JavaScript: HackerRank Apples and Oranges Tree falling by the house problem solved

JSHRApplesAndOrangesFallingSolved.js

// my solution using easy to refactor methods (in case you wanted to either share logic or compare new types of fruit)
function countApplesAndOranges(s, t, a, b, apples, oranges) {
    const fLoc = function (treeLoc, arr2d) {
       return arr2d.map(fruitLoc => (treeLoc + fruitLoc));
    }
    const fRange = function (s, t, arr2d) {
        let a, b;
        a = 0; b = 0;
        arr2d.forEach((f, i) => {
            if (i === 0) { // apple count
                f.forEach(loc => s <= loc && loc <= t ? a++ : null);
            }
            if (i === 1) { // orange count
                f.forEach(loc => s <= loc && loc <= t ? b++ : null);
            }
        });
        return [a, b];
    }
    console.log(fRange(s, t, [fLoc(a, apples), fLoc(b, oranges)]).join('\n'));
}

// shorter solutions...

// using filter
function countApplesAndOranges2(s, t, a, b, apples, oranges) {
  console.log(apples.filter(d => s - a <= d && d <= t - a).length);
  console.log(oranges.filter(d => s - b <= d && d <= t - b).length);
}

// using reduce
function countApplesAndOranges3(s, t, a, b, apples, oranges) {
  console.log( apple.reduce((sum, d) => sum + (s - a <= d && d <= t - a), 0));
  console.log(orange.reduce((sum, d) => sum + (s - b <= d && d <= t - b), 0));
}

// using filter and map
function countApplesAndOranges4 {
  function addBy(num) {
    return (d) => d + num;
  }
  function isScored(d) {
    return s <= d && d <= t;
  }
  const larry = apples.map(addBy(a)).filter(isScored).length;
  const rob = oranges.map(addBy(b)).filter(isScored).length;
  console.log(larry, rob);
}

// using Array.prototype (bad because a new version of ECMA could come out with a 'sum' helper for arrays)
function countApplesAndOranges5 {
  Array.prototype.sum = function(f) {
      return this.reduce((s, v) => s + f(v), 0);
  }
  console.log( apple.sum(d => s - a <= d && d <= t - a));
  console.log(orange.sum(d => s - b <= d && d <= t - b));
}

how you are getting i === 0 & i===1 count ?

how you are getting i === 0 & i===1 count ?

how you are getting i === 0 & i===1 count ?

i is the index of the 2d f 'fruit' array containing the apples and oranges data
The index of the fruit array will be 0 for apple data since it is assumed that apple data will always come first
The index of the fruit array will be 1 for orange data since it is assumes apple data will always come second

        arr2d.forEach((f, i) => { // f stands for fruit
            if (i === 0) { // apple count
                f.forEach(loc => s <= loc && loc <= t ? a++ : null);
            }
            if (i === 1) { // orange count
                f.forEach(loc => s <= loc && loc <= t ? b++ : null);
            }
        });