freeCodeCamp/curriculum/challenges/en/08-coding-interview-prep/rosetta-code/fibonacci-sequence.en.md

---
title: Fibonacci sequence
id: 597f24c1dda4e70f53c79c81
challengeType: 5
---

## Description
<section id='description'>
<p>Write a function to generate the  <big> n<sup>th</sup> </big>  Fibonacci number.</p>
///<p>The <big> n<sup>th</sup> </big>  Fibonacci number is given by :
///<p>F<sub>n</sub> = F<sub>n-1</sub> + F<sub>n-2</sub></p>
///<p>The first two terms of the series are 0, 1.</p>
///<p>Hence, the series is : 0, 1, 1, 2, 3, 5, 8, 13...</p>
///
</section>

## Instructions
<section id='instructions'>

</section>

## Tests
<section id='tests'>

```yml
- text: <code>fibonacci</code> is a function.
  testString: 'assert(typeof fibonacci === ''function'', ''<code>fibonacci</code> is a function.'');'
- text: <code>fibonacci(2)</code> should return a number.
  testString: 'assert(typeof fibonacci(2) == ''number'', ''<code>fibonacci(2)</code> should return a number.'');'
- text: <code>fibonacci(3)</code> should return 1.")
  testString: 'assert.equal(fibonacci(3),1,"<code>fibonacci(3)</code> should return 1.");'
- text: <code>fibonacci(5)</code> should return 3.")
  testString: 'assert.equal(fibonacci(5),3,"<code>fibonacci(5)</code> should return 3.");'
- text: <code>fibonacci(10)</code> should return 34.")
  testString: 'assert.equal(fibonacci(10),34,"<code>fibonacci(10)</code> should return 34.");'

```

</section>

## Challenge Seed
<section id='challengeSeed'>

<div id='js-seed'>

```js
function fibonacci(n) {
  // Good luck!
}
```

</div>



</section>

## Solution
<section id='solution'>


```js
function fibonacci(n) {
  let a = 0, b = 1, t;
  while (--n > 0) {
    t = a;
    a = b;
    b += t;
  }
  return a;
}

```

</section>