jest.spy on a global function is ignored when the spy is called from another function - jestjs

Question

I'm working on a legacy JS web project that doesn't use import/require etc. So if I want to write tests, I need to somehow load all the code before the test is executed (I'm using a custom testEnvironment for that).
I've created a sample repo here.
Here are the main files:
// ./src/index.js
function spyOnMe() {
return "Hello World!";
}
function main() {
const text = spyOnMe();
return text;
}
// ./src/index.spec.js
it('should spyOn spyOnMe', () => {
const mockedValue = 'Thanks for helping!';
jest.spyOn(window, 'spyOnMe').mockReturnValue(mockedValue);
expect(spyOnMe()).toBe(mockedValue); // OK
const result = main();
expect(result).toBe(mockedValue); // KO
});
// ./jest.config.js
module.exports = {
clearMocks: true,
coverageProvider: "v8",
testEnvironment: "./jest.env.js",
};
// ./jest.env.js
const JSDOMEnvironment = require("jest-environment-jsdom");
const vm = require("vm");
const fs = require("fs");
class MyEnv extends JSDOMEnvironment.default {
constructor(config, context) {
super(config, context);
this.loadContext();
}
loadContext() {
const js = fs.readFileSync("./src/index.js", "utf8");
const context = vm.createContext();
context.document = this.global.document;
context.window = this.global.window;
vm.runInContext(js, context, {
filename: "./src/index.js",
displayErrors: true,
});
Object.assign(this.global, context);
}
}
module.exports = MyEnv;
The issue is in the index.spec.js:
The first expect returns Thanks for helping!
The second one returns "Hello world!"
Why is that?

Answer 1

I found a fix but I don't really understand why it works:
In jest.env.js, I should replace this line:
- const context = vm.createContext();
+ const context = vm.createContext(this.global);