced patterns require manual review after conversion.