Tips for importing a library in a TypeScript file that expands a JavaScript prototype

Question

Tips for importing a library in a TypeScript file that expands a JavaScript prototype

After following the instructions provided in this question, I am experimenting with integrating Moment.js to enhance the capabilities of the Date prototype within a TypeScript project.

The process of extending the Date prototype appears successful, as outlined below:

interface Date {
    myExtension: () => string;
}

if (!Date.prototype.myExtension) {
    Date.prototype.myExtension = function(): string {
        return 'a certain value obtained from a date';
    };
}

However, upon including Moment.js by inserting the subsequent line in my script, TypeScript flags an issue with my code and states that

Property 'myExtension' does not exist on type 'Date'

.

import * as moment from 'moment';

I wonder if there is an alternative approach for declaring the importation of Moment.js?

typescript import momentjs prototype

Answer 1

Answer №1

Unclear how the moment library connects to your query - assuming you aim to enhance global types for the Date.prototype, utilizing internal implementation by moment.

To enrich a global type definition of a built-in type like Date, you can either 1.) define the types in a separate .d.ts file or 2.) within one of your existing file modules.

1. Global Declaration File

Create a file named global.d.ts (naming is flexible). Include the Date interface, which will merge with the preexisting Date interface from lib.d.ts. Ensure that there are no top-level export or import declarations to append types to the global namespace/scope. global.d.ts will automatically be included in projects with a tsconfig.json.

// global.d.ts:
interface Date {
  myExtension: () => string;
}

2. File Module

Surround the definitions in a global context using declare global:

// inside some file module with export/import
...
declare global {
  interface Date {
    myExtension: () => string;
  }
}

Test it out in App.ts

// utilize moment for implementation
import * as moment from "moment"; 

if (!Date.prototype.myExtension) {
  Date.prototype.myExtension = function(): string {
    return "some value derived from a date";
  };
}

const d = new Date();
console.log(d.myExtension()); // some value derived from a date

Answer 2

Unclear how the moment library connects to your query - assuming you aim to enhance global types for the Date.prototype, utilizing internal implementation by moment.

To enrich a global type definition of a built-in type like Date, you can either 1.) define the types in a separate .d.ts file or 2.) within one of your existing file modules.

1. Global Declaration File

Create a file named global.d.ts (naming is flexible). Include the Date interface, which will merge with the preexisting Date interface from lib.d.ts. Ensure that there are no top-level export or import declarations to append types to the global namespace/scope. global.d.ts will automatically be included in projects with a tsconfig.json.

// global.d.ts:
interface Date {
  myExtension: () => string;
}

2. File Module

Surround the definitions in a global context using declare global:

// inside some file module with export/import
...
declare global {
  interface Date {
    myExtension: () => string;
  }
}

Test it out in App.ts

// utilize moment for implementation
import * as moment from "moment"; 

if (!Date.prototype.myExtension) {
  Date.prototype.myExtension = function(): string {
    return "some value derived from a date";
  };
}

const d = new Date();
console.log(d.myExtension()); // some value derived from a date

Tips for importing a library in a TypeScript file that expands a JavaScript prototype

Answer №1

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