JS for(in) - use with JS objects

gistfile1.txt

for(in) loops through the enumerable properties of an object, not the indexes of an array

console.log('The primary use case for for(in) is to enumerate the properties of an object and will result in unexpected results if used with an Array');
  console.log('for(in) gives you the index positions of the items if used in the array and not an objecct:');
  console.log('the results below are from using a for(in) with var myNewArray = ["a", "b", "c", "d"]');
  
  var myNewArray = ["a", "b", "c", "d"];

  for (var i in myNewArray) {
    console.log('i', i);
  };
  // prints 0 1 2 3
  // Which is generally unexpected. The primary use case for for(in) is to enumerate the properties of an object:

  console.log('\nbut if used on the object: var o = { x: 10, y: 20 }; you get:');
  var o = { x: 10, y: 20 };
  for (var i in o) {
    console.log('property(s) of object o:', i);
  };
  // prints x y

  console.log('\nin general, it\'s recommended that you use Object.keys to get the set of keys for an object, and then use the normal array enumeration:');
  console.log('Object.keys(o).forEach...');
  var o = { x: 10, y: 20 };
  Object.keys(o).forEach(function (keyName) {
    console.log('property(s) of object o:', keyName);
  });
// prints x y