freeCodeCamp/curriculum/challenges/english/08-coding-interview-prep/rosetta-code/fibonacci-sequence.english.md

title	id	challengeType
Fibonacci sequence	597f24c1dda4e70f53c79c81	5

Description

Write a function to generate the n^th Fibonacci number.

///

The n^th Fibonacci number is given by : ///

F_n = F_n-1 + F_n-2

///

The first two terms of the series are 0, 1.

///

Hence, the series is : 0, 1, 1, 2, 3, 5, 8, 13...

///

Instructions

Tests

tests:
  - text: <code>fibonacci</code> is a function.
    testString: assert(typeof fibonacci === 'function', '<code>fibonacci</code> is a function.');
  - text: <code>fibonacci(2)</code> should return a number.
    testString: assert(typeof fibonacci(2) == 'number', '<code>fibonacci(2)</code> should return a number.');
  - text: <code>fibonacci(3)</code> should return 1.")
    testString: assert.equal(fibonacci(3),1,"<code>fibonacci(3)</code> should return 1.");
  - text: <code>fibonacci(5)</code> should return 3.")
    testString: assert.equal(fibonacci(5),3,"<code>fibonacci(5)</code> should return 3.");
  - text: <code>fibonacci(10)</code> should return 34.")
    testString: assert.equal(fibonacci(10),34,"<code>fibonacci(10)</code> should return 34.");

Challenge Seed

function fibonacci(n) {
  // Good luck!
}

Solution

function fibonacci(n) {
  let a = 0, b = 1, t;
  while (--n > 0) {
    t = a;
    a = b;
    b += t;
  }
  return a;
}