The use of semicolons in the front of a function ($,undefined) in jquery _jquery

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

In general, there are semicolons in front of some JQuery functions, preceded by a semicolon can be used for many purposes:

1, to prevent the integration of multiple files into a file, high compression appears syntax errors.

2, this is an anonymous function, the General JS Library is the implementation of this anonymous function to protect the internal variables (function () {}) ().

3, because undefined is a window property, declared as a local variable, in the function if there are variables and undefined to compare, the program can not search undefined to window, can improve program performance.