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Discussion on: Javascript academy #1: Primitive value vs reference value

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Akash Kava • Edited on

In the same Stackoverflow link given in the last this is the line.

Well for starters we're talking about JavaScript, and JavaScript doesn't have a stack or a heap. It's a dynamic language and all the variables in JavaScript are dynamic. To explain the difference I'll compare it to C....

I have integrated V8 for Android at github.com/web-atoms/xamarin-v8 and I can guarantee, that nothing except small integer (31 bits, one bit to differentiate heap allocated number) lives in the stack, V8Number is the exact class which is allocated on the heap which stores the value, all JavaScript values are on heap and they are all derived from V8Value class, please refer to the source code of V8.

Now, v8 analyzes source code and does some shortcut, like adding two numbers,

function add(a) {
   var c = 2;
   return a + c;
}
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In Above function, a is unknown, so it is of type V8Value in C++. However, V8 sees that we are adding literal 2 which is kept in variable c, so V8 intelligently puts 2 in stack and then combines it with a. As long as compiler can track the type, it can keep it on stack, otherwise it needs to create reference. As everything passed as an argument to other method in C++ are handled by references.

So only for certain operations V8 puts things on stack, otherwise everything lives in heap, when you call a method add(2, 4) , all though you know that you are passing 2 and 4 but function doesn't , everything is dynamic. So compiler has to allocate V8Number, and pass reference to add method.

To understand how complex it is to put primitives on stack, try creating a small javascript engine and you will see how it is possible and how it is complex enough to not do it.

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