Implementing TypeScript type declarations for merging core types

Question

Implementing TypeScript type declarations for merging core types

Is there a way to perform type declaration merging in TypeScript for built-in types when using imports?

I am currently attempting to merge interfaces as per the guidelines outlined in this documentation: https://www.typescriptlang.org/docs/handbook/declaration-merging.html

When I try merging without any import statements, it works successfully:

interface Function {
  applyParams?(aa: string[]): string
}

function f() {}

const a:  Function  = f

a.applyParams && a.applyParams(["1", "2"]);

However, when I include an import statement at the beginning of the file, I encounter an error, as shown below:

import { MyType } from "./MyType";

interface Function {
  applyParams?(aa: string[]): MyType;
}

function f() {}

const a: Function = f;

a.applyParams && a.applyParams(["1", "2"]);

The error message reads: TS2559: Type '() => void' has no properties in common with type 'Function'.

typescript prototype declaration

Answer 1

Answer №1

If you want your definition to work, make sure to enclose it within a global block.

import { MyType } from "./MyType";

declare global {
  interface Function {
    applyParams?(aa: string[]): MyType;
  }
}

function f() {}

const a: Function = f;

a.applyParams && a.applyParams(["1", "2"]);

The reason for this is likely because TypeScript handles files with imports differently than those without them. Files with imports are treated as modules and have local scopes, while files without imports can be seen as declaration files with global scope by default.

Answer 2

If you want your definition to work, make sure to enclose it within a global block.

import { MyType } from "./MyType";

declare global {
  interface Function {
    applyParams?(aa: string[]): MyType;
  }
}

function f() {}

const a: Function = f;

a.applyParams && a.applyParams(["1", "2"]);

The reason for this is likely because TypeScript handles files with imports differently than those without them. Files with imports are treated as modules and have local scopes, while files without imports can be seen as declaration files with global scope by default.

Implementing TypeScript type declarations for merging core types

Answer №1

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