Rewired function is not instanceof Function in Jest context

[sguillia]

Hi

Here's a very simple example

index.js

function foo() { }
module.exports = { foo }

index.test.js

const rewire = require("rewire")
const index = rewire("./index")
const foo = index.foo

console.log("typeof:", typeof foo)
console.log("instanceof Function:", foo instanceof Function)

Run with Node

$ node index.test.js
typeof: function
instanceof Function: true

Run with Jest

$ jest index.test.js
...
typeof: function
instanceof Function: false
...

I'm really curious on why this happens, whether this is a bug, and how a function can be something else than an instance of Function.

[nktnet1]

This is related to #164.

The reason is covered in this article by Manuel Spigolon:

• https://backend.cafe/should-you-use-jest-as-a-testing-library

I wrote a niche library called jewire to work around this issue:

Hope this helps :).