23rd November, 2017

Mocking dependencies with TypeScript and WebPack

I wanted to be able to override third party dependencies in some TypeScript repositories and feed my code mocked dependencies for unit testing.

Let's say we've got a TS module like

import * as moment from 'moment';

export class Calendar {
    public now(): void {
        let now = moment().format('LLLL');

This is our Calendar class which has a dependency on MomentJS.

I want to make some unit tests, but not actually call MomentJS, because I want to check I'm passing the right values in.

Luckily, Typescript supports overriding module imports.


    "compilerOptions": {
        "paths": {
            "moment": ["test/mock-modules/moment"],

The TypeScript documentation gives great examples.

In this example, I'm remapping to a module which is test/mock-modules/moment relative to baseUrl in tsconfig.json.

Since TypeScript doesn't remap imports, we need to add an alias to WebPack config:

resolve: {
    alias: {
        "moment": "../test/mock-modules/moment"

Hopefully this helps someone.


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The opinions expressed here are my own and not those of my employer.