TypeScript vs JavaScript : Quand et comment convertir

Looking for a TS to JS converter? Whether you need to convert TypeScript to JavaScript online for a quick snippet or plan a full migration, this guide has you covered. TypeScript has become the default choice for new JavaScript projects, but millions of lines of JS code still power production apps. Below you will find our free online TypeScript to JavaScript converter, key differences between TS and JS, migration strategies, and what actually happens when TypeScript compiles to JavaScript.

Convert TS to JS online instantly (free tool) →

1. TypeScript vs JavaScript: Key Differences

TypeScript is a strict syntactical superset of JavaScript. Every valid JS file is already valid TS. TypeScript adds an optional static type system that catches errors at compile time rather than runtime.

The critical insight: TypeScript types exist only at compile time. After compilation, all type annotations are removed and you're left with standard JavaScript. There is zero runtime overhead.

2. When to Use Each

Choose TypeScript when:

• Your project has more than a few hundred lines of code
• Multiple developers work on the same codebase
• You're building libraries or APIs consumed by others
• You want better IDE support (autocomplete, refactoring)
• Your project will be maintained long-term

Choose JavaScript when:

• Quick prototypes or scripts (under 200 lines)
• Simple serverless functions or small utilities
• Your team has no TypeScript experience and deadlines are tight
• Configuration files and build scripts
• You need maximum ecosystem compatibility (some older tools struggle with TS)

Bottom line: For any project you'll maintain beyond a month, TypeScript pays for itself through fewer bugs and better refactoring confidence.

3. Gradual Migration Strategy

You don't have to convert an entire project at once. TypeScript supports gradual adoption:

1. Step 1: Add tsconfig.json with "allowJs": true and "strict": false
2. Step 2: Rename files one by one from .js to .ts
3. Step 3: Add types to the renamed files, starting with public-facing APIs
4. Step 4: Gradually enable stricter compiler options
5. Step 5: Enable "strict": true when most files are converted

This approach lets you keep shipping features while improving type safety incrementally.

Pro tip: Start by adding types to shared utility functions and data models. These provide the most value because they're imported throughout the codebase.

4. What TypeScript Compiles To

Understanding the TypeScript compiler output demystifies the whole system. Here's what happens to common TS features:

Type annotations are simply removed:

// TypeScript
function greet(name: string, age: number): string {
  return `Hello ${name}, you are ${age}`;
}

// Compiled JavaScript
function greet(name, age) {
  return `Hello ${name}, you are ${age}`;
}

Interfaces and type aliases are completely erased — they produce zero JavaScript output:

// TypeScript
interface User {
  id: number;
  name: string;
  email: string;
}
type Status = 'active' | 'inactive';

// Compiled JavaScript
// (nothing — both are completely erased)

Enums are one of the few TS features that produce runtime code:

// TypeScript
enum Direction {
  Up = "UP",
  Down = "DOWN",
  Left = "LEFT",
  Right = "RIGHT",
}

// Compiled JavaScript
var Direction;
(function (Direction) {
  Direction["Up"] = "UP";
  Direction["Down"] = "DOWN";
  Direction["Left"] = "LEFT";
  Direction["Right"] = "RIGHT";
})(Direction || (Direction = {}));

5. Common TypeScript Syntax Removed During Compilation

These TypeScript-only constructs are stripped entirely during compilation and produce no JavaScript output:

• type aliases — type User = {{ name: string }}
• interface declarations — interface Props {{ title: string }}
• Type annotations — const x: number = 5 becomes const x = 5
• Generic type parameters — function identity<T>(x: T) becomes function identity(x)
• as type assertions — value as string becomes value
• ! non-null assertions — element!.click() becomes element.click()
• declare blocks — ambient declarations for external code

Important: Some TS features like enum, namespace, and parameter decorators do produce runtime JavaScript. These are the exception, not the rule.

6. Project Setup Tips

Whether you're starting fresh or migrating, these tsconfig.json settings are recommended:

{
  "compilerOptions": {
    "target": "ES2022",
    "module": "ESNext",
    "moduleResolution": "bundler",
    "strict": true,
    "noUncheckedIndexedAccess": true,
    "forceConsistentCasingInFileNames": true,
    "skipLibCheck": true,
    "esModuleInterop": true,
    "resolveJsonModule": true,
    "isolatedModules": true
  }
}

Key settings explained:

• "strict": true — Enables all strict type checks (the whole point of using TS)
• "noUncheckedIndexedAccess": true — Array and object index access returns T | undefined
• "forceConsistentCasingInFileNames" — Prevents bugs on case-sensitive file systems (Linux)
• "skipLibCheck": true — Skips type-checking node_modules for faster builds

For Next.js, Vite, and other frameworks, the framework's default tsconfig.json is usually a good starting point. Only customize what you need.

7. Free Online TS to JS Converter Tool

Need to convert TS to JS online quickly? Whether you want to see what your TypeScript compiles to or convert a TS snippet for a JS-only project, our free TypeScript to JavaScript converter handles it instantly:

• Strips all type annotations, interfaces, and type aliases
• Converts enums to their JavaScript equivalents
• Preserves all runtime logic and formatting
• Shows you exactly what the TypeScript compiler produces

Paste your TypeScript code and see the clean JavaScript output immediately. Great for learning, debugging, and sharing code with JS-only teams.

Convert TS to JS online instantly (free tool) →

Frequently Asked Questions

Does TypeScript make my code run slower?

No. TypeScript has zero runtime performance overhead. All type information is removed during compilation. The resulting JavaScript runs at exactly the same speed as hand-written JavaScript. The only cost is a slightly longer build step.

Can I use TypeScript without a build step?

Recent Node.js versions (22.6+) support running TypeScript directly with --experimental-strip-types. Deno and Bun support TypeScript natively. For browsers, you still need a compilation step, which tools like Vite handle transparently.

Should I convert all my JavaScript files to TypeScript at once?

No. Gradual migration is strongly recommended. Start with "allowJs": true in your tsconfig, rename files one at a time, and add types progressively. Converting everything at once creates a massive PR that's hard to review and may introduce bugs.

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