Spy not working when depdendent service is destructured

[batjko]

Hi there,

Following the issue noted in #106, I found out why my spy won't work, despite following the documentation properly:

Scenario: You're testing a module and its function called MyModule.getStuff(), which itself depends on another service that you therefore want to stub, say using expect.spyOn(otherService, 'getData');.

Issue: The spy will not be called if MyModule's getStuff() is imported using ES6 destructuring, e.g.:

/* MyModule.js */

// will be caught by the spy:
const otherService = require('./otherService');

// won't be caught by the spy:
const { getData } = require('./otherService');

That's what stumped me on the issue for a good day. Changing the import back to the oldschool version made my test with the spy work just fine.

[ljharb]

It's impossible to spy on named imports. You can only spy on object properties, and a named import is a direct binding to the thing being exported.

That said, destructuring works just fine, unless the module has cached a reference to the function directly. In other words, if the module is intentionally being robust against someone mutating the object, it's also intentionally preventing anyone from spying on that thing.

[batjko]

I'm rather new to this, so forgive my ignorance regarding the workings of spies. But why can a spy not be directly attached to a the destructured function?

[ljharb]

@batjko because the mechanism by which all kinds of spies in JS functions is by replacing an object property with a spy function. There's nothing in JS that allows replacing a direct reference to a value with another one when it's not in scope (via reassignment)

[batjko]

That explains a few things. Thanks!

[didaquis]

Maybe this can help: https://stackoverflow.com/questions/60149391/how-to-use-jasmine-js-spy-on-a-required-function