TypeScript Writing and Using Type Declarations (`.d.ts` Files)

Table of Contents

1. Fundamental Concepts
   • What are .d.ts files?
   • Why use .d.ts files?
2. Usage Methods
   • Creating .d.ts files
   • Importing and using type declarations
3. Common Practices
   • Declaration merging
   • Global type declarations
4. Best Practices
   • Keeping declarations up - to - date
   • Using third - party type definitions
5. Conclusion
6. References

Fundamental Concepts

What are .d.ts files?

.d.ts files are TypeScript declaration files. They are used to provide type information for JavaScript code. These files do not contain any executable code; instead, they only hold type definitions. For example, if you have a JavaScript library that you want to use in a TypeScript project, you can create a .d.ts file to define the types of the functions, classes, and variables in that library.

Why use .d.ts files?

• Error prevention: By providing type information, TypeScript can catch type - related errors during the compilation process, reducing the number of runtime errors.
• Code readability: Type declarations make the code more self - explanatory. Developers can easily understand the expected types of function parameters and return values.
• Interoperability: They allow TypeScript projects to work seamlessly with existing JavaScript libraries.

Usage Methods

Creating .d.ts files

Let’s say we have a simple JavaScript function in a file named math.js:

// math.js
function add(a, b) {
    return a + b;
}

To create a type declaration for this function, we can create a math.d.ts file:

// math.d.ts
declare function add(a: number, b: number): number;

The declare keyword is used to tell TypeScript that we are just declaring the type of an entity, not providing an implementation.

Importing and using type declarations

In a TypeScript file, we can use the type declarations from the .d.ts file.

// main.ts
/// <reference path="math.d.ts" />
const result = add(1, 2);
console.log(result);

The /// <reference path="math.d.ts" /> directive is used to tell the TypeScript compiler to include the type declarations from the math.d.ts file.

Common Practices

Declaration merging

Declaration merging allows you to combine multiple declarations of the same name into one. For example, let’s say we have two declarations for a class in different .d.ts files:

// file1.d.ts
declare class MyClass {
    constructor();
    method1(): void;
}

// file2.d.ts
declare class MyClass {
    method2(): void;
}

TypeScript will merge these two declarations into one:

class MyClass {
    constructor() {}
    method1() {}
    method2() {}
}

Global type declarations

You can use global type declarations to make types available throughout your project without the need to import them explicitly. For example, if you want to declare a global variable type:

// global.d.ts
declare global {
    var myGlobalVariable: string;
}

Now, you can use myGlobalVariable anywhere in your TypeScript project without importing it.

Best Practices

Keeping declarations up - to - date

As the JavaScript code evolves, the type declarations in the .d.ts files need to be updated accordingly. Make sure to review and update the declarations whenever the underlying JavaScript code changes.

Using third - party type definitions

For popular JavaScript libraries, there are often third - party type definitions available on DefinitelyTyped ( https://definitelytyped.org/) . You can install these type definitions using npm. For example, to install the type definitions for React:

npm install --save-dev @types/react

This will provide accurate type information for the React library in your TypeScript project.

Conclusion

TypeScript’s .d.ts files are a powerful tool for providing type information to JavaScript code. They help in error prevention, code readability, and interoperability. By understanding the fundamental concepts, usage methods, common practices, and best practices of writing and using .d.ts files, developers can make the most of TypeScript’s static typing capabilities and build more robust and maintainable applications.

References

• TypeScript official documentation: https://www.typescriptlang.org/docs/
• DefinitelyTyped: https://definitelytyped.org/