Description

My needs arose from wanting to test a function that internally created some objects from 3rd party libraries. I want to intercept calls to the constructors of classes in those 3rd party libraries so that I can a) verify that the function being tested actually instantiates those objects with the desired parameters, and b) return fake/mock instances of those objects for testability. I thought that td.replace() is a perfect candidate for this, eg. td.replace(Some3rdPartyClass.prototype, 'constructor'); this worked perfectly with any other methods of classes, I can mock pretty much all methods, static as well as instance, except for 'constructor'. Maybe this was not meant to be but I can't find any documentation that state otherwise.

Issue

I tried to replace the constructors like so:

import { SomeClass } from 'some-library';
import * as td from 'testdouble';
const someMockInstance = {};
const constructor = td.replace(SomeClass.prototype, 'constructor');
td.when(constructor()).thenReturn(someMockInstance);
const instance =  new SomeClass();
expect(instance).to.equal(someMockInstance);

This will never pass, and when I ran the debugger, new SomeClass() always invoked the real constructor and not my mock constructor.

Maybe what I tried to do is misguided as there are not many who asked about this out in the wild.

Environment

• node -v output: v18.12.1
• npm -v (or yarn --version) output: 8.19.2
• npm ls testdouble (or yarn list testdouble) version: testdouble@3.16.8

Failing Test

• Fork the repo
• Add a failing test (probably to the `/regression/src' directory)
• Submit a pull request for the failing test or link to your branch here

Example Repo

• Create a minimal repository that reproduces the issue
• Make sure that a fresh clone can run only npm it and observe the issue
• Link to that repo here

Runkit Notebook

• Create a Runkit notebook
• Invoke var td = require('testdouble') at the top
• Verify the behavior your issue is concerned with by clicking "Run"
• Link to the Runkit here

Code-fenced Examples