How the interface is augmented in JS and typescript files

In TypeScript, "augmented" typically refers to the process of extending or enhancing an existing interface or type with additional properties or methods. This is often done by using declaration merging, where multiple declarations with the same name are merged into a single definition by the TypeScript compiler.

Explanation with an Example

In the context of the provided code snippet:

declare namespace _ {
    // eslint-disable-next-line @typescript-eslint/no-empty-interface -- (This will be augmented)
    interface LoDashStatic {}
}

The comment // eslint-disable-next-line @typescript-eslint/no-empty-interface -- (This will be augmented) indicates that the LoDashStatic interface is initially empty but will be extended later on. This means that other parts of the code or additional declaration files will add properties or methods to this interface.

Declaration Merging

Declaration merging allows multiple declarations of the same interface to be combined into a single interface. For example:

Initial Declaration

declare namespace _ {
    interface LoDashStatic {}
}

Augmenting the Interface

In another part of the code or another file, you can augment the LoDashStatic interface by adding new properties or methods:

declare namespace _ {
    interface LoDashStatic {
        map<T, U>(array: T[], iteratee: (value: T) => U): U[];
        chunk<T>(array: T[], size?: number): T[][];
    }
}

Result of Augmentation

After augmentation, the LoDashStatic interface will include all the properties and methods from the initial declaration and the subsequent augmentations:

declare namespace _ {
    interface LoDashStatic {
        map<T, U>(array: T[], iteratee: (value: T) => U): U[];
        chunk<T>(array: T[], size?: number): T[][];
    }
}

Practical Use Case

For libraries like Lodash, which have many utility functions, it's common to split type definitions into multiple files for better organization and maintainability. These files can augment the main interface by adding specific type definitions related to different functionalities.

Example of Augmentation in Practice

1. Main Declaration File (lodash.d.ts):

   declare namespace _ {
       // eslint-disable-next-line @typescript-eslint/no-empty-interface -- (This will be augmented)
       interface LoDashStatic {}
   }
   
   declare const _: _.LoDashStatic;
   
   export = _;
   export as namespace _;
   

2. Additional Type Definition File (lodash.array.d.ts):

   declare namespace _ {
       interface LoDashStatic {
           chunk<T>(array: T[], size?: number): T[][];
       }
   }
   

3. Additional Type Definition File (lodash.collection.d.ts):

   declare namespace _ {
       interface LoDashStatic {
           map<T, U>(array: T[], iteratee: (value: T) => U): U[];
       }
   }