Find the sequence of a given Fibonacci number.

findfibonaccisequence.js

/*
  Say you are given a number from Fibonacci series - 13, find its sequence number
  Input - 13, Output - 7
  Input - 21, Output - 8
*/

function findFibSeq(num) {
	var a = 1, b = 1, sum = 0, n = 2;
	
  //continue to run until sum and provided number is not equal
	while(sum !== num) {
		if(sum > num) { 
			return -1; //given number is not a number from fibonacci series eg. 15
		}
		sum = a + b;
		a = b;
		b = sum;
		n++; 
	}
	return n;  //return the index once found
}