js的闭包

Javascript 中的闭包其实很改语言中很难理解的一部分。 感谢 Stuart 提供了那么棒的 PPT ，它深入浅出的解释了 Javascript 的闭包技术。

而我「自作主张」地在这份 PPT 中加入了部分的注释（希望没有影响阅读），主要的内容在这里简要整理下。

闭包的概念

「官方」的解释

「闭包」，是指拥有多个变量和绑定了这些变量的环境的
表达式（通常是一个函数），因而这些变量也是该表达式
的一部分。           

但不要咬文嚼字，我们简单的可以理解为

闭包是个函数，而它「记住了周围发生了什么」。表现为由「一个函数」体中定义了「另个函数」
           

function outer () {
    ...
    function inner () {
        ...
    }
}           

闭包的作用

1. setTimeout/setInterval
2. 回调函数（callback）
3. 事件句柄（event handle）

模块化代码

function Container(param) {
    var sercert = 3;
    this.member = param;
    var that    = this; // 注意这行

    function dec() {
        return (sercert > 0) ? --secret : false;
    }

    this.service = function() {
        return (dec()) ? that.member : null;
    }
}           

其中 dec 为私有，that 变量引用 Container 同时 service 函数构成了个闭包。

注意 Exploer 下闭包的「特殊情况」

1. 循环问题
2. 垃圾回收

最后，PPT 可以 在这里下载 ，同时需要进一步有关闭包的信息，为之漫笔翻译了份很 详实的文档 ，强烈建议看下。

阮一峰：http://www.ruanyifeng.com/blog/2009/08/learning_javascript_closures.html

A closure is the combination of a function bundled together (enclosed) with references to its surrounding state (the lexical environment). In other words, a closure gives you access to an outer function’s scope from an inner function. In JavaScript, closures are created every time a function is created, at function creation time.

Lexical scoping

Consider the following example code:

function init() {
  var name = 'Mozilla'; // name is a local variable created by init
  function displayName() { // displayName() is the inner function, a closure
    alert(name); // use variable declared in the parent function
  }
  displayName();
}
init();           

init()

 creates a local variable called 

name

 and a function called 

displayName()

. The 

displayName()

 function is an inner function that is defined inside 

init()

 and is available only within the body of the 

init()

 function. Note that the 

displayName()

 function has no local variables of its own. However, since inner functions have access to the variables of outer functions, 

displayName()

 can access the variable 

name

 declared in the parent function, 

init()

.

Run the code using this JSFiddle link and notice that the 

alert()

 statement within the 

displayName()

 function successfully displays the value of the 

name

 variable, which is declared in its parent function. This is an example of lexical scoping, which describes how a parser resolves variable names when functions are nested. The word lexical refers to the fact that lexical scoping uses the location where a variable is declared within the source code to determine where that variable is available. Nested functions have access to variables declared in their outer scope.

Closure

Consider the following code example:

function makeFunc() {
  var name = 'Mozilla';
  function displayName() {
    alert(name);
  }
  return displayName;
}

var myFunc = makeFunc();
myFunc();
           

Running this code has exactly the same effect as the previous example of the 

init()

 function above. What's different (and interesting) is that the 

displayName()

 inner function is returned from the outer function before being executed.

At first glance, it might seem unintuitive that this code still works. In some programming languages, the local variables within a function exist for just the duration of that function's execution. Once 

makeFunc()

 finishes executing, you might expect that the name variable would no longer be accessible. However, because the code still works as expected, this is obviously not the case in JavaScript.

The reason is that functions in JavaScript form closures. A closure is the combination of a function and the lexical environment within which that function was declared. This environment consists of any local variables that were in-scope at the time the closure was created. In this case, 

myFunc

 is a reference to the instance of the function 

displayName

 created when 

makeFunc

 is run. The instance of 

displayName

 maintains a reference to its lexical environment, within which the variable 

name

 exists. For this reason, when 

myFunc

 is invoked, the variable 

name

 remains available for use, and "Mozilla" is passed to 

alert

Here's a slightly more interesting example—a 

makeAdder

 function:

function makeAdder(x) {
  return function(y) {
    return x + y;
  };
}

var add5 = makeAdder(5);
var add10 = makeAdder(10);

console.log(add5(2));  // 7
console.log(add10(2)); // 12
           

In this example, we have defined a function 

makeAdder(x)

, that takes a single argument 

x

, and returns a new function. The function it returns takes a single argument 

y

, and returns the sum of 

x

 and 

y

In essence, 

makeAdder

 is a function factory. It creates functions that can add a specific value to their argument. In the above example, the function factory creates two new functions—one that adds five to its argument, and one that adds 10.

add5

add10

 are both closures. They share the same function body definition, but store different lexical environments. In 

add5

's lexical environment, 

x

 is 5, while in the lexical environment for 

add10

, 

x

 is 10.

Practical closures

Closures are useful because they let you associate data (the lexical environment) with a function that operates on that data. This has obvious parallels to object-oriented programming, where objects allow you to associate data (the object's properties) with one or more methods.

Consequently, you can use a closure anywhere that you might normally use an object with only a single method.

Situations where you might want to do this are particularly common on the web. Much of the code written in front-end JavaScript is event-based. You define some behavior, and then attach it to an event that is triggered by the user (such as a click or a keypress). The code is attached as a callback (a single function that is executed in response to the event).

For instance, suppose we want to add buttons to a page to adjust the text size. One way of doing this is to specify the font-size of the 

body

 element (in pixels), and then set the size of the other elements on the page (such as headers) using the relative 

em

 unit:

body {
  font-family: Helvetica, Arial, sans-serif;
  font-size: 12px;
}

h1 {
  font-size: 1.5em;
}

h2 {
  font-size: 1.2em;
}
           

Such interactive text size buttons can change the 

font-size

 property of the 

body

 element, and the adjustments are picked up by other elements on the page thanks to the relative units.

Here's the JavaScript:

function makeSizer(size) {
  return function() {
    document.body.style.fontSize = size + 'px';
  };
}

var size12 = makeSizer(12);
var size14 = makeSizer(14);
var size16 = makeSizer(16);
           

size12

size14

, and 

size16

 are now functions that resize the body text to 12, 14, and 16 pixels, respectively. You can attach them to buttons (in this case hyperlinks) as demonstrated in the following code example.

document.getElementById('size-12').onclick = size12;
document.getElementById('size-14').onclick = size14;
document.getElementById('size-16').onclick = size16;
           

<a href="#" id="size-12">12</a>
<a href="#" id="size-14">14</a>
<a href="#" id="size-16">16</a>
           

Run the code using JSFiddle.

Emulating private methods with closures

Languages such as Java allow you to declare methods as private, meaning that they can be called only by other methods in the same class.

JavaScript does not provide a native way of doing this, but it is possible to emulate private methods using closures. Private methods aren't just useful for restricting access to code. They also provide a powerful way of managing your global namespace.

The following code illustrates how to use closures to define public functions that can access private functions and variables. Note that these closures follow the Module Design Pattern.

var counter = (function() {
  var privateCounter = 0;
  function changeBy(val) {
    privateCounter += val;
  }

  return {
    increment: function() {
      changeBy(1);
    },

    decrement: function() {
      changeBy(-1);
    },

    value: function() {
      return privateCounter;
    }
  };
})();

console.log(counter.value());  // 0.

counter.increment();
counter.increment();
console.log(counter.value());  // 2.

counter.decrement();
console.log(counter.value());  // 1.
           

In previous examples, each closure had its own lexical environment. Here though, there is a single lexical environment that is shared by the three functions: 

counter.increment

counter.decrement

counter.value

The shared lexical environment is created in the body of an anonymous function, which is executed as soon as it has been defined (also known as an IIFE). The lexical environment contains two private items: a variable called 

privateCounter

, and a function called 

changeBy

. You can't access either of these private members from outside the anonymous function. Instead, you can access them using the three public functions that are returned from the anonymous wrapper.

Those three public functions are closures that share the same lexical environment. Thanks to JavaScript's lexical scoping, they each have access to the 

privateCounter

 variable and the 

changeBy

 function.

Notice that I defined an anonymous function that creates a counter, and then I call it immediately and assign the result to the 

counter

 variable. You could store this function in a separate variable 

makeCounter

, and then use it to create several counters.

var makeCounter = function() {
  var privateCounter = 0;
  function changeBy(val) {
    privateCounter += val;
  }
  return {
    increment: function() {
      changeBy(1);
    },

    decrement: function() {
      changeBy(-1);
    },

    value: function() {
      return privateCounter;
    }
  }
};

var counter1 = makeCounter();
var counter2 = makeCounter();

alert(counter1.value());  // 0.

counter1.increment();
counter1.increment();
alert(counter1.value()); // 2.

counter1.decrement();
alert(counter1.value()); // 1.
alert(counter2.value()); // 0.
           

Notice how the two counters maintain their independence from one another. Each closure references a different version of the 

privateCounter

 variable through its own closure. Each time one of the counters is called, its lexical environment changes by changing the value of this variable. Changes to the variable value in one closure don't affect the value in the other closure.

Using closures in this way provides benefits that are normally associated with object-oriented programming. In particular, data hiding and encapsulation.

Closure Scope Chain

Every closure has three scopes:

• Local Scope (Own scope)
• Outer Functions Scope
• Global Scope

A common mistake is not realizing that, in the case where the outer function is itself a nested function, access to the outer function's scope includes the enclosing scope of the outer function—effectively creating a chain of function scopes. To demonstrate, consider the following example code.

// global scope
var e = 10;
function sum(a){
  return function(b){
    return function(c){
      // outer functions scope
      return function(d){
        // local scope
        return a + b + c + d + e;
      }
    }
  }
}

console.log(sum(1)(2)(3)(4)); // log 20

// You can also write without anonymous functions:

// global scope
var e = 10;
function sum(a){
  return function sum2(b){
    return function sum3(c){
      // outer functions scope
      return function sum4(d){
        // local scope
        return a + b + c + d + e;
      }
    }
  }
}

var s = sum(1);
var s1 = s(2);
var s2 = s1(3);
var s3 = s2(4);
console.log(s3) //log 20
           

In the example above, there's a series of nested functions, all of which have access to the outer functions' scope. In this context, we can say that closures have access to all outer function scopes.

Creating closures in loops: A common mistake

Prior to the introduction of the 

let

 keyword in ECMAScript 2015, a common problem with closures occurred when you created them inside a loop. To demonstrate, consider the following example code.

<p id="help">Helpful notes will appear here</p>
<p>E-mail: <input type="text" id="email" name="email"></p>
<p>Name: <input type="text" id="name" name="name"></p>
<p>Age: <input type="text" id="age" name="age"></p>
           

function showHelp(help) {
  document.getElementById('help').innerHTML = help;
}

function setupHelp() {
  var helpText = [
      {'id': 'email', 'help': 'Your e-mail address'},
      {'id': 'name', 'help': 'Your full name'},
      {'id': 'age', 'help': 'Your age (you must be over 16)'}
    ];

  for (var i = 0; i < helpText.length; i++) {
    var item = helpText[i];
    document.getElementById(item.id).onfocus = function() {
      showHelp(item.help);
    }
  }
}

setupHelp();
           

Try running the code in JSFiddle.

The 

helpText

 array defines three helpful hints, each associated with the ID of an input field in the document. The loop cycles through these definitions, hooking up an 

onfocus

 event to each one that shows the associated help method.

If you try this code out, you'll see that it doesn't work as expected. No matter what field you focus on, the message about your age will be displayed.

The reason for this is that the functions assigned to 

onfocus

 are closures; they consist of the function definition and the captured environment from the 

setupHelp

 function's scope. Three closures have been created by the loop, but each one shares the same single lexical environment, which has a variable with changing values (

item

). This is because the variable 

item

 is declared with 

var

 and thus has function scope due to hoisting. The value of 

item.help

 is determined when the 

onfocus

 callbacks are executed. Because the loop has already run its course by that time, the 

item

 variable object (shared by all three closures) has been left pointing to the last entry in the 

helpText

 list.

One solution in this case is to use more closures: in particular, to use a function factory as described earlier:

function showHelp(help) {
  document.getElementById('help').innerHTML = help;
}

function makeHelpCallback(help) {
  return function() {
    showHelp(help);
  };
}

function setupHelp() {
  var helpText = [
      {'id': 'email', 'help': 'Your e-mail address'},
      {'id': 'name', 'help': 'Your full name'},
      {'id': 'age', 'help': 'Your age (you must be over 16)'}
    ];

  for (var i = 0; i < helpText.length; i++) {
    var item = helpText[i];
    document.getElementById(item.id).onfocus = makeHelpCallback(item.help);
  }
}

setupHelp();
           

Run the code using this JSFiddle link.

This works as expected. Rather than the callbacks all sharing a single lexical environment, the 

makeHelpCallback

 function creates a new lexical environment for each callback, in which 

help

 refers to the corresponding string from the 

helpText

 array.

One other way to write the above using anonymous closures is:

function showHelp(help) {
  document.getElementById('help').innerHTML = help;
}

function setupHelp() {
  var helpText = [
      {'id': 'email', 'help': 'Your e-mail address'},
      {'id': 'name', 'help': 'Your full name'},
      {'id': 'age', 'help': 'Your age (you must be over 16)'}
    ];

  for (var i = 0; i < helpText.length; i++) {
    (function() {
       var item = helpText[i];
       document.getElementById(item.id).onfocus = function() {
         showHelp(item.help);
       }
    })(); // Immediate event listener attachment with the current value of item (preserved until iteration).
  }
}

setupHelp();           

If you don't want to use more closures, you can use the 

let

 keyword introduced in ES2015 :

function showHelp(help) {
  document.getElementById('help').innerHTML = help;
}

function setupHelp() {
  var helpText = [
      {'id': 'email', 'help': 'Your e-mail address'},
      {'id': 'name', 'help': 'Your full name'},
      {'id': 'age', 'help': 'Your age (you must be over 16)'}
    ];

  for (let i = 0; i < helpText.length; i++) {
    let item = helpText[i];
    document.getElementById(item.id).onfocus = function() {
      showHelp(item.help);
    }
  }
}

setupHelp();           

This example uses 

let

 instead of 

var

, so every closure binds the block-scoped variable, meaning that no additional closures are required.

Another alternative could be to use 

forEach()

 to iterate over the 

helpText

 array and attach a listener to each 

<p>

, as shown:

function showHelp(help) {
  document.getElementById('help').innerHTML = help;
}

function setupHelp() {
  var helpText = [
      {'id': 'email', 'help': 'Your e-mail address'},
      {'id': 'name', 'help': 'Your full name'},
      {'id': 'age', 'help': 'Your age (you must be over 16)'}
    ];

  helpText.forEach(function(text) {
    document.getElementById(text.id).onfocus = function() {
      showHelp(text.help);
    }
  });
}

setupHelp();           

Performance considerations

It is unwise to unnecessarily create functions within other functions if closures are not needed for a particular task, as it will negatively affect script performance both in terms of processing speed and memory consumption.

For instance, when creating a new object/class, methods should normally be associated to the object's prototype rather than defined into the object constructor. The reason is that whenever the constructor is called, the methods would get reassigned (that is, for every object creation).

Consider the following case:

function MyObject(name, message) {
  this.name = name.toString();
  this.message = message.toString();
  this.getName = function() {
    return this.name;
  };

  this.getMessage = function() {
    return this.message;
  };
}
           

Because the previous code does not take advantage of the benefits of using closures in this particular instance, we could instead rewrite it to avoid using closure as follows:

function MyObject(name, message) {
  this.name = name.toString();
  this.message = message.toString();
}
MyObject.prototype = {
  getName: function() {
    return this.name;
  },
  getMessage: function() {
    return this.message;
  }
};
           

However, redefining the prototype is not recommended. The following example instead appends to the existing prototype:

function MyObject(name, message) {
  this.name = name.toString();
  this.message = message.toString();
}
MyObject.prototype.getName = function() {
  return this.name;
};
MyObject.prototype.getMessage = function() {
  return this.message;
};
           

In the two previous examples, the inherited prototype can be shared by all objects and the method definitions need not occur at every object creation. See Details of the Object Model for more.

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• Last modified: Jun 23, 2020, by MDN contributors