So technically, a smi can exist on the stack since they don’t need additional storage allocated on the heap, depending how the variables are declared: A bunch of other languages like OCaml and Ruby does this too. And it is not unique to V8 or JavaScript. In V8, integers ranging from -2³¹ to 2³¹-1 on a 64-bit architecture (the V8 term is smi) are heavily optimized so they can be encoded inside of a pointer directly without the need to allocate additional storage for it. I wrote another post covering how JavaScript variables are implemented in more details. V8_INLINE static internal ::Address * GetRoot (v8 ::Isolate * isolate, int index )
If you don’t want to really dig into V8’s source code, there is an easy way that I can prove this to you. (Almost) Everything is on the heap #Ĭontrary to common belief, primitive values are also allocated on the heap, just like objects.
Stack vs heap javascript code#
If you want the ground truth, you can always look up the source code for the VM - at least for V8 it is all open-sourced.Īll of the examples in this post are based on V8’s implementation. You can trivially check this yourself by doing memory profiling in Chrome DevTools. However being an implementation detail doesn’t mean it is a myth. What is interpreted or compiled is not the languages but instead implementations - we can easily build simple AST interpreter for JavaScript, or a Stack-based virtual machine, or static LLVM compiler to native code. Asking how JavaScript handles memory allocation is like asking if JavaScript is a compiled or interpreted language. These are considered implementation details. In fact, I doubt you can find anything about memory layout in any language specification - even for C++, which is considered much more low-level than JavaScript, does not have the terms defined in its standard. You cannot find the term “Stack” or “Heap” used in the ECMAScript specification. The stack only stores temporary, function-local and small variables (mostly pointers) and that's largely unrelated to JavaScript types.įirst of all, the JavaScript language itself doesn’t mandate memory layout.
TL DR: #Īll JavaScript values are allocated on the heap accessed by pointers no matter if they are objects, arrays, strings or numbers (except for small integers i.e. I am typing in this post so that I can link to it and save myself some time in the future. This idea is false, at least this is not how the language is implemented in the majority of JavaScript engines I have seen. There are a wealth of resources online claiming that in JavaScript primitive values are allocated on the stack while objects are allocated on the heap. Also this is not going to be a hardcore memory-related post which I am not really qualified to talk about. V8 is very complex, and it has multiple ways to run code, and its various parts and pipelines that have been rewritten many times over the years so what I described today might become outdated tomorrow. Maybe a more accurate title should be “the JavaScript memory model that is implemented in the current version of V8 demystified (with a lot of oversimplification)”. I admit this title is a little clickbaity. Why primitive values are not allocated on the stack
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