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**The `break` keyword in JavaScript is used to stop a loop or a `switch` block earlier than it would finish naturally.** Control immediately moves to the next line after the construct that was interrupted.

## Theory

### TL;DR
- `break` immediately ends the nearest enclosing loop or `switch` block.
- In loops it is used to stop iterating once the result has been found.
- In a `switch` it prevents execution from falling through into the following `case` clauses.
- `break` may only appear inside a loop or a `switch`, otherwise you get a `SyntaxError`.
- To skip a single iteration instead of ending the loop, use `continue`.
- With a label, `break label` exits the outer loop rather than the inner one.

### Quick example
```javascript
for (let i = 0; i < 10; i++) {
  if (i === 5) {
    break; // stop the loop when i is 5
  }
  console.log(i);
}

console.log('Loop finished');
```

Result:

```text
0
1
2
3
4
Loop finished
```

The loop stops as soon as `i` becomes `5`, and the remaining iterations never run.

### `break` in `for`, `while` and `do...while` loops
`break` ends the loop immediately and passes control to the next line of code after it. It is the main tool for searching: once the element you need is found, there is no point in scanning the rest.

```javascript
const users = [
  { id: 1, name: 'Alice' },
  { id: 2, name: 'Bob' },
  { id: 3, name: 'Maria' },
];

let found = null;

for (const user of users) {
  if (user.id === 2) {
    found = user;
    break; // no reason to keep scanning
  }
}

console.log(found); // { id: 2, name: 'Bob' }
```

The same works in `while` and `do...while`. One important detail: `break` leaves only one loop, the nearest one, so in nested loops it stops the inner loop only.

### `break` in a `switch`
Here `break` is used to leave the `switch` block after the required `case` has run. Without it, execution falls through into the next `case`.

```javascript
const day = 2;

switch (day) {
  case 1:
    console.log('Monday');
    break;
  case 2:
    console.log('Tuesday');
    break;
  case 3:
    console.log('Wednesday');
    break;
  default:
    console.log('Another day');
}
```

Result:

```text
Tuesday
```

If `case 2` had no stop in it, every following `case` would run, up to the nearest `break` or the end of the block.

### Labelled `break` in nested loops
When you need to leave several levels of nesting at once, label the loop and break out of that label.

```javascript
outer: for (let i = 0; i < 3; i++) {
  for (let j = 0; j < 3; j++) {
    if (i * j === 2) break outer; // leave both loops at once
    console.log(i, j);
  }
}
```

Result:

```text
0 0
0 1
0 2
1 0
1 1
```

An alternative to a label is extracting the nested loops into a separate function and leaving it with `return`, which is often easier to read.

### Comparison with `continue`

| Keyword | What it does | Where it works |
| --- | --- | --- |
| `break` | Stops the loop completely, or exits a `switch` | Loops, `switch` |
| `continue` | Skips the rest of the current iteration | Loops only |

### Common mistakes
- Using `break` inside `forEach`, `map` or `filter`. These are methods, not loops, so it is a `SyntaxError`; for an early exit use `for...of`, `some` or `find`.
- Forgetting `break` in a `switch` branch and getting fall-through, that is, several branches running instead of one.
- Assuming `break` in nested loops leaves all of them. It leaves only the nearest one; anything further needs a label.
- Confusing `break` with `continue`: the first ends the loop, the second only skips a step.
- Placing `break` inside an `if` block that is not in a loop. Outside a loop or a `switch` it is a syntax error.
- Expecting `break` to cancel a `finally` block: code in `try ... finally` still runs before control leaves.

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