This is my reference when phone screening.

fibonacchi-recursive.js

var fibonacci = function (position) {
  return position < 1 ? 0 :
    position <= 2 ? 1 :
      fibonacci(position - 1) + fibonacci(position - 2);
};

fibonacci-basic.js

var fibonacci = function (position) {
  var previous = 0,
    tmpCurrent,
    current = (position > 0 ? 1 : 0);

  for (var i = 1; i < position; ++i) {
    tmpCurrent = current;
    current += previous;
    previous = tmpCurrent;
  }
  return current;
};

fibonacciTest.js

(function fibonacciTest() {
  var tests = [0, 0, 0, 1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89], // This array will be shifted from 0 to -2.
    len = tests.length,
    i = 0,
    answerA, answerB, prefixMessage;

  for (; i < len; ++i) {
    answerA = fibonacci(i - 2);
    answerB = tests[i];

    prefixMessage = ("fibonacci(" + (i - 2) + ") = " + answerA);

    if (answerA === answerB) {
      console.log(prefixMessage, "✓");
    } else {
      console.error(prefixMessage, "✗ - It should be: " + answerB);
      return false;
    }
  }
  return true;
}());