---
id: 5900f3851000cf542c50fe98
title: 'Problem 25: 1000-digit Fibonacci number'
challengeType: 5
forumTopicId: 301897
dashedName: problem-25-1000-digit-fibonacci-number
---
# --description--
The Fibonacci sequence is defined by the recurrence relation:
Fn = Fn−1 + Fn−2, where F1 = 1 and F2 = 1.
Hence the first 12 terms will be:
F1 = 1
F2 = 1
F3 = 2
F4 = 3
F5 = 5
F6 = 8
F7 = 13
F8 = 21
F9 = 34
F10 = 55
F11 = 89
F12 = 144
The 12th term, F12, is the first term to contain three digits.
What is the index of the first term in the Fibonacci sequence to contain `n` digits?
# --hints--
`digitFibonacci(5)` should return a number.
```js
assert(typeof digitFibonacci(5) === 'number');
```
`digitFibonacci(5)` should return 21.
```js
assert.strictEqual(digitFibonacci(5), 21);
```
`digitFibonacci(10)` should return 45.
```js
assert.strictEqual(digitFibonacci(10), 45);
```
`digitFibonacci(15)` should return 69.
```js
assert.strictEqual(digitFibonacci(15), 69);
```
`digitFibonacci(20)` should return 93.
```js
assert.strictEqual(digitFibonacci(20), 93);
```
# --seed--
## --seed-contents--
```js
function digitFibonacci(n) {
return n;
}
digitFibonacci(20);
```
# --solutions--
```js
const digitFibonacci = (n) => {
const digits = (num) => {
return num.toString().length;
};
let f1 = 1;
let f2 = 1;
let index = 3;
while (true) {
let fn = f1 + f2;
if (digits(fn) === n) return index;
[f1, f2] = [f2, fn];
index++;
}
};
```