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### Detailed explanation

Immutability is one of the key principles of **pure and predictable functions**.
If a function mutates the data passed to it (an array, an object, etc.), it **affects external state** → it becomes **impure** and **hard to debug**.

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### Example with mutation (not immutable)

```javascript
function addItem(arr, item) {
  arr.push(item); // mutates the original array
  return arr;
}

const numbers = [1, 2, 3];
const result = addItem(numbers, 4);

console.log(numbers); // [1, 2, 3, 4] - the original data changed!
```

This is a **mutable** approach - the function *changes* its arguments.

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### Immutable version

```javascript
function addItem(arr, item) {
  return [...arr, item]; // creates a new array
}

const numbers = [1, 2, 3];
const result = addItem(numbers, 4);

console.log(numbers); // [1, 2, 3] - the original data is preserved
console.log(result);  // [1, 2, 3, 4]
```

The function **did not change the input data**,
it **returned a new copy** - that is **immutability**.

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### Examples of immutable operations

| Data type | Immutable way |
|---|---|
| Array | `map()`, `filter()`, `concat()`, spread (`[...]`) |
| Object | `{ ...obj, key: newValue }` |
| String | Always immutable (strings cannot be changed in JS) |

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### Examples of mutable operations (to avoid in pure functions)

| Data type | Operation |
|---|---|
| Array | `push()`, `pop()`, `splice()`, `sort()`, `reverse()` |
| Object | Direct assignment `obj.key = value` |

---

### Why immutability matters

Makes functions **pure and predictable**
Simplifies **debugging and testing**
Rules out **side effects**
Allows operations to be **safely parallelized**
Makes it easier to **roll back and compare states** (for example, in React, Redux)

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### SUMMARY

| Property | Immutable function |
|---|---|
| Changes input data | No |
| Creates new structures | Yes |
| Side effects | None |
| Predictability | High |
| Examples | `map`, `filter`, `reduce` |

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