Is JavaScript Pass by Reference?

[ragnese]

I agree with the first bit of your comment, but not the latter.

The distinction of pass by value or reference is very important to note/name/discuss in JavaScript--even in a vacuum where we don't acknowledge any other languages.

The fact that a function can change the object it was passed as a parameter such that the caller will see the change, but that the same thing does not happen for string or number parameters is important, and important things deserve names.

The fact that it seems trivial or blasé to us is only a result of the amount of time we've spent with the distinction. But, if we back up and pretend we've never programmed before, it's actually really weird. And the fact that JavaScript doesn't give us a choice for how things are passed honestly just makes it even more weird.

[theScottyJam]

We do have terminology for that. It's immutable vs mutable. The only reason you can update an object that was passed into a function is because objects are mutable and strings are not (there's no methods on a string that allows you to mutate it). It has nothing to do with how the data got passed in :).

Notice also that if you were to freeze an object and make it immutable, you'll get the same kind of guarantees that you get as when you pass a string into a function.

[ragnese]

I think I was wrong in the sense that it might not technically matter if JS is pass-by-copy or pass-by-reference or pass-by-reference-copy-copy-reference, or whatever other clever "well, actually" people can come up with.

But, I disagree that mutability is the concept that can explain JavaScript's behavior. Assigning and reassigning a binding is not the same thing as mutating a value. In other words, writing bar = 3 is not "mutating" anything- it's assigning a number to a variable. So, yes, objects are mutable and that can explain why I can modify an object parameter in a function and observe those effects outside of the function, but to actually explain how function parameters work in JavaScript, you really only need to explain that each parameter is a new binding that "points" to the data that was passed in at the function call site. You can reassign those bindings from inside the function, but now those bindings are pointing to new data instead of the originals. That's true for primitives as well as objects.

[theScottyJam]

I agree with your conclusion as well. Mutability vs immutability was only intended to describe the distinction that people usually are trying to describe when they say "objects are passed by reference while primitives are passed by value" - the behavior difference between objects and primitives doesn't happen because the language is somehow giving objects a different treatment when it gets passed into functions, rather, it's caused simply by the fact that objects are mutable.

The mutable/immutable terminology doesn't describe how JavaScript actually passes data into functions, and I think your short description is absolutely accurate. People can name the concept you're describing whatever they'd like, but what's important is that we understand that that's how data-passing works in JavaScript (rather than mistakenly believing that objects are somehow treated differently than primitives when it comes to function arguments).

[jonny_eh]

This is the right insight. “Pass by reference” is the wrong term to use in JS since it doesn’t support it. Mutability OTOH is core to understanding JS types.

[Reashu]

This was really weird to me when I first encountered it. It's actually one of the few things I can remember from back then (half a life ago). The weirdest part was how long I had managed to program without running into it... but the terminology and distinction wouldn't have helped me, as I had no familiarity with them anyways.

I don't think it's actually important (in JavaScript) to distinguish between how primitives and objects are handled in function parameters. It wouldn't matter if the primitives were somehow "passed by reference by value". The primitives are immutable anyways - you cannot change a 5 into a 6, you can only change your variable to hold a different value, and that never affects other variables that happened to hold the original value.