The +function() {} notation is primarily used to force the parser to treat whatever follows the + as an expression. This is used for functions that are invoked immediately, for example,
+function() { alert("Demo!"); }();However, + before a function is one of the symbol. You can add other options also like !, -, ~, also.
Parentheses can also be used as shown below:
(function() { alert("Demo!"); })();You can also use it like this:
(function() { alert("Demo!"); }());