Call Stack and Execution Context

AmanGupta1703 — Thu, 26 Jun 2025 13:33:52 +0000

What is a Callstack?

It is part of the JavaScript engine.
A part of the JavaScript engine that keeps track of function calls.
It is a data structure that operates on a LIFO (Last-In-First-Out) basis to remove an execution context from the Call Stack.
- The first item pushed to the stack is the Global Execution Context. Each time a function is invoked, a new Function Execution Context is added (pushed) on top.
Once a function completes, its context is popped off the stack.

It is the environment in which JavaScript code is evaluated and executed.
It goes through two main phases:
1. Creation Phase
2. Execution Phase
The engine scans the code and allocates memory for variables and functions.
Functions are hoisted with their full definitions.
Variables declared with var are hoisted and initialised with undefined.
Variables declared with let and const are hoisted but remain uninitialized — they are in the Temporal Dead Zone (TDZ).
Primitive values are stored in the Call Stack; reference values (like objects) are stored in the Heap.

The code is executed line by line.
Variables declared with var, let, and const are assigned their actual values.
Accessing a let or const variable before its declaration results in a ReferenceError due to the TDZ.

The TDZ for a let or const variable starts at the beginning of its scope and ends when the variable is declared.
Accessing the variable in this zone will throw a ReferenceError.