Detailed explanation of Javascript closure examples_javascript skills

What is a closure

What is a closure? Closure is Closure, which is a new feature that static languages ​​do not have. But closure is not something too complicated to understand. In short, closure is:

A closure is a collection of local variables of a function, but these local variables will continue to exist after the function returns.

Closure is the "stack" of a function that is not released after the function returns. We can also understand that these function stacks are not allocated on the stack but on the heap. When another function is defined within a function, it will Generating closures The second definition above is a supplement to the first definition, extracting the subject, predicate and object of the first definition - a closure is the set of 'local variables' of a function. It's just that this local variable can be accessed after the function returns. (This is not an official definition, but this definition should be more helpful for you to understand closures)

It is very important to understand the closure of Javascript. This article tries to understand this concept with the simplest example.

function greet(sth){
  return function(name){
    console.log(sth + ' ' + name);
  }
}
//hi darren
greet('hi')('darren');

Or it can be written like this:

var sayHi = greet('hi');
sayHi('darren');

The question we want to ask is: Why can the internal function of greet use the sth variable?

Its internal operation is roughly as follows:

→ Create global context
→ Execute the var sayHi = greet('hi'); statement to create a greet context, and the variable sth is stored in the greet context.
→ Continue to execute the statements in the greet function and return an anonymous function. Although the greet context disappears from the stack, the sth variable still exists in a certain space in the memory.
→ Continue to execute sayHi('darren'); creates a sayHi context and tries to search for the sth variable, but there is no sth variable in the context of sayHi. The sayHi context will follow a scope chain to find the memory corresponding to the sth variable. The outer function is like a closure, and its inner function can use the variables of the outer function.

A simple example of closure

function buildFunctions(){
  var funcArr = [];
  for(var i = 0; i < 3; i++){
    funcArr.push(function(){console.log(i)});
  }
  return funcArr;
}
var fs = buildFunctions();
fs[0](); //3
fs[1](); //3
fs[2](); //3

The above, why is the result not 0, 1, 2?

--Because i serves as a closure variable, its current value is 3 and is used by the internal function. To achieve the desired effect, you can create an independent context for each traversal during traversal so that it is not affected by closures. The self-triggering function can implement independent context.

function buildFunctions(){
  var funcArr = [];

  for(var i = 0; i < 3; i++){
    funcArr.push((function(j){
      return function(){
       console.log(j);
      };
    }(i)));
  }

  return funcArr;
}
var fs = buildFunctions();
fs[0](); //0
fs[1](); //1
fs[2](); //2

The two examples in this article exactly reflect two aspects of closure: one is that the internal function uses closure variables, and the other is that the internal function is written in a self-triggering function to avoid being affected by the closure.

As a local variable, it can be accessed by the code within the function. This is no different from a static language. The difference with closures is that local variables can still be accessed by code outside the function after the function execution ends. This means that the function must return a "reference" pointing to the closure, or assign this "reference" to an external variable to ensure that the local variables in the closure can be accessed by external code. Of course, the entity containing this reference should be an object, because in Javascript, except for basic types, everything else is an object. Unfortunately, ECMAScript does not provide relevant members and methods to access local variables in closures. But in ECMAScript, the inner function () inner function defined in the function object is a local variable that can directly access the external function. Through this mechanism, we can complete access to the closure in the following way.

function greeting(name) {
   var text = 'Hello ' + name; // local variable
   // 每次调用时，产生闭包，并返回内部函数对象给调用者
   return function () { alert(text); }
}
var sayHello=greeting( "Closure" );
sayHello() // 通过闭包访问到了局部变量text

上述代码的执行结果是：Hello Closure，因为sayHello()函数在greeting函数执行完毕后，仍然可以访问到了定义在其之内的局部变量text。

好了，这个就是传说中闭包的效果，闭包在Javascript中有多种应用场景和模式，比如Singleton，Power Constructor等这些Javascript模式都离不开对闭包的使用。

ECMAScript闭包模型

ECMAScript到底是如何实现闭包的呢？想深入了解的亲们可以获取ECMAScript 规范进行研究，我这里也只做一个简单的讲解，内容也是来自于网络。

在ECMAscript的脚本的函数运行时，每个函数关联都有一个执行上下文场景(Execution Context) ，这个执行上下文场景中包含三个部分

文法环境（The LexicalEnvironment）
变量环境（The VariableEnvironment）
this绑定

其中第三点this绑定与闭包无关，不在本文中讨论。文法环境中用于解析函数执行过程使用到的变量标识符。我们可以将文法环境想象成一个对象，该对 象包含了两个重要组件，环境记录(Enviroment Recode)，和外部引用(指针)。环境记录包含包含了函数内部声明的局部变量和参数变量，外部引用指向了外部函数对象的上下文执行场景。全局的上下文 场景中此引用值为NULL。这样的数据结构就构成了一个单向的链表，每个引用都指向外层的上下文场景。

例如上面我们例子的闭包模型应该是这样，sayHello函数在最下层，上层是函数greeting，最外层是全局场景。如下图：

因此当sayHello被调用的时候，sayHello会通过上下文场景找到局部变量text的值，因此在屏幕的对话框中显示出”Hello Closure”
变量环境(The VariableEnvironment)和文法环境的作用基本相似，具体的区别请参看ECMAScript的规范文档。

闭包的样列

前面的我大致了解了Javascript闭包是什么，闭包在Javascript是怎么实现的。下面我们通过针对一些例子来帮助大家更加深入的理解闭包，下面共有5个样例，例子来自于JavaScript Closures For Dummies(镜像)。

例子1:闭包中局部变量是引用而非拷贝

function say667() {
  // Local variable that ends up within closure
  var num = 666;
  var sayAlert = function() { alert(num); }
  num++;
  return sayAlert;
}
var sayAlert = say667();
sayAlert()

因此执行结果应该弹出的667而非666。

例子2：多个函数绑定同一个闭包，因为他们定义在同一个函数内。

function setupSomeGlobals() {
  // Local variable that ends up within closure
  var num = 666;
  // Store some references to functions as global variables
  gAlertNumber = function() { alert(num); }
  gIncreaseNumber = function() { num++; }
  gSetNumber = function(x) { num = x; }
}
setupSomeGolbals(); // 为三个全局变量赋值
gAlertNumber(); //666
gIncreaseNumber();
gAlertNumber(); // 667
gSetNumber(12);//
gAlertNumber();//12

例子3：当在一个循环中赋值函数时，这些函数将绑定同样的闭包

function buildList(list) {
  var result = [];
  for (var i = 0; i < list.length; i++) {
    var item = 'item' + list[i];
    result.push( function() {alert(item + ' ' + list[i])} );
  }
  return result;
}
function testList() {
  var fnlist = buildList([1,2,3]);
  // using j only to help prevent confusion - could use i
  for (var j = 0; j < fnlist.length; j++) {
    fnlist[j]();
  }
}

testList的执行结果是弹出item3 undefined窗口三次，因为这三个函数绑定了同一个闭包，而且item的值为最后计算的结果，但是当i跳出循环时i值为4，所以list[4]的结果为undefined.

例子4：外部函数所有局部变量都在闭包内，即使这个变量声明在内部函数定义之后。

function sayAlice() {
  var sayAlert = function() { alert(alice); }
  // Local variable that ends up within closure
  var alice = 'Hello Alice';
  return sayAlert;
}
var helloAlice=sayAlice();
helloAlice();

执行结果是弹出”Hello Alice”的窗口。即使局部变量声明在函数sayAlert之后，局部变量仍然可以被访问到。

例子5：每次函数调用的时候创建一个新的闭包

function newClosure(someNum, someRef) {
  // Local variables that end up within closure
  var num = someNum;
  var anArray = [1,2,3];
  var ref = someRef;
  return function(x) {
    num += x;
    anArray.push(num);
    alert('num: ' + num +
    '\nanArray ' + anArray.toString() +
    '\nref.someVar ' + ref.someVar);
  }
}
closure1=newClosure(40,{someVar:'closure 1'});
closure2=newClosure(1000,{someVar:'closure 2'});
closure1(5); // num:45 anArray[1,2,3,45] ref:'someVar closure1'
closure2(-10);// num:990 anArray[1,2,3,990] ref:'someVar closure2'

闭包的应用

Singleton 单件：

var singleton = function () {
  var privateVariable;
  function privateFunction(x) {
    ...privateVariable...
  }
  return {
    firstMethod: function (a, b) {
      ...privateVariable...
    },
    secondMethod: function (c) {
      ...privateFunction()...
    }
  };
}();

这个单件通过闭包来实现。通过闭包完成了私有的成员和方法的封装。匿名主函数返回一个对象。对象包含了两个方法，方法1可以方法私有变量，方法2访 问内部私有函数。需要注意的地方是匿名主函数结束的地方的'()'，如果没有这个'()'就不能产生单件。因为匿名函数只能返回了唯一的对象，而且不能被 其他地方调用。这个就是利用闭包产生单件的方法。