Functional programming

functional.js

// the functional programming version
const find2 = (f => f(f))(f =>  
  (next => (x, y, i = 0) =>  
    (i >= x.length) ? null : 
      (x[i] === y) ? i : 
        next(x, y, i+1))((...args) =>  
          (f(f))(...args)));

let arr = [0, 1, 2, 3];
console.log(find2(arr, 2));
console.log(find2(arr, 5));

iteration.js

function find3 (arr, x) {
  function $find3 (arr, x, i) {
    if (i >= array.length) {
      return -1;
    }
    if (arr[i] === x) {
      return i;
    }
    return $find3 (arr, x, i + 1);
  }
}

var arr = [0, 1, 2, 3];
console.log(find3(arr, 2));
console.log(find3(arr, 5));

traditional.js

let arr = [0, 1, 2, 3]

// find the first value in array x that equals to y
// return index, or null if not found
const find1 = (x, y) => {
  for (let i = 0; i < x.length; i++) {
    if (x[i] === y) {
      return i;
    }
  }
  return null;
}

console.log(find1(arr, 2));
console.log(find1(arr, 5));