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Declaration merging

Declaration merging allows you to combine multiple declarations with the same name into a single definition. This is particularly useful for extending existing types, interfaces, and modules without modifying their original definitions.

Merging interfaces​

Interfaces with the same name are automatically merged:

interface Box {
  height: number;
  width: number;
}

interface Box {
  scale: number;
}

// Resulting Box interface:
// interface Box {
//   height: number;
//   width: number;
//   scale: number;
// }

Module augmentation​

You can extend existing modules by using declaration merging:

// Existing module
declare module "myModule" {
  export function someFunction(): void;
}

// Augmentation
declare module "myModule" {
  export function newFunction(): void;
}

Module augmentation allows us to add type definitions for new functions, classes, or variables added to an existing module/npm package.

This is common with third-party libraries where we might have a custom configurations from which type definitions should be generated.

Examples:

Global augmentation​

You can add declarations to the global scope from within a module:

// In a module file
declare global {
  interface Array<T> {
    customMethod(): T;
  }
}

Declaration merging is a powerful tool for extending and modifying types in TypeScript, allowing for greater flexibility and expressiveness in your type definitions.