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The **call stack** is a data structure that the JavaScript engine uses to track the execution of functions in a program.

### How it works

While executing code, JavaScript needs to know:

- **which function is running right now**,
- **where it was called from**,
- **where to return after it finishes**.

For this it uses the **call stack** - a structure that works on the **LIFO** principle (Last In - First Out).

### What this looks like in practice

Example:

```javascript
function a() {
  console.log('a');
  b();
}

function b() {
  console.log('b');
  c();
}

function c() {
  console.log('c');
}

a();
```

#### Step by step:

1. `a()` is called -> pushed onto the stack.
**Stack:** `[a]`
2. `a()` calls `b()` -> `b()` is added.
**Stack:** `[a, b]`
3. `b()` calls `c()` -> `c()` is added.
**Stack:** `[a, b, c]`
4. `c()` finishes -> it is removed.
**Stack:** `[a, b]`
5. `b()` finishes -> it is removed.
**Stack:** `[a]`
6. `a()` finishes -> it is removed.
**Stack:** `[]` (empty)

### What happens with infinite recursion

If a function calls itself with no exit:

```javascript
function recurse() {
  recurse();
}

recurse();
```

Every call is added to the stack, but is never removed.
In the end, a **"Stack overflow"** error occurs, meaning the call stack has overflowed.

### Relation to the Event Loop

JavaScript is **single-threaded**, meaning that at any given moment only **one function from the stack** is executing.
When the stack is empty, the engine can take the next task from the **callback queue** and start a new Event Loop cycle.

### Summary

| Term | Description |
| --- | --- |
| **Call Stack** | Where the engine stores the execution context of functions |
| **LIFO** | Last called - first finished |
| **Stack Overflow** | Error when the stack overflows |
| **Relation to Event Loop** | When the stack is empty, callbacks and promises are processed |

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