curriculum/challenges/english/blocks/project-euler-problems-1-to-100/5900f3851000cf542c50fe98.md
The Fibonacci sequence is defined by the recurrence relation:
<div style='padding-left: 4em;'>F<sub>n</sub> = F<sub>n−1</sub> + F<sub>n−2</sub>, where F<sub>1</sub> = 1 and F<sub>2</sub> = 1.</div>Hence the first 12 terms will be:
<div style='padding-left: 4em; display: inline-grid; grid-template-rows: auto; row-gap: 7px;'><div>F<sub>1</sub> = 1</div><div>F<sub>2</sub> = 1</div><div>F<sub>3</sub> = 2</div><div>F<sub>4</sub> = 3</div><div>F<sub>5</sub> = 5</div><div>F<sub>6</sub> = 8</div><div>F<sub>7</sub> = 13</div><div>F<sub>8</sub> = 21</div><div>F<sub>9</sub> = 34</div><div>F<sub>10</sub> = 55</div><div>F<sub>11</sub> = 89</div><div>F<sub>12</sub> = 144</div></div>The 12th term, F<sub>12</sub>, is the first term to contain three digits.
What is the index of the first term in the Fibonacci sequence to contain n digits?
digitFibonacci(5) should return a number.
assert.isNumber(digitFibonacci(5));
digitFibonacci(5) should return 21.
assert.strictEqual(digitFibonacci(5), 21);
digitFibonacci(10) should return 45.
assert.strictEqual(digitFibonacci(10), 45);
digitFibonacci(15) should return 69.
assert.strictEqual(digitFibonacci(15), 69);
digitFibonacci(20) should return 93.
assert.strictEqual(digitFibonacci(20), 93);
function digitFibonacci(n) {
return n;
}
digitFibonacci(20);
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++;
}
};