How do I filter out null entries in an array of objects?

Question

I'm just learning JavaScript, and I was trying to understand this example of filter() on Mozilla , but I don't get how this example works, because when I try to do the same thing on my example, it just returns an empty array.

Here's the example from the link:

var arr = [
  { id: 15 },
  { id: -1 },
  { id: 0 },
  { id: 3 },
  { id: 12.2 },
  { },
  { id: null },
  { id: NaN },
  { id: 'undefined' }
];

var invalidEntries = 0;

function filterByID(obj) {
  if ('id' in obj && typeof(obj.id) === 'number' && !isNaN(obj.id)) {
    return true; // <--- WHY? 
  } else {
    invalidEntries++;
    return false;
  }
}

var arrByID = arr.filter(filterByID);

console.log('Filtered Array\n', arrByID); 
// Filtered Array
// [{ id: 15 }, { id: -1 }, { id: 0 }, { id: 3 }, { id: 12.2 }]

console.log('Number of Invalid Entries = ', invalidEntries); 
// 4

This is my example:

obj = [{0: 31}, {1: null}, {2: null}, {3, 28}]

function isNotNull(obj) {
    for (var prop in obj) {
        if (obj.prop != null) {
            return true;
        }
    } 
}

console.log(ages.filter(isNotNull)) // -> []

Answer 1

To quote the page you linked:

callback Function to test each element of the array. Invoked with arguments (element, index, array). Return true to keep the element, false otherwise.

Answer 2

true as return value means the object will be accepted in the result set. false means it is considered invalid.

Answer 3

you need to use array syntax to dynamically lookup properties:

ages = [{0: 31}, {1: null}, {2: null}, {3: 28}]

    function isNotNull(obj) {
        for (var prop in obj) {
            if (obj[prop] != null) {
                return true;
            }
        } 
    }

    console.log(ages.filter(isNotNull)) // -> [{"0":31},{"3":28}]

note the obj[prop] instead of obj.prop