js 的自执行

// Either of the following two patterns can be used to immediately invoke
  // a function expression, utilizing the function's execution context to
  // create "privacy."

  (function(){ /* code */ }()); // Crockford recommends this one
  (function(){ /* code */ })(); // But this one works just as well

  // Because the point of the parens or coercing operators is to disambiguate
  // between function expressions and function declarations, they can be
  // omitted when the parser already expects an expression (but please see the
  // "important note" below).

  var i = function(){ return 10; }();
  true && function(){ /* code */ }();
  0, function(){ /* code */ }();

  // If you don't care about the return value, or the possibility of making
  // your code slightly harder to read, you can save a byte by just prefixing
  // the function with a unary operator.

  !function(){ /* code */ }();
  ~function(){ /* code */ }();
  -function(){ /* code */ }();
  +function(){ /* code */ }();

  // Here's another variation, from @kuvos - I'm not sure of the performance
  // implications, if any, of using the `new` keyword, but it works.
  // http://twitter.com/kuvos/status/18209252090847232

  new function(){ /* code */ }
  new function(){ /* code */ }() // Only need parens if passing arguments