This document provides a comprehensive guide on how to convert JavaScript (JS) code into TypeScript (TS). TypeScript is a superset of JavaScript that adds static typing, which can help catch errors at compile time and improve code quality. This guide will cover the necessary steps and best practices for a smooth transition from JS to TS.

Step 1: Install TypeScript

Before you start converting your code, ensure that TypeScript is installed in your project. You can install it globally or as a development dependency.

npm install -g typescript
or
npm install --save-dev typescript

Step 2: Rename Files

Change the file extensions of your JavaScript files from .js to .ts. For example, rename app.js to app.ts.

Step 3: Add a TypeScript Configuration File

Create a tsconfig.json file in the root of your project. This file will contain the configuration settings for the TypeScript compiler.

{
"compilerOptions": {
"target": "es6",
"module": "commonjs",
"strict": true,
"esModuleInterop": true,
"skipLibCheck": true,
"forceConsistentCasingInFileNames": true
},
"include": ["src/**/*"],
"exclude": ["node_modules", "**/*.spec.ts"]
}

Step 4: Start Adding Types

Begin adding type annotations to your variables, function parameters, and return types. Here are some common types you can use:

string
number
boolean
any
void
null and undefined
Custom types and interfaces

Example

JavaScript Code:

function add(a, b) {
return a + b;
}

TypeScript Code:

function add(a: number, b: number): number {
return a + b;
}

Step 5: Handle Any Errors

After adding types, you may encounter type errors. TypeScript will provide feedback on where types are mismatched or where type safety is violated. Address these errors by refining your types or adjusting your code.

Step 6: Use Interfaces and Types

For complex objects, consider using interfaces or type aliases to define the shape of your data.

Example

Using an Interface:

interface User {
name: string;
age: number;
}
function greet(user: User): string {
return `Hello, ${user.name}`;
}

Step 7: Leverage TypeScript Features

Take advantage of TypeScript features such as enums, generics, and tuples to enhance your code. These features can help you write more robust and maintainable code.

Example of an Enum:

enum Direction {

Up,

Down,

Left,

Right,

}

function move(direction: Direction) {

// Implementation

}

Step 8: Test Your Code

After converting your code, thoroughly test it to ensure that everything works as expected. You can use testing frameworks like Jest or Mocha to run your tests.

Conclusion

Converting JavaScript code to TypeScript can significantly improve the quality and maintainability of your codebase. By following the steps outlined in this document, you can make the transition smoothly and take full advantage of TypeScript's features. Happy coding!