[bluefirebrand]

No, you understand perfectly well!

This is the same conversation I had with the new hire I mentioned. And as the other comment here mentioned, it's JavaScript. You simply overwrite the object method with the mock at runtime of the test, then restore it after the test finishes running.

Most JS testing frameworks do this under the hood I believe, without any change in how you write your code. It's a fundamental difference in how JavaScript handles things versus something stricter like C#.

It's a dangerous footgun if you use it carelessly, of course. But a useful language feature when applied carefully.

[mdpye]

Right. Facepalm. I kind of missed the bit where we shifted paradigms.

My personal preference would still be to be explicit, to echew secret mutable state even in the tests, but that's just me. My personal baggage makes phrases like "then restore it after the test finishes" bring me out in a cold sweat ;)

I guess I sympathise with your colleague, but I agree it's not idiomatic, and not being idiomatic is not helpful in a team.