JS闭包原理与应用经典示例

function foo() {
   var a = 10;
   function bar() {
    a *= 2;
    return a;
   }
   return bar;
}
var baz = foo(); // baz is now a reference to function bar.
console.log(baz()); // returns 20.
console.log(baz()); // returns 40.
console.log(baz()); // returns 80.
var blat = foo(); // blat is another reference to bar.
console.log(blat()); // returns 20, because a new copy of a is being used.

var Book = function(newIsbn, newTitle, newAuthor) { // implements Publication
 // Private attributes.
 var isbn, title, author;
 // Private method.
 function checkIsbn(isbn) {
  //...
  return true;
 }
 // Privileged methods.
 this.getIsbn = function() {
  return isbn;
 };
 this.setIsbn = function(newIsbn) {
  if(!checkIsbn(newIsbn)) throw new Error('Book: Invalid ISBN.');
  isbn = newIsbn;
 };
 this.getTitle = function() {
  return title;
 };
 this.setTitle = function(newTitle) {
  title = newTitle || 'No title specified';
 };
 this.getAuthor = function() {
  return author;
 };
 this.setAuthor = function(newAuthor) {
  author = newAuthor || 'No author specified';
 };
 // Constructor code.
 this.setIsbn(newIsbn);
 this.setTitle(newTitle);
 this.setAuthor(newAuthor);
};
// Public, non-privileged methods.
Book.prototype = {
 display: function() {
  //...
 }
};
var mybook=new Book("myisbtn","mytittle","myauthor");
console.log(mybook.getAuthor());

var Book = (function() {
 // Private static attributes.
 var numOfBooks = 0;
 // Private static method.
 function checkIsbn(isbn) {
  // ...
  return true;
 }
 // Return the constructor.
 return function(newIsbn, newTitle, newAuthor) { // implements Publication
  // Private attributes.
  var isbn, title, author;
  // Privileged methods.
  this.getIsbn = function() {
   return isbn;
  };
  this.setIsbn = function(newIsbn) {
   if(!checkIsbn(newIsbn)) throw new Error('Book: Invalid ISBN.');
   isbn = newIsbn;
  };
  this.getTitle = function() {
   return title;
  };
  this.setTitle = function(newTitle) {
   title = newTitle || 'No title specified';
  };
  this.getAuthor = function() {
   return author;
  };
  this.setAuthor = function(newAuthor) {
   author = newAuthor || 'No author specified';
  };
  // Constructor code.
  numOfBooks++; // Keep track of how many Books have been instantiated
         // with the private static attribute.
  if(numOfBooks > 1) throw new Error('Book: Only 1 instances of Book can be '
    + 'created.');
  this.setIsbn(newIsbn);
  this.setTitle(newTitle);
  this.setAuthor(newAuthor);
 }
})();
// Public static method.
Book.convertToTitleCase = function(inputString) {
 //...
 console.log("convertToTitleCase");
};
// Public, non-privileged methods.
Book.prototype = {
 display: function() {
  //...
  console.log("display");
 }
};
var mybook=new Book("myisbtn","mytittle","myauthor");
console.log(mybook.getAuthor());  //myauthor
mybook.display();          //display
//mybook.convertToTitleCase();   //mybook.convertToTitleCase is not a function
var mybook2= new Book("my2","tittle2","myauthor2");
console.log(mybook2.getAuthor());  //Only 1 instances of Book can be created.