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**In short:**
A **stateless function** is a function that **does not store or change internal data between calls**.
It **always behaves the same way** for the same set of input arguments.

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### Detailed explanation

A "stateless" function remembers nothing about past calls and does not depend on external changes.
Every call is an **isolated computation**, fully determined by its input parameters.

That is, such a function:

- has no **internal memory** (no counters, caches, flags);
- does not use **global variables**;
- does not change anything outside itself (the DOM, files, network requests).

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### Example of a stateless function

```javascript
function sum(a, b) {
  return a + b;
}

console.log(sum(2, 3)); // 5
console.log(sum(2, 3)); // 5 - always the same
```

Each call is fully independent - the result depends only on the input data.

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### Example of a function *with* state

```javascript
let counter = 0;

function increment() {
  counter++;
  return counter;
}

increment(); // 1
increment(); // 2 - the result depends on the previous call
```

Here the function "remembers" state (`counter`),
so it is **stateful**.

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### Where stateless functions matter

- In **functional programming** - the foundation of pure computations.
- In **React** (for example, "stateless components") - components that simply render data without their own state.
- In **testable code** - predictable results without dependence on the environment.

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### SUMMARY

| Trait | Stateless function |
|---|---|
| Stores data between calls | No |
| Changes external state | No |
| Depends only on arguments | Yes |
| Always returns the same result for the same input | Yes |
| Example | `Math.max(a, b)` |

---

**In one phrase:**

> A stateless function is a function that has **no "memory" of past calls**,
> and its result always depends **only on the input data**, not on context.

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