All you want to know about — Call Stack and Memory Heap in Javascript

Javascript engine is powerful and it does many things, but primarily it does two work that is to read our code and execute it .

To perform the above operation javascript needs two things: —

  1. A place where it can write and store information such as variables, objects, etc.
  2. To keep track of where is our code execution is or in simple words, it keeps track of what happens line by line in our code execution.

So, here comes the game of Memory Heap and Call Stack.

To store information or to allocate memory, javascript engine uses Memory Heap and to track code execution of code line by line javascript engine use Call Stack

Taa daaaa, Blog is done. Everything is over and I think you understood what you need to tell in an interview.

I am just kidding. Let’s dive deep and understand both of the concepts in detail.

Memory Heap

Memory heap is a large region in memory that the JS engine uses to allocate memory to the variable, objects, etc that used in the program.


The above code says that please allocate memory for the number variable and point to the value 1 in our memory heap.

The same goes for the string variable, it says that please allocate memory for the string variable and point to the value “hello world” in our memory heap.

Summary: Memory heap is a region of memory that javascript engine uses to store data/information and it stores it in an unordered manner. You can think of it as a bunch of boxes that have its name to identify it and it is arranged randomly. If you want to know how these boxes or memory of data is arranged for performance enhancement, just forget about it, the Javascript engine handles it all.

Now, I think we are done with Memory Heap, we can jump to Call Stack

Call Stack is a region on the memory that works in last in first out mode. A real-time example will be a stack of stones as shown in the above picture and if you need to add more stones we have to add them at the top to increase the height of the stack and if you want to remove them, we have to remove it from the top. so it works as last in first out principle.

Now let jump into some examples to understand this -

It's a simple program doing some calculations. so here, when we call calculate function the js engine pushes calculate to the call stack. Js engines execute the code line by line and when it encounters minusTwo function, it pushes mintusTwo on top of callstack and after complete execution of minusTwo function call stack returns to calculate function, do the execution of code and we get our result 4 in console

I know it may sound confusing as code is never good to explain by writing, let me attach some screenshots for your reference

In the above attachment, you can observe the whole process, how JS engine is doing the code execution line by line and pushing function to call stack one by one and after executing one function it is poping off from call stack. It’s a awesome thing what can JS engine do to track our code execution.

In the above attachments, you can see the two functions going to the call stack one by one and one and popping out from it after execution of the code.

This is the way it works in most languages — we have call stacks and memory heaps. Now, since the Javascript engine implementations vary, where variables are allocated is not 100% the same all the time.

Simple variables can be stored on the Call Stack and complex data structures such as arrays objects etc are stored in the Memory Heap.

In short, the story is

Javascript engine uses two places to remember things, one is memory heap and another is call stack. Memory Heap is used for memory allocation and Call stack is used to track execution of code line by line.

I tried my best to explain but if you still have any doubts you can connect me through LinkedIn.

The article is inspired by Andrei Neagoie udemy course of Advance Javascript. It is an awesome course and you can check it.

Thank you for your time and have a nice day

A Software Developer and a Fitness enthusiast.