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Discussion on: Javascript call by value or by reference, actually by sharing

kingychiu profile image
Anthony Chiu • Edited on

Thanks for this explanation here and also the "variables just point to values" mindset: in this post.

(Sorry for the long comment below in advance)

So let me try to summarize what I have learned here (hopefully is correct):

  1. For primitive types a=1, JS variable holds a reference (address) to the actual memory location that stores the value.
  2. For non-primitive types b=[1,2,3], the variable holds a reference (address) to the locations storing other references to the actual values of 1, 2, 3.
  3. For primitive types, the value are immutable, and the reassignment is copy(value) on write!! like a=1;b=a; b and a hold the same references/address to the same memory location. A new memory is allocated only if we try to assign b=2 for example. (what if we assign b=1? since 1 is immutable, will a=1;b=1; result in both a and b hold references tht point to the same place that storing that immutable 1?

It is shocking that even for primitive types, the JS variable only holds a reference (address in JS) to the actual value.

So basically, 90%+ of the materials out there are wrong, like this

A variable is a container for a value, like a number we might use in a sum, or a string that we might use as part of a sentence.

from developer.mozilla.org/en-US/docs/L...

I mainly have two questions here:
First: I learned most of the knowledge of a programming language internal from C++. Any reference materials that I can read about the virtual memory level of JavaScript? Like the stack vs heap in C++. For example in this blog, it describes let name = 'John'; stores the value in the stack frame of the stack, which is wrong based on the discussion here (right?)

Second: The second question I have is entirely the opposite. If I don't want to touch on any details about virtual memory, how can I learn or teach others about JavaScript? Since JavaScript is a high-level programming language, we should not care about memory (unless we need to optimize). But it seems like it is impossible to teach the correct concept without touching the memory and addresses.

I appreciate the new metaphor you suggested here for JavaScript, but now I worry if that variable metaphor idea will differ in different languages (Java vs JavaScript or even others). What is the point of learning that metaphor (I believe metaphor is an abstraction that should be helpful in quickly picking up different languages)? All in all, my second question is more about what is the right level of abstraction to learn/teach a programming language without touching the internals but learning/teaching it correctly?

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peerreynders profile image
peerreynders

storing other references to the actual values of 1, 2, 3.

That depends as the runtime can optimize. See Elements kinds in V8. Also there are Typed Arrays which hold the actual (uniformly typed) values.

It is shocking that even for primitive types, the JS variable only holds a reference (address in JS) to the actual value.

Remember that the runtime is free to optimize whichever way it seems fit as long as the observable behaviour aligns with the language specification. However it is dictated that the conceptual behaviour of a value of a primitive type is that of an "immutable datum represented directly at the lowest level of the language". Immutability is easily explained with Strings; you can't change a string, you can only create a new one, so on "assignment" the let name is essentially being rebound to the new string. But Boolean, Null Undefined, Number, BigInt, and Symbol values all behave the same way by virtue of being primitive types. I seem to remember coming across some V8 source code where primitive JavaScript values (at least in some context) were represented by C++ classes.

So basically, 90%+ of the materials out there are wrong

It's a simplified mental model which mostly works and is easy to grasp for beginners.

which is wrong based on the discussion here (right?)

Again the runtime implementation isn't dictated. V8 has the "Ignition" interpreter and the "Turbofan" compiler so actual JS execution could be accomplished in very different ways depending on the level of optimization.

if that variable metaphor idea will differ in different languages (Java vs JavaScript or even others).

My metaphor isn't original. Given that in JavaScript primitive values are immutable it's unfortunate that the term "assignment" is being used as that implies PLOP (PLace-Oriented Programming), i.e. (re)placing a value inside a location giving rise to the notion of a "variable" as the value inside the location can vary. In value-oriented programming (typically practiced in functional languages)

let number = 1983;
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is binding the name number to the value 1983 while

number = number * 10;
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is rebinding the name number to the result of the number * 10 expression. With that terminology it is the binding that changes, not the value.

I only stumbled upon this because I got annoyed at constβ€”until I discovered:

"The const declaration creates a read-only reference to a value. It does not mean the value it holds is immutableβ€”just that the variable identifier cannot be reassigned."

So people keep saying that const works differently for primitive values and objects (overlooking completely that primitive values are already immutable). This is a perspective that becomes necessary because of the assignment/variable oversimplification that people learn in the beginning. The truth is much simpler:

const doesn't act on the value but on the bindingβ€”it's the binding that is constant and unchangeable. As a result the const behaviour is absolutely identical for primitive values and objects as the binding being constant does not affect the internal mutability of objects (which includes arrays).

If I don't want to touch on any details about virtual memory

I hope I conveyed that going to that level isn't actually necessary. Once you eliminate the notion of variables and assignments and replace them entirely with names and bindings, everything behaves entirely consistently. It's just that beginners material insists on starting with variables and assignments and then never adjust that initial mental model.

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kingychiu profile image
Anthony Chiu

Thanks for your reply, let me study more with the given directions.

My imdidate response to this

Remember that the runtime is free to optimize whichever way it seems fit as long as the observable behaviour aligns with the language specification.

Yeah, it makes a lot of sense. For example, in C/C++, the "Optimization" I learned is like out-of-order execution (including speculation) and unfolding loops. I remember back then, I was shocked as well. Now I realize it is not just the JavaScript tutorials problem. One of the utimate problems is that the teaching materials don't separate what the language specifications are. But for non-specified behaviors, they are subjected to be changed/optimized. I think this is also a missing mental model puzzle here.

Back then, our university C++/Java course should also focus more on the language specifications first and make a clear separation between abstraction and implementation. Now I am confused about what I have learned (for all languages what their specifications/promises are).