> I know what happens by reading the function. Pretty much any function that calls a method on some object mutates its argument. It makes little difference if you abstract this mutation behind the method, or do it directly, or do it via a helper function that takes the object as an argument.

Are we talking about functions in general, or small accumulator functions here? I would argue that mutating arguments in general is a really bad pattern that certainly will lead to weird and subtle bugs in the long run. It is also completely unnecessary – you have inputs and outputs for a reason. You might know what happens if you read the function yourself, but you can never assume that everyone else will.

> No. Most of those are usually more wasteful, because if you can't mutate objects, you'll have to throw them away at every mutation.

No, you don’t. This is an implementation detail. Libraries like immutable.js [1] (and several others, I believe) makes it possible to work with immutable data structures without cloning anything.

> Anyway, it is harmful to avoid mutation at all costs.

We can more or less agree on this one :) My comment was unnecessary bastant. I have used mutations in reducer aggregators myself. But, avoiding mutation removes a source of bugs, without sacrificing readability or functionality.

[1] https://immutable-js.github.io/immutable-js/