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";

但是,如果有人告诉我正确的语法是什么,它会SyntaxError: missing ; before statement在网上抛出set a(b){？

Answer 1

有几种方法可以做到这一点.

构造函数和 Object.defineProperty

要在像你一样的构造函数中执行它,你可以使用Object.defineProperty或Object.defineProperties.例如:

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

实例:

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t在Testing资本中创造了第一个,因为这是JavaScript中构造函数的压倒性惯例.)

构造函数属性和Object.defineProperty使用原型

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;
    }
});

实例:

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化程序

您尝试使用的语法是对象初始值设定项,而不是构造函数.我们可以用一个对象初始化器更换的Testing.prototype,就像这样:

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;
    }
};

实例:

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,而不是可以用来构建一堆这些对象的函数:

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

实例:

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>

构建器功能和Object.create原型

构造函数(您使用的函数new)并不是城里唯一一个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以下方式使用构建器功能:

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

您不使用new这些,因为它们自己创建对象:

实例:

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>

构建器功能,没有原型

最后但并非最不重要的是,您可以使用构建器函数而不使用原型:

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

您不使用new这些:

实例:

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>