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.
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()
。
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.
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
that is 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
函数：
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
and
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.
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
特性为
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
，和
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 .
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
，和
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
函数。
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 .
Every closure has three scopes:
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.
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 .
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();
此范例使用
let
而不是
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();
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.