Take this TypeScript class (the whole procedure also works in pure JavaScript):

export class UserDBClient {
  getName(userId: number): string {
  	// some magic
    return userId
  }
}

You might mock the method getName within your unit tests like this:

import UserDBClient from 'src/user-db-client'

jest.spyOn(UserDBClient.prototype, 'getName').mockResolvedValue('Homer Simpson')

// some unit tests

But what if the module exports single functions instead of a class containing the methods? Take this example:

export function getName(userId: number): string {
  // some magic
  return userId
}

Well, you can leverage a very simple trick to still use jest.spyOn. You just need to import the entire module into one variable within your test file and use it as if it was a class:

import * as UserDBClient from 'src/user-db-client'

jest.spyOn(UserDBClient.prototype, 'getName').mockResolvedValue('Homer Simpson')

// some unit tests

This is a very small example but over the past years I have seen multiple projects from different companies where the presence of exported functions had caused troubles when writing tests because developers struggled to mock them correctly.

When I was asked for help regarding unit tests in Jest, around 75% of the time this (in slight variations) was the solution to their problem.