Expanding a Typescript class with a new method through its prototype

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I'm encountering an issue while attempting to add a method to my Typescript class using prototype. Visual Studio is giving me a warning that the function does not exist in the target type.

I came across some information suggesting that I need to declare an additional interface for my type, which would include the definition of the method I want to add. However, it's not entirely clear to me how I should proceed after importing my type with import. Simply trying:

import { EcommerceCartItem } from "../classes/EcommerceCartItem";

interface EcommerceCartItem {
    myMethod: any
}

EcommerceCartItem.prototype.myMethod = function () {
    return null;
};

...causes a conflict between the import declaration and the local declaration of EcommerceCartItem. So, what would be the correct approach in this situation?

typescriptprototype

Answer №1

In order for your interface augmentation to be recognized, you need to declare it within the correct module:

import { ShoppingCartItem } from "../models/ShoppingCartItem";
declare module "../models/ShoppingCartItem" {
    interface ShoppingCartItem {
        customMethod: any
    }
}

ShoppingCartItem.prototype.customMethod = function () {
    return null;
};

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