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# Question 35

### Question

How would you use generators to create a custom iterator?

### Answer

Generators are perfect for writing custom iterators in JavaScript.

Here's how you would use them to build your own iterator:

```javascript
function* fibonacci() {
  let a = 0;
  let b = 1;
  while (true) { // Infinite loop
    yield a;          // Yield the current Fibonacci number
    [a, b] = [b, a + b]; // Update for the next iteration
  }
}

const fibGenerator = fibonacci();

console.log(fibGenerator.next()); // Output: { value: 0, done: false }
console.log(fibGenerator.next()); // Output: { value: 1, done: false }
console.log(fibGenerator.next()); // Output: { value: 1, done: false }
console.log(fibGenerator.next()); // Output: { value: 2, done: false }
// ... and so on
```

**Explanation:**

1. **`function* fibonacci()`:** The key is using `function*`. This declares a generator function.
2. **Variables:** We initialize variables `a` and `b` to start the Fibonacci sequence (0 and 1).
3. **`while (true)` Loop:** Generates an infinite loop for continuous Fibonacci numbers.
4. **`yield a;`:** The `yield` keyword pauses execution and "yields" the current value of `a`. When you call `next()` on the generator, it resumes from this point, giving you the yielded value.
5. **`[a, b] = [b, a + b];`:** Updates `a` and `b` for the next Fibonacci number in the sequence.

**Using the Generator:**

* Create an instance of your generator: `const fibGenerator = fibonacci();`.
* Call `next()` repeatedly to get each value from the sequence.
  * `next()` returns an object with `value` (the yielded number) and `done` (true when the sequence is exhausted).

**Benefits:**

* **Memory Efficiency:** Generators don't calculate all Fibonacci numbers at once; they generate them on demand, saving memory.
* **Readability:** The code clearly shows the logic for generating the sequence.
* **Lazy Evaluation:** Values are produced only when needed.