The compilation of TypeScript extending DataType can sometimes result in errors

Question

The compilation of TypeScript extending DataType can sometimes result in errors

I have written a custom extension in my extensions/date.ts file which adds a method to the Date interface:

interface Date {
  addDays: (days: number) => Date
}

Date.prototype.addDays = function(days: number): Date {
  if (!days) return this;
  let date = this;
  date.setDate(date.getDate() + days);
  return date;
};

The extension is imported at the beginning of the server.ts file:

import './extensions/date';
import * as dotenv from 'dotenv';
dotenv.config()
...rest imports

The extension works correctly and the app compiles successfully. However, sometimes after making changes and having the app automatically recompiled using nodemon and run with node-ts, the app breaks and TypeScript no longer recognizes Date.addDays() as a valid function.

To temporarily solve this issue, I used to move the import statement higher up in the file, but now I have reached a limit.

The error message is the familiar 2339:

Property 'addDays' does not exist on type 'Date'. (2339)

Has anyone encountered this problem before? Any suggestions for a workaround?

typescript typescript-typings

Answer 1

Answer №1

To update the global object with your custom Date interface, you need to create a file, such as date.ts, and add the following code:

export {}

declare global {
  interface Date {
    addDays(days: number):  Date
  }
}

Date.prototype.addDays = function(days: number): Date {
  if (!days) return this;
  let date = this;
  date.setDate(date.getDate() + days);
  return date;
};

The export {} statement is a workaround to prevent the File date.ts is not a module. error.

Next, in your main file, you can import the date.ts file by adding import './date' and then use the function

let fn = (arg: Date) => arg.addDays(1);

without any issues.

import * as x from './date';

let fn = (arg: Date) => arg.addDays(1);

Answer 2

To update the global object with your custom Date interface, you need to create a file, such as date.ts, and add the following code:

export {}

declare global {
  interface Date {
    addDays(days: number):  Date
  }
}

Date.prototype.addDays = function(days: number): Date {
  if (!days) return this;
  let date = this;
  date.setDate(date.getDate() + days);
  return date;
};

The export {} statement is a workaround to prevent the File date.ts is not a module. error.

Next, in your main file, you can import the date.ts file by adding import './date' and then use the function

let fn = (arg: Date) => arg.addDays(1);

without any issues.

import * as x from './date';

let fn = (arg: Date) => arg.addDays(1);

The compilation of TypeScript extending DataType can sometimes result in errors

Answer №1

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