solutions.js

var arr = ["apple", ["apple","fix","apple"],["Apple", "apple"]];


function recurCount([...arr]) {
  const WORD = 'apple';
  
  if (arr.length === 0) {
    return 0;
  } else {
    let tmp = arr.pop();
    if (Array.isArray(tmp)) {
      return recurCount(tmp) + recurCount(arr);
    } else if (tmp === WORD)  {
      return 1 + recurCount(arr);
    } else {
      return 0 + recurCount(arr);
    }
  }
}

//https://stackoverflow.com/questions/5278580/non-recursive-depth-first-search-algorithm
function loopCount(arr) {
  let n = 0;
  let stack = [];
  const WORD = 'apple';
  stack.push(arr);                                                                                                                                                                                              
  
  while(stack.length) {
    let current = stack.pop();
    if (Array.isArray(current)) {
      current.forEach(n => stack.push(n));
    } else if (current === WORD) {
      n++;
    }
  }
  return n;
}

console.log(recurCount(arr));
console.log(loopCount(arr));