# JavaScript Using generator function for fibonacci sequence

Here's a nice use-case for *generator* functions.

The following is an implementation of *fibonacci sequence* of arbitrary size (see https://jsfiddle.net/cj83gkno/). Generator function makes this really easy.

```js
/*
Fibonacci sequence of arbitrary length
0, 1, 1, 2, 3, 5, 8, 13, ...
*/

function* fibonacci() {
	// we need to store both current and next
  let current=0;
  let next=1;
  
  // infinite loop to generate yields
  while (true) {
  	// first extract optional value to next(v)
    // as you will see lalter, this has a function.
    // also, yield current
	const v=yield current;
    
    [current, next]=[next, next+current];
    
    // here's why
    // if next(true) is passed then reset sequence.
    if (v) {
      current=0;
      next=1;
    }
  }
}

// let's run it
const seq=fibonacci();
console.log(seq.next());
console.log(seq.next());
console.log(seq.next());
console.log(seq.next());
console.log(seq.next());
console.log(seq.next());
console.log(seq.next());

// reset seq
console.log(seq.next(true));
console.log(seq.next());
console.log(seq.next());
```

Output
```
{
  done: false,
  value: 0
}
{
  done: false,
  value: 1
}
{
  done: false,
  value: 1
}
{
  done: false,
  value: 2
}
{
  done: false,
  value: 3
}
{
  done: false,
  value: 5
}
{
  done: false,
  value: 8
}
{
  done: false,
  value: 0
}
{
  done: false,
  value: 1
}
{
  done: false,
  value: 1
}
```
Firstly, whenever the code execution reaches `yield` it is paused there until `next()` is called.

Particular note on `const v=yield current;`. Here `v` is the value passed to `next(v)` and even if the old value of `v` is already *bound* when execution is paused, it gets updated to the new `v` passed to `next(v)` when execution resumes. So, in `if (v) {...}`, `v` is not the old value but the new one passed fresh to `next(v)`.