为js对象分配setter / getter

Question

我试图为我的js“class”分配一个setter：

function testing(){
    this.value = "content";
    set a(b){
        this.value = b;
    };
}
var test1 = new testing();
test1.a = "nope";

但它在行SyntaxError: missing ; before statement上抛出set a(b){可能有人告诉我正确的语法是什么吗？

Answer 1

有几种方法可以做到这一点。

构造函数和Object.defineProperty

要在您拥有的构造函数中执行此操作，您可以使用Object.defineProperty或Object.defineProperties。例如：

function Testing(){
    this.value = "content";
    Object.defineProperty(this, "a", {
        set: function(b){
            this.value = b;
        }
    });
}

直播示例：

function Testing() {
  this.value = "content";
  Object.defineProperty(this, "a", {
    set: function(b) {
      this.value = b;
    }
  });
}
var t = new Testing();
snippet.log("t.value before: " + t.value);
t.a = "new content";
snippet.log("t.value after: " + t.value);

<!-- Script provides the `snippet` object, see http://meta.stackexchange.com/a/242144/134069 -->
<script src="http://tjcrowder.github.io/simple-snippets-console/snippet.js"></script>

（注意：我在t首都中创建了第一个Testing，因为这是JavaScript中构造函数的压倒性约定。）

使用原型

的构造函数属性和Object.defineProperty

或者您可以在原型上定义setter：

function Testing(){
    this.value = "content";
}
Object.defineProperty(Testing.prototype, "a", {
    set: function(b){
        this.value = b;
    }
});

直播示例：

function Testing(){
    this.value = "content";
}
Object.defineProperty(Testing.prototype, "a", {
    set: function(b){
        this.value = b;
    }
});
var t1 = new Testing();
snippet.log("t1.value before: " + t1.value);
t1.a = "new content";
snippet.log("t1.value after: " + t1.value);
var t2 = new Testing();
snippet.log("t2.value before: " + t2.value);
t2.a = "new content";
snippet.log("t2.value after: " + t2.value);

<!-- Script provides the `snippet` object, see http://meta.stackexchange.com/a/242144/134069 -->
<script src="http://tjcrowder.github.io/simple-snippets-console/snippet.js"></script>

构造函数和原型

的对象初始值设定项

您尝试使用的语法是对象初始值设定项，而不是构造函数。我们可以使用对象初始值设定项替换 Testing.prototype，如下所示：

function Testing(){
    this.value = "content";
}
Testing.prototype = {
    constructor: Testing, // The old object had this, so let's maintain it
    set a(b){
        this.value = b;
    }
};

直播示例：

function Testing(){
    this.value = "content";
}
Testing.prototype = {
    constructor: Testing, // The old object had this, so let's maintain it
    set a(b){
        this.value = b;
    }
};
var t1 = new Testing();
snippet.log("t1.value before: " + t1.value);
t1.a = "new content";
snippet.log("t1.value after: " + t1.value);
var t2 = new Testing();
snippet.log("t2.value before: " + t2.value);
t2.a = "new content";
snippet.log("t2.value after: " + t2.value);

<!-- Script provides the `snippet` object, see http://meta.stackexchange.com/a/242144/134069 -->
<script src="http://tjcrowder.github.io/simple-snippets-console/snippet.js"></script>

一次性对象的对象初始值设定项

这只是创建一个一次性对象，而不是可用于构建一堆这些对象的函数：

var t = {
  value: "content",
  set a(b) {
    this.value = b;
  }
};

直播示例：

var t = {
  value: "content",
  set a(b) {
    this.value = b;
  }
};
snippet.log("t.value before: " + t.value);
t.a = "new content";
snippet.log("t.value after: " + t.value);

<!-- Script provides the `snippet` object, see http://meta.stackexchange.com/a/242144/134069 -->
<script src="http://tjcrowder.github.io/simple-snippets-console/snippet.js"></script>

构建器函数和原型

的Object.create

构造函数（与new一起使用的函数）并不是城里唯一用于创建具有原型的对象的游戏。如果您愿意，也可以通过Object.create：

使用构建器功能执行此操作

var testingPrototype = {
    set a(b){
        this.value = b;
    }
};
function testing(){
    var obj = Object.create(testingPrototype);
    obj.value = "content";
    return obj;
}

你不要使用new，因为他们自己创建了对象：

直播示例：

var testingPrototype = {
    set a(b){
        this.value = b;
    }
};
function testing(){
    var obj = Object.create(testingPrototype);
    obj.value = "content";
    return obj;
}
var t1 = testing();
snippet.log("t1.value before: " + t1.value);
t1.a = "new content";
snippet.log("t1.value after: " + t1.value);
var t2 = testing();
snippet.log("t2.value before: " + t2.value);
t2.a = "new content";
snippet.log("t2.value after: " + t2.value);

<!-- Script provides the `snippet` object, see http://meta.stackexchange.com/a/242144/134069 -->
<script src="http://tjcrowder.github.io/simple-snippets-console/snippet.js"></script>

构建器函数，没有原型

最后但并非最不重要的是，您可以使用构建器功能而不使用原型：

function testing() {
    return {
        value: "content",
        set a(b){
            this.value = b;
        }
    };
}

您不会将new与以下内容一起使用：

直播示例：

function testing(){
    return {
        value: "content",
        set a(b){
            this.value = b;
        }
    };
}
var t1 = testing();
snippet.log("t1.value before: " + t1.value);
t1.a = "new content";
snippet.log("t1.value after: " + t1.value);
var t2 = testing();
snippet.log("t2.value before: " + t2.value);
t2.a = "new content";
snippet.log("t2.value after: " + t2.value);

<!-- Script provides the `snippet` object, see http://meta.stackexchange.com/a/242144/134069 -->
<script src="http://tjcrowder.github.io/simple-snippets-console/snippet.js"></script>

Answer 2

在这样的例子中，集合需要成为对象的一部分：

这样做的一种方式是：

javascript

http://jsfiddle.net/GarryPas/1s7v0rdn/