How to spyOn a function that is not part of the class under test with Jest and Typescript

I'm trying to spy on a function that come from uuidv4 package but I didn't figure out how to do that.

This is my User Class:

import { uuid } from 'uuidv4';
import { IUser } from '../interfaces/IUser';

export class User implements IUser {
  constructor(
    public name: string,
    public email: string,
    public password: string,
    public id?: string,
  ) {
    this.id = id ? id : uuid();
  }
}

What I'm trying to do is spy on that uuid() method that is called on the constructor of the User.ts. I tried something like this:

import { User } from './User';

describe('User', () => {
  it('should call uuid() when no id is provided', () => {
    const sut = new User('Foo', 'foo@bar.com', '12345');
    const spy = jest.spyOn(sut, 'uuid');
    expect(spy).toHaveBeenCalledTimes(1);
  });
});

But it didn't work. Anyone knows how could I do this?

You don't need to mock or install spy on the uuid to test the implementation detail. You can test the with Using a regular expression to check user.id is a UUID v4 or not.

Using a regular expression:

If you want to perform the verification on your own using a regular expression, use the regex property, and access its v4 or v5 property

index.ts:

import { uuid } from 'uuidv4';

interface IUser {
  id?: string;
}

export class User implements IUser {
  constructor(public name: string, public email: string, public password: string, public id?: string) {
    this.id = id ? id : uuid();
  }
}

index.test.ts:

import { User } from './';
import { regex } from 'uuidv4';

describe('User', () => {
  it('should call uuid() when no id is provided', () => {
    const user = new User('Foo', 'foo@bar.com', '12345');
    expect(regex.v4.test(user.id!)).toBeTruthy();
  });
});

 PASS  stackoverflow/72130740/index.test.ts (13.885 s)
  User
    ✓ should call uuid() when no id is provided (2 ms)

Test Suites: 1 passed, 1 total
Tests:       1 passed, 1 total
Snapshots:   0 total
Time:        16.53 s

Also take a look at the uuid test case