Can you explain why the commas become the member of new array that is result of the sum of two arrays?

Question

var array1 = [1,2];
var array2 = [3,4];
var newArray = array1 + array2;

console.log(newArray.length);
The result is 6

Answer 1

Using the + operator to sum arrays most probably coerces the arrays into string and includes the commas between the elements in the string representation thats why it has a larger length.
If you check the typeof the newArray you'll find it's a string. A better way is to use arr.concat(new).

Answer 2

You cannot use the + operator for arrays. This operator converts arrays into strings and concatenates them.

Use concat:

[1, 2].concat([3, 4])

Answer 3

var array1 = [1,2];
var array2 = [3,4];
var newArray = [...array1, ...array2]; // I like do that
// concat also works...

array1 + array2 // This is simple join, "1,23,4" and have 6 characters.