JavaScript String concatenation behavior with null or undefined values

Question

As you may know, in JavaScript \'\' + null = \"null\" and \'\' + undefined = \"undefined\" (in most browsers I can test: Firefox, Chrome and IE). I

Answer 1

To add null and '' they need to meet a minimum common type criterium which in this case is a string type.

null is converted to "null" for this reason and as they are string the two are concatenated.

The same happens with numbers:

4 + '' = '4'

as there is a string in there which can't be converted to any number, so the 4 will be converted to string instead.