What is a call stack?

In a nutshell

A call stack is a stack data structure that keeps track of function execution within a program.

When a function is invoked, it gets pushed onto the stack. When it returns, it’s popped off the stack. Here’s a visualization of how a call stack works:

Many programming languages print the call stack when an exception occurs to help you debug issues.

Debugging tools like the Chrome DevTools debugger or PHP’s Xdebug also let you inspect the call stack at any given breakpoint. This can be invaluable in many cases (e.g., you want to understand why your code unexpectedly reaches a particular execution path).