Introduction

TypeScript, a superset of JavaScript, adds static typing and modern tooling to enhance code reliability and maintainability. Migrating an existing JavaScript project to TypeScript can improve developer experience, catch errors early, and future-proof your codebase. This guide walks you through the process step-by-step.

Why Migrate to TypeScript?

Type Safety: Catch errors during development, not runtime.
Better Tooling: Autocompletion, refactoring, and navigation in IDEs.
Scalability: Easier to manage large codebases.
Community Support: Widely adopted in modern frameworks like React, Angular, and Node.js.

Step 1: Install TypeScript

Begin by installing TypeScript as a development dependency:

npm install --save-dev typescript

Step 2: Initialize tsconfig.json

Create a tsconfig.json file to configure TypeScript settings:

{
  "compilerOptions": {
    "target": "es6",
    "module": "commonjs",
    "strict": true,
    "esModuleInterop": true,
    "outDir": "./dist"
  }
}

Step 3: Add TypeScript Scripts to package.json

Update your package.json scripts:

"scripts": {
  "build": "tsc",
  "watch": "tsc --watch",
  "pretest": "tsc"
}

Step 4: Convert JavaScript Files to TypeScript

Rename .js files to .ts and update import statements. For example:

// Before (JavaScript)
import { sum } from './utils';

const result = sum(2, 3);

// After (TypeScript)
import { sum } from './utils';

const result = sum(2, 3);

TypeScript will infer types for existing variables and functions. Use any sparingly to avoid type safety issues.

Step 5: Add Type Definitions for External Libraries

For libraries without .d.ts files, install type definitions:

npm install --save-dev @types/node

Step 6: Enable Strict Type Checking

Set strict: true in tsconfig.json to enforce type checking, including:

strictNullChecks
strictFunctionTypes
noImplicitAny

Step 7: Use Type Annotations

Explicitly define types for variables, function parameters, and return values:

function greet(name: string): void {
  console.log(`Hello, ${name}`);
}

greet('Alice');

Step 8: Handle JavaScript Files in TypeScript Projects

If your project includes .js files (e.g., third-party code), configure tsconfig.json:

"include": [
  "src/**/*.ts",
  "third-party/*.js"
]

Step 9: Test Your TypeScript Code

Use Jest or Mocha with TypeScript support:

npm install --save-dev jest @types/jest ts-jest

Create a test file:

// utils.test.ts
import { sum } from './utils';

test('sum function works', () => {
  expect(sum(2, 3)).toBe(5);
});

Step 10: Build and Deploy

Run the build script:

npm run build

Ensure the output directory (dist) contains compiled JavaScript files.

Conclusion

Migrating to TypeScript enhances code quality and developer productivity. Start small, enable strict checks, and gradually refactor your project.

FAQs

How long does migration take?

Depends on project size. Start with critical files first.

Can I mix JavaScript and TypeScript?

Yes, but configure tsconfig.json to include both.

Is TypeScript slower than JavaScript?

TypeScript adds a build step, but runtime performance is identical.

Cover Image Prompt

A split-screen image showing JavaScript code on one side and TypeScript code on the other, with a glowing TypeScript logo in the center, symbolizing the migration process.

Keywords

TypeScript migration, JavaScript to TypeScript, TypeScript best practices, TypeScript setup, TypeScript benefits