FibonacciNumber.js

/**
 * @function fibonacci
 * @description Fibonacci is the sum of previous two fibonacci numbers.
 * @param {Integer} N - The input integer
 * @return {Integer} fibonacci of N.
 * @see [Fibonacci_Numbers](https://en.wikipedia.org/wiki/Fibonacci_number)
 */
const fibonacci = (N) => {
 if (!Number.isInteger(N)) {
 throw new TypeError('Input should be integer')
 }

 // memoize the last two numbers
 let firstNumber = 0
 let secondNumber = 1

 for (let i = 1; i < N; i++) {
 const sumOfNumbers = firstNumber + secondNumber
 // update last two numbers
 firstNumber = secondNumber
 secondNumber = sumOfNumbers
 }

 return N ? secondNumber : firstNumber
}

export { fibonacci }