Javascript - simple exercises

Question

I have a task to write a function getEvenAverage, which should take only one argument - array. This function should return an average value of even numbers from this array. If in the array there aren't any even numbers the function should return null.

I'd really appreciate any feedback :-)

function getEvenAverage(tab) {
  {
    if (i % 2 === 0) {
      for (var i = 0; i < tab.length; i++) {
        sum += parseInt(tab[i], 10);
      }
      var avg = sum / tab.length;
    } else
      console.log('null');
  }
}

Answer 1

You say you need to return something, so return it. Also move your if statement inside your for loop, and fix a few other syntax errors. And as pointed out in the comments, you should divide sum by the number of even numbers to get your avg:

function getEvenAverage(tab) {
  var sum = 0;
  var evens = 0;
  for (var i = 0; i < tab.length; i++) {
    if (i % 2 === 0) {
      sum += parseInt(tab[i], 10);
      evens++;
    } 
  }
  if (evens == 0) {
    console.log("null");
    return null;
  } else {
    var avg = sum / evens;
    return avg;
  }
}

console.log(getEvenAverage([1, 2, 3]));

Answer 2

You could also do it with the array reduce, with a single array traversal

const reducer = (acc, val) => {
  let {
    sum,
    count
  } = acc;
  return (val % 2 === 0 ? {
    sum: sum + val,
    count: count + 1
  } : acc);
};

const getEvenAverage = (input) => {
  const initialValue = {
    sum: 0,
    count: 0
  };
  const output = input.reduce(reducer, initialValue);

  if (output.count === 0) {
    return null;
  } else {
    return output.sum / output.count;
  }
};

console.log(getEvenAverage([1, 2, 3]));