objects in javascript

Question

Primitive values are stored in a stack in javascript but objects are stored in a heap. I understand why to store primitives in stack but any reason why objects are stored in heaps?

Answer 1

The size of objects can grow dynamically. Therefore, you would need to adjust their memory requirements. That is why, they are stored in heap.

Answer 2

Actually, in JavaScript even primitives are stored in the heap rather than on a stack. When control enters a function, an execution context (an object) for that call to the function is created, which has a variable object. All vars and arguments to the function (plus a couple of other things) are properties of that anonymous variable object, exactly like other properties of named objects. A call stack is used, but the spec doesn't require the stack be used for "local" variable storage, and JavaScript's closures would make using a stack a'la C, C++, etc. for that impractical. Details in the spec.

Instead, a chain (linked list) is used. When you refer to an unqualified symbol, the interpreter checks the variable object for the current execution context to see if it has a property for that name. If so, it gets used; if not, the next variable object in the scope chain is checked (note that this is in the lexical order, not the call order like a call stack), and so on until the global execution context is reached (the global execution context has a variable object just like any other execution context does). The variable object for the global EC is the only one we can directly access in code: this points to it in global scope code (and in any function called without this being explicitly set). (On browsers, we have another way of accessing it directly: The global variable object has a property called window that it uses to point to itself.)

Re your question of why objects are stored in the heap: Because they can be created and released independently of one another. C, C++, and others that use a stack for local variables can do so because variables can (and should) be destroyed when the function returns. A stack is a nice efficient way to do that. But objects aren't created an destroyed in that straightforward a way; three objects created at the same time can have radically different lifecycles, so a stack doesn't make sense for them. And since JavaScript's locals are stored on objects and those objects have a lifecycle that's (potentially) unrelated to the function returning...well, you get the idea. :-)

Answer 3

Both primitive values and objects are always stored in some other object - they are properties of some object.

There is not one primitive value / object that is not a property of another object. (The only exception here is the global object).