javascriptは直ちに関数を実行します。

http://benalman.com/news/2010/11/immediately-invoked-function-expression/
 

// Because this function returns another function that has access to the
// "private" var i, the returned function is, effectively, "privileged."

function makeCounter() {
  // `i` is only accessible inside `makeCounter`.
  var i = 0;

  return function() {
    console.log( ++i );
  };
}

// Note that `counter` and `counter2` each have their own scoped `i`.

var counter = makeCounter();
counter(); // logs: 1
counter(); // logs: 2

var counter2 = makeCounter();
counter2(); // logs: 1
counter2(); // logs: 2

i; // ReferenceError: i is not defined (it only exists inside makeCounter)

 
Immedial tely-Invoked Function Expression(IIIIIIIIIIIIIIIIIFSE)
Fortunatey，the SyntxError「fix」is simple.The most widely accepted way to tell the parser to expect a function expression is just to wrap in parens，because in JavaScript，parens can’t contantintement.Athentis.142it knows to parse it as a function expression and not a function declaration.
 

// 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

 
 
Saving state with closures
Just like when arggments may be passed when functititionarare invoked by their named dentiinininininininininininininininininininininininininininininininininininininininininininininininininininininininininindedededededededentininininininininininininininininininininininininininininininininininininininininininininininininininininininininininininininininininininininininininininininininininininininina closure）an Immediately-Invoked Function Expression can be used to"lock in"values and effectively state.
 

// This doesn't work like you might think, because the value of `i` never
// gets locked in. Instead, every link, when clicked (well after the loop
// has finished executing), alerts the total number of elements, because
// that's what the value of `i` actually is at that point.

var elems = document.getElementsByTagName( 'a' );

for ( var i = 0; i < elems.length; i++ ) {

  elems[ i ].addEventListener( 'click', function(e){
    e.preventDefault();
    alert( 'I am link #' + i );
  }, 'false' );

}

// This works, because inside the IIFE, the value of `i` is locked in as
// `lockedInIndex`. After the loop has finished executing, even though the
// value of `i` is the total number of elements, inside the IIFE the value
// of `lockedInIndex` is whatever the value passed into it (`i`) was when
// the function expression was invoked, so when a link is clicked, the
// correct value is alerted.

var elems = document.getElementsByTagName( 'a' );

for ( var i = 0; i < elems.length; i++ ) {

  (function( lockedInIndex ){

    elems[ i ].addEventListener( 'click', function(e){
      e.preventDefault();
      alert( 'I am link #' + lockedInIndex );
    }, 'false' );

  })( i );

}

// You could also use an IIFE like this, encompassing (and returning) only
// the click handler function, and not the entire `addEventListener`
// assignment. Either way, while both examples lock in the value using an
// IIFE, I find the previous example to be more readable.

var elems = document.getElementsByTagName( 'a' );

for ( var i = 0; i < elems.length; i++ ) {

  elems[ i ].addEventListener( 'click', (function( lockedInIndex ){
    return function(e){
      e.preventDefault();
      alert( 'I am link #' + lockedInIndex );
    };
  })( i ), 'false' );

}