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Creating Custom Type Definitions

When a library does not have existing type definitions, you can create your own custom type definitions to enable type-safe usage in TypeScript projects. Custom type definitions are written in .d.ts files.

Key Topics

Why Create Custom Type Definitions?

Custom type definitions allow you to enable TypeScript support for libraries that do not provide built-in types. This ensures better type checking and IntelliSense in your projects.

Basic Structure of a Type Definition File

A type definition file typically includes interfaces, types, and function declarations that describe the structure of a library.

// custom-library.d.ts
declare module "custom-library" {
    export function customFunction(param: string): number;
    export const customValue: boolean;
}

Explanation: The declare module syntax is used to define the shape of a module, specifying its exported members and their types.

Using Custom Type Definitions

To use custom type definitions, save the .d.ts file in your project and import the library as usual.

// Import and use the custom types
import { customFunction, customValue } from "custom-library";

console.log(customFunction("Test")); // Outputs a number
console.log(customValue); // Outputs a boolean

Output

A number

true

Explanation: The custom type definitions enable TypeScript to provide IntelliSense and type-checking when using the custom-library.

Key Takeaways

  • Custom Definitions: Create .d.ts files to add TypeScript support for untyped libraries.
  • Declare Modules: Use declare module to define the structure of the library.
  • Enable IntelliSense: Custom types improve code readability and reduce errors.
  • Reusable: Share custom definitions across projects for consistent type safety.
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