025.js

/*
Problem 25: 1000-digit Fibonacci number
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?
*/

const digitFibonacci = n => {
  const fibs = [1, 1];
  while (fibs[fibs.length - 1].toString().length < n) {
    fibs.push(fibs[fibs.length - 2] + fibs[fibs.length - 1]);
  }
  return fibs.length;
}

console.log(digitFibonacci(20));