JavaScript to TypeScript Converter – Complete Guide

What Is a JavaScript to TypeScript Converter?

A JavaScript to TypeScript Converter takes plain JavaScript code and adds TypeScript type annotations, converts CommonJS modules to ES module syntax, and transforms module.exports to export default. Migrating a JavaScript codebase to TypeScript is one of the most common modernization tasks for web development teams, and this tool automates the repetitive parts of that process.

Our converter runs entirely in your browser using pattern-based analysis. Your source code is never uploaded to any server, making it safe for proprietary and sensitive codebases.

How the Conversion Works

The converter applies a series of intelligent transformations to your JavaScript code:

Type Inference from Literals

Variables assigned to literal values receive inferred type annotations:

const name = "Alice" → const name: string = "Alice"
const age = 30 → const age: number = 30
const active = true → const active: boolean = true
const items = [] → const items: any[] = []
const config = {} → const config: Record<string, any> = {}

Function Parameter Types

Function declarations and arrow functions receive any type annotations for untyped parameters, signaling to developers where specific types should be added:

function greet(name) → function greet(name: any): any
const add = (a, b) => → const add = (a: any, b: any): any =>

Module Conversion

CommonJS require() calls are converted to ES module import statements:

const express = require('express') → import express from 'express'
const { readFile } = require('fs') → import { readFile } from 'fs'
module.exports = app → export default app

Step-by-Step Usage

Paste your JavaScript into the left input panel.
The converter instantly transforms it to TypeScript in the right panel.
Use 📄 Sample to load example JavaScript code.
Click 📋 Copy to copy the TypeScript output.
Review the added types and refine any annotations with specific types.

Common Use Cases

Codebase Migration

When migrating a Node.js or React project from JavaScript to TypeScript, use this tool to handle the bulk of the mechanical conversion work. Focus your manual effort on replacing any types with specific interfaces.

Learning TypeScript

If you are learning TypeScript, paste your JavaScript code and see how types are added. This helps you understand where TypeScript expects type annotations.

Code Review Preparation

Before reviewing a TypeScript migration PR, use this tool to generate a baseline conversion and compare it against the submitted changes.

Legacy Code Modernization

Convert old CommonJS modules (require, module.exports) to modern ES module syntax (import, export) as part of a modernization effort.

What the Converter Does NOT Do

This tool provides a starting point, not a complete migration:

Complex type inference: It does not analyze control flow, function return values, or variable reassignment patterns.
Generic types: It does not infer generic type parameters.
Interface generation: It does not create TypeScript interfaces from object shapes (use our JSON to TypeScript tool for that).
Third-party types: It does not add @types/* package references.

After conversion, you should review each any annotation and replace it with a specific type.

Best Practices for JS-to-TS Migration

Start with strict: false in tsconfig.json and enable strict checks gradually.
Rename files from .js to .ts (or .jsx to .tsx) one at a time.
Install type definitions for your dependencies: npm i -D @types/node @types/express.
Use any initially, then progressively replace with specific types.
Enable noImplicitAny as one of the last strict flags.

Related Tools

TypeScript to JavaScript — Strip types from TypeScript
JSON to TypeScript — Generate TS interfaces from JSON data
TypeScript to JSON — Create JSON from TS interfaces
JSON Viewer — Explore JSON data structures
JSON Validator — Check JSON syntax

🟡 JavaScript to TypeScript Converter

Related Developer Utilities

JavaScript to TypeScript Converter — Developer Guide