jQuery; function of $ the use of the semicolon before undefined

;(function($){$.extend($.fn...

1 usually has a semicolon in front of some JQuery functions, which can be used for a variety of purposes:

1. Prevent syntax errors in high compression after multiple files are integrated into one file.

2, this is an anonymous function, 1 js library all use this kind of self-executing anonymous function to protect the internal variables (function(){})().

3. Because undefined is an attribute of window, after declaring it as a local variable, if there is another variable in the function to compare with undefined, the program can not search undefined to window, which can improve the performance of the program.