JavaScript Closures

JavaScript Closures

 

Closures

Scope chain

var global = 1;
function outer() {//函數做用域
    var outer_local = 2;

    function inner() {//一個內部函數inner()has access to all variables
        var inner_local = 3;
        return inner_local + outer_local + global;
    }

    return inner();
}

console.log(outer());//6

從這個示例中能夠看到inner()函數能夠訪問到全部的變量，這就是做用域鏈的效果。

 

Breaking the chain with a closure

var a = "global variable";
var F = function () {
    var b = "local variable";
    var N = function () {
        //The function N has access to its private space, to the F() function's space, and to the global space.
        var c = "inner local";
        return b;
    };
    return N;
};

The function N has access to its private space, to the F() function's space, and to the global space. So, it can see b. Since F() is callable from the global space (it's a global function), you can call it and assign the returned value to another global variable. The result: a new global function that has access to the F() function's private space:

var inner = F();
var b = inner();
console.log(b);//local variable

另外一個閉包的例子：

var inner; // placeholder
var F = function () {
    var b = "local variable";
    var N = function () {
        return b;
    };
    inner = N;
};
F();

A new function, N(),is defined inside F() and assigned to the global inner. During definition time, N() was inside F(), so it had access to the F() function's scope. inner() 

will keep its access to the F() function's scope, even though it's part of the global space:

var b = inner();
console.log(b);//local variable

 

A definition and closure

Every function can be considered a closure. This is because every function maintains a secret link to the environment (the scope) in which it was created. But, most of the time this scope is destroyed unless something interesting happens (as shown above) that causes this scope to be maintained.

Based on what you've seen so far, you can say that a closure is created when a function keeps a link to its parent scope even after the parent has returned. And, every function is a closure because, at the very least, every function maintains access to the global scope, which is never destroyed.

 

再看一個閉包的例子，這個閉包的例子是有關函數的參數的：

Function parameters behave like local variables to this function（函數的參數就像函數做用域中的本地變量同樣）, but they are implicitly（隱式的） created (you don't need to use var for them). You can create a function that returns another function, which in turn returns its parent's parameter:

function F(param) {
    var N = function () {
        return param;
    };
    param++;
    return N;
}

var inner = F(123);
var result = inner();
console.log(result);

Notice how param++was incremented after the function was defined and yet, when called, inner() returned the updated value. This demonstrates that 

the function maintains a reference to the scope where it was defined, not to the variables and their values found in the scope during the function definition.

 

Closures in a loop

function F() {
    var arr = [], i;
    for (i = 0; i < 3; i++) {
        arr[i] = function () {
            return i;
        };
    }
    return arr;
}

var arr = F();
console.log(arr[0]());//3
console.log(arr[1]());//3
console.log(arr[2]());//3

三次循環，每次循環保存當前循環的變量i，但從結果來看，沒有獲得咱們想要的效果：

程序應該這樣改：

function F() {
    var arr = [], i;
    for (i = 0; i < 3; i++) {
        arr[i] = (function (x) {
            return function () {
                return x;
            };
        }(i));
    }
    return arr;
}

var arr = F();
console.log(arr[0]());//0
console.log(arr[1]());//1
console.log(arr[2]());//2

Here, instead of just creating a function that returns i, you pass the ivariable's current value to another immediate function. In this function, ibecomes the local value x, and xhas a different value every time.

 

Alternatively（另外）, you can use a "normal" (as opposed to an immediate) inner function to achieve the same result. The key is to use the middle function to "localize" the value of iat every iteration:

function F() {
    function binder(x) {
        return function () {
            return x;
        };
    }

    var arr = [], i;
    for (i = 0; i < 3; i++) {
        arr[i] = binder(i);
    }
    return arr;
}

var arr = F();
console.log(arr[0]());//0
console.log(arr[1]());//1
console.log(arr[2]());//2

 

Getter/Setter-Closures 

var getValue, setValue;
(function () {
    var secret = 0;
    getValue = function () {
        return secret;
    };
    setValue = function (v) {
        if (typeof v === "number") {
            secret = v;
        }
    };
}());

console.log(getValue());//0
setValue(123);
console.log(getValue());//123

In this case, the function that contains everything is an immediate function. It defines setValue()and getValue()as global functions, while the secretvariable remains local and inaccessible directly.

 

Iterator-Closures 

The last closure example (also the last example in the chapter) shows the use of a closure to accomplish an iterator functionality.

function setup(x) {
    var i = 0;
    return function () {
        return x[i++];
    };
}

var next = setup(['a', 'b', 'c']);
console.log(next());//a
console.log(next());//b
console.log(next());//c

For this example, let's just use a simple array and not a complex data structure. Here's an initialization function that takes an input array and also defines a secret pointer, i, that will always point to the next element in the array.

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