Extending Array's prototype not building in typescript

Question

In my TypeScript-based application I want to add some functionality to the Array-prototype.

Therefore I added a file Array.d.ts to my project which contains:

interface Array<T> {
    selectMany<TOut>(selectListFn: (t: T) => TOut[]): TOut[];
}

Then I created another file under /Scripts/Extentsions/Array.ts where I've implemented the function:

Array.prototype.selectMany = Array.prototype.selectMany || function<TIn, TOut>(selectListFn: (t: TIn) => TOut[]): TOut[] {
    return this.reduce((out, inx) => {
        out.push(...selectListFn(inx));
        return out;
    }, new Array<TOut>());
}

In another file I'm going to use the "extension" on an DivisionResource[]-array where DivisionResource is just the concrete type of the array's elements. Intellisense shows no errors but when running webpack it throws

TS2339: Property 'selectMany' does not exist on type 'DivisionResource[]'.

I don't get the point what is missing here.

Umut Özel · Accepted Answer · 2018-05-09 11:50:15Z

1

You should wrap merging for global types with "global", like this;

declare global {
    interface Array<T> {
        selectMany<TOut>(selectListFn: (t: T) => TOut[]): TOut[];
    }
}

https://www.typescriptlang.org/docs/handbook/declaration-merging.html

You could also try adding a triple slash reference of declaration file to .ts file.

/// <reference path="../Array.d.ts" />

edited May 9, 2018 at 11:50

answered May 9, 2018 at 10:49

Umut Özel

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2 Comments

KingKerosin Over a year ago

This throws

TS2669: Augmentations for the global scope can only be directly nested in external modules or ambient module declarations.

Umut Özel Over a year ago

You are right, I got that error too (with TS 2.6.2). Could you try adding a reference of definition file to .ts file? /// <reference path="./Array.d.ts" />

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