In Figure 1 below, when function **A** ① calls function **B** ②, which in turn calls function **C** ③, the JavaScript engine must keep track where function **B** was at the moment that **C** was called, so that it can continue executing **B** when function **C** returns ④ . The same applies to function **A** when function **B** returns ⑤.

For this purpose the JavaScript engine maintains a [call stack](https://developer.mozilla.org/en-US/docs/Glossary/Call_stack) as shown in Figure 1, The call stack is a first-in, first-out data structure. When a function is called it is pushed onto the call stack and when it returns it is popped of the call stack.