• Array(5) gives you an array with length 5 but no values, hence you can't iterate over it.

• Array.apply(null, Array(5)).map(function () {}) gives you an array with length 5 and undefined as values, now it can be iterated over.

• Array.apply(null, Array(5)).map(function (x, i) { return i; }) gives you an array with length 5 and values 0,1,2,3,4.

• Array(5).forEach(alert) does nothing, Array.apply(null, Array(5)).forEach(alert) gives you 5 alerts

• ES6 gives us Array.from so now you can also use Array.from(Array(5)).forEach(alert)

• If you want to initialize with a certain value, these are good to knows...
  Array.from('abcde'), Array.from('x'.repeat(5))
  or Array.from({length: 5}, (v, i) => i) // gives [0, 1, 2, 3, 4]

Array.size() is not a valid method

Always use the length property

There is a library or script adding the size method to the array prototype since this is not a native array method. This is commonly done to add support for a custom getter. An example of using this would be when you want to get the size in memory of an array (which is the only thing I can think of that would be useful for this name).

Underscore.js unfortunately defines a size method which actually returns the length of an object or array. Since unfortunately the length property of a function is defined as the number of named arguments the function declares they had to use an alternative and size was chosen (count would have been a better choice).