Typescript: add function to momentjs' prototype

Question

I'm trying to add a function to the momentjs prototype. In Javascript, the code was like this:

Object.getPrototypeOf(moment()).isWeekend = function() {
    return this.isoWeekday() >= 6;
};

How do I do this in typescript? I've read that i need to duplicate the interface and and my function to it, but that's not working:

module moment {
    interface Moment {
        isWeekend(): boolean
    }

    Moment.prototype.isWeekend = () => {
        this.isoWeekday() >= 6;
    };
}

Fenton · Accepted Answer · 2015-07-31 07:53:41Z

2

You need to place the actual extension outside of the module, and you need to export the interface...

module moment {
    export interface Moment {
        isWeekend(): boolean
    }
}

(<any>moment).fn.isWeekend = function() {
    this.isoWeekday() >= 6;
};

edited Jul 31, 2015 at 7:53

answered Jul 30, 2015 at 14:10

Fenton

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4 Comments

Bart van den Burg Over a year ago

I've tried to apply this, but i get "moment.isWeekend is undefined" when I try to use it

Fenton Over a year ago

It looks like with MomentJS you don't extend the prototype, but instead add to a fn property. I have updated the example. You also need to use a function, not an arrow-function as otherwise you will affect the scope of this.

Bart van den Burg Over a year ago

My mistake was that I didn't have to change the original JavaScript code, just add the TypeScript declaration.

Rookian Over a year ago

@Fenton I get the error: [ts] cannot use 'moment' namespace as a value.

Bibby Chung · Accepted Answer · 2018-01-30 14:23:51Z

1

It's working for me.

import * as moment from 'moment';
declare module 'moment' {
  export interface Moment {
    toTaiwaneseYear(): number;
  }
}

(moment.fn as any).toTaiwaneseYear = function () {
  const _self = this as moment.Moment;
  return _self.year() - 1911;
}

reference:

https://medium.com/my-life/extension-method-in-typescript-66d801488589

answered Jan 30, 2018 at 14:23

Bibby Chung

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