On: Deep-copying in JavaScript

[pjmlp]

I doubt there would be any serious compiler design lecture about implementing a JavaScript engine that would use anything other than "pass by value" to decribe how arguments are placed into the stack upon calling a function.

The problem is trying to apply terms without understanding what they actually mean.

[Stratoscope → pjmlp]

This is exactly the problem I've been talking about.

A compiler design lecture about implementing a JavaScript engine should not use the term "pass by value" to describe JavaScript's behavior.

Most JavaScript engines to date have been written in C/C++. Function parameters are passed by value in these languages, unless you use a reference parameter in C++.

This means that if the argument is a struct, the function receives a copy of the struct, not a reference to the original struct that was passed in.

Consider this C code:

  typedef struct {
      int bar;
  } Foo;
  
  void fun( Foo boo ) {
      boo.bar = 43;
  }
  
  int main() {
      Foo foo;
      foo.bar = 42;
  
      printf( "Before call: %d\n", foo.bar );
      fun( foo );
      printf( "After call: %d\n", foo.bar );
  
      return 0;
  }

This will print:

  Before call: 42
  After call: 42

fun() is passed a copy of the struct, so modifying the struct inside the function only affects its local copy. That's what "pass by value" means in C.

Compare with the similar JavaScript:

  function fun( boo ) {
      boo.bar = 43;
  }
  
  function main() {
      let foo = { bar: 42 }
  
      console.log( "Before call:", foo.bar );
      fun( foo );
      console.log( "After call:", foo.bar );
  }
  
  main();

This will log:

  Before call: 42
  After call: 43

The function doesn't receive a copy of the object, it receives the object itself, under a new name.

Using the term "pass by value" for JavaScript's behavior would only invite confusion.

[pjmlp → Stratoscope]

You are doing pass by value on JavaScript example.

For it to be pass by reference you need to change boo inside fun(), but you are changing a field of boo.bar using the boo address passed by value.

When JavaScript starts having pass by reference, this code will hold true. Until then it won't, regardless how people re-invent CS concepts on HN.

  function fun( boo ) {
      boo = { bar: 42 }
  }
  
  function main() {
      let foo = null;
  
      console.log( "Before call:",  foo);
      fun( foo );
      console.log( "After call:", foo );
  }
  
  main();

This will log:

  Before call: null
  After call: { bar: 42 }

[zzzcpan → pjmlp]

Placing arguments into the stack is an implementation detail and it can be done either way.

For example, we can hold a reference to a struct representing a primitive value and place it onto the stack. If we let our callee modify this struct we get a behavior you understand as pass-by-reference. Otherwise we can copy this struct into a new struct on write and replace the reference on the stack. This way we are passing references, but implementing pass-by-value behavior. We can do the same for non-primitive data structures, etc.

So these things are about behavior, not implementation. And Javascript does have a behavior where it passes certain things as references implicitly stored in variables. It's in no way pass by value.

[pjmlp → zzzcpan]

When JavaScript starts having pass by reference, this code will hold true.

  function fun( boo ) {
      boo = { bar: 42 }
  }
  
  function main() {
      let foo = null;
  
      console.log( "Before call:",  foo);
      fun( foo );
      console.log( "After call:", foo );
  }
  
  main();

This will log:

  Before call: null
  After call: { bar: 42 }

[zzzcpan → pjmlp]

I don't know why it's so hard to understand. It's not the essence of passing by reference, you are mixing implementation details with semantics.

If you can modify something that you didn't __explicitly__ use as a reference, you have an implicit pass by reference behavior. It doesn't matter if you can't modify the root reference itself of a nested structure, but only the children. You still can modify something that you didn't explicitly specify, so it is still pass by reference.

And by the way, CS has no precise terminology about this, CS prefers to neglect usability side of programming languages altogether.

[shkkmo → zzzcpan]

> It's not the essence of passing by reference, you are mixing implementation details with semantics.

> And by the way, CS has no precise terminology about this,

That is not correct. You are the one mixing up terms. Pass-by-reference and Pass-by-value have specific meanings.

There are two acceptable terms for how Javascript passes variables, those are "pass-by-sharing" or "pass-reference-by-value" https://en.wikipedia.org/wiki/Evaluation_strategy#Call_by_sh...

[zzzcpan → shkkmo]

But they are not actual terms and are not even conventions, just some things somebody used somewhere that you choose to stand behind. And I choose not to and prefer to use more common understanding of them.

See, it's all about usability and how users understand things. If one way of passing things leaves them with no side effect and another way lets called functions modify shared state without them realizing - these are the big distinctions. It really adds nothing if we call two slightly different variations of passing references by different names instead of more common and intuitive "pass-by-reference".

[shkkmo → zzzcpan]

> But they are not actual terms and are not even conventions, just some things somebody used somewhere that you choose to stand behind. And I choose not to and prefer to use more common understanding of them.

Yes, they are actual terms. You are choosing to be wrong and spread misinformation.

> It really adds nothing if we call two slightly different variations of passing references by different names instead of more common and intuitive "pass-by-reference".

Yes, it really does. The "slightly different variation" has a huge impact on how you write code. PHP, for example, does pass-reference-by-value by default (same behavior as Javascript) but it can also do pass-by-reference if you specify that in the function definition. see: https://www.tehplayground.com/nzi7SxTxXrrlqTZf

You used to be able to specify pass-by-reference when calling a function, but that was deprecated in 5.3 and removed in 5.4. It was removed precisely because it is important the the person writing a function knows if the variables he is using are pass-by-reference or pass-reference-by-value.

Since it is all about usability and helping users understand things, please stop trying to remove important meaning from the established term "pass-by-reference".

[zzzcpan → shkkmo]

> The "slightly different variation" has a huge impact on how you write code.

From my years of experience with a language that has both variations (Perl) - it doesn't have much impact on how you write code at all, except making the language a bit more flexible and expressive in rare situations.

I consider usability of programming languages to be rather important and would like wrong ideas not to be claimed as terminology there nor conventions. Wrong people shouldn't be pushing wrong terms into the field they have no understanding of.