קסם JavaScript

// Imperative: The Fastest ( + very simple, no new pointers or excess allocs ):
function fib(n) {
  var a = 1,
    b = 1,
    c = 0;
  for (var i = 1; i < n - 1; ++i) {
    c = a + b;
    a = b;
    b = c;
  }
  return b;
}

// Recursive: (FIREFOX or BABELJS Only) ES6 function definition with
//  parameter defaults used to set initial (internal/recursive) values
function fib(n, current = 0, a = 1, b = 1, c = 0) {
  current++;
  c = a + b;
  a = b;
  b = c;
  return current >= n ? b : fib(n, current, a, b, c);
}

// Text-book-Bad Example - poor function scope w/ multiple mutable external values
function fib(n) {
  if (!arr) {
    var arr = [1, 1];
    n = n - 2;
  } // Bad
  if (n === -1) {
    return [arr[0]];
  }
  if (n === 0) {
    return arr;
  }
  var proc = function() {
    --n;
    arr.push(arr[arr.length - 1] + arr[arr.length - 2]);
    return n === 0 ? arr : proc();
    // Bad: inner recursive function not needed, hint: variables used are from parent function scope
  };
  var ans = proc();
  return ans[ans.length - 1];
}

// Example Using bluebird Promises and it's
var Promise = require("bluebird"),
  fs = Promise.promisifyAll(require("fs")),
  less = Promise.promisifyAll(require("less"));

function writeFileData(data) {
  return fs.writeFileAsync("/tmp/output.css", data);
}
// Bluebird makes something like this perhaps uncomfortably simple and succinct:
fs.readFileAsync("./style.less") // Call promisified readFile()
  .then(less.renderAsync) // Hand off to less.render
  .then(writeFileData); // Function to recieve the css contents (1st parameter)