JavaScript 访问器


目录

    显示目录


JavaScript 访问器(Getter 和 Setter)

ECMAScript 5 (ES5 2009) 引入了 Getter 和 Setters。

Getter 和 Setter 允许您定义对象访问器(计算 特性)。


JavaScript Getter(get 关键字)

此示例使用 lang 属性来 get 语言 属性的值。

例子

// Create an object:
const person = {
  firstName: "John",
  lastName: "Doe",
  language: "en",
  get lang() {
    return this.language;
    }
};

// Display data from the object using a getter:
document.getElementById("demo").innerHTML = person.lang;

自己尝试一下 →

<!DOCTYPE html>
<html>
<body>

<h2>JavaScript Getters and Setters</h2>

<p>Getters and setters allow you to get and set object properties via methods.</p>

<p>This example uses a lang property to get the value of the language property:</p>

<p id="demo"></p>

<script>
// Create an object:
const person = {
  firstName: "John",
  lastName: "Doe",
  language: "en",
  get lang() {
    return this.language;
  }
};

// Display data from the object using a getter:
document.getElementById("demo").innerHTML = person.lang;
</script>

</body>
</html>

JavaScript Setter(set 关键字)

此示例使用 lang 属性来设置 语言 属性的值。

例子

const person = {
  firstName: "John",
  lastName: "Doe",
  language: "",
    set lang(lang) {
    this.language = lang;
    }
};

// Set an object 
property using a setter:
person.lang = "en";

// Display data from the object:
document.getElementById("demo").innerHTML = person.language;

自己尝试一下 →

<!DOCTYPE html>
<html>
<body>

<h2>JavaScript Getters and Setters</h2>

<p>Getters and setters allow you to get and set properties via methods.</p>

<p>This example uses a lang property to set the value of the language property.</p>

<p id="demo"></p>

<script>
// Create an object:
const person = {
  firstName: "John",
  lastName: "Doe",
  language: "NO",
  set lang(value) {
    this.language = value;
  }
};

// Set a property using set:
person.lang = "en";

// Display data from the object:
document.getElementById("demo").innerHTML = person.language;
</script>

</body>
</html>


JavaScript 函数还是 Getter?

这两个例子有什么区别?

实施例1

const person = {
  firstName: "John",
  lastName: "Doe",
  fullName: function() {
    return this.firstName + " " + 
this.lastName;
  }
};
// Display data from the object using a method:
  document.getElementById("demo").innerHTML = person.fullName();

自己尝试一下 →

<!DOCTYPE html>
<html>
<body>

<h2>JavaScript Object Methods</h2>

<p>Object data can be accessed using property stored as a function.</p>

<p id="demo"></p>

<script>
// Create an object:
const person = {
  firstName: "John",
  lastName: "Doe",
  fullName: function() {
    return this.firstName + " " + this.lastName;
  }
};

// Display data from the object using a method:
document.getElementById("demo").innerHTML = person.fullName();
</script>

</body>
</html>

实施例2

const person = {
  firstName: "John",
  lastName: "Doe",
  get fullName() {
    return this.firstName + " " + 
this.lastName;
  }
};
// Display data from the object using a getter:
  document.getElementById("demo").innerHTML = person.fullName;

自己尝试一下 →

<!DOCTYPE html>
<html>
<body>

<h2>JavaScript Object Methods</h2>

<p>Object data can be accessed using a getter.</p>

<p id="demo"></p>

<script>
// Create an object:
const person = {
  firstName: "John",
  lastName: "Doe",
  get fullName() {
    return this.firstName + " " + this.lastName;
  }
};
// Display data from the object using a getter:
document.getElementById("demo").innerHTML = person.fullName;
</script>

</body>
</html>

示例 1 将 fullName 作为函数访问:person.fullName()。

示例 2 将 fullName 作为属性访问:person.fullName。

第二个示例提供了更简单的语法。


数据质量

使用 getter 和 setter 时,JavaScript 可以确保更好的数据质量。

在本例中,使用 lang 属性返回值 大写的 语言 属性:

例子

// Create an object:
const person = {
  firstName: "John",
  lastName: "Doe",
  language: "en",
  get lang() {
    return this.language.toUpperCase();
    }
};

// Display data from the object using a getter:
document.getElementById("demo").innerHTML = person.lang;

自己尝试一下 →

<!DOCTYPE html>
<html>
<body>

<h2>JavaScript Getters and Setters</h2>

<p>Getters and setters allow you to get and set properties via methods.</p>

<p id="demo"></p>

<script>
// Create an object:
const person = {
  firstName: "John",
  lastName: "Doe",
  language: "en",
  get lang() {
    return this.language.toUpperCase();
  }
};
// Display data from the object using a getter:
document.getElementById("demo").innerHTML = person.lang;
</script>

</body>
</html>

在本例中,使用 lang 属性存储大写字母 语言 属性中的值:

例子

const person = {
  firstName: "John",
  lastName: "Doe",
  language: "",
  set lang(lang) {
      this.language = lang.toUpperCase();
    }
};

// Set an object 
property using a setter:
person.lang = "en";

// Display data from the object:
document.getElementById("demo").innerHTML = person.language;

自己尝试一下 →

<!DOCTYPE html>
<html>
<body>

<h2>JavaScript Getters and Setters</h2>

<p>Getters and setters allow you to get and set properties via methods.</p>

<p id="demo"></p>

<script>
// Create an object:
const person = {
  firstName: "John",
  lastName: "Doe",
  language: "",
  set lang(lang) {
    this.language = lang.toUpperCase();
  }
};

// Set a property using set:
person.lang = "en";

// Display data from the object:
document.getElementById("demo").innerHTML = person.language;
</script>

</body>
</html>

为什么使用 Getter 和 Setter?

  • 它提供了更简单的语法

  • 它允许属性和方法具有相同的语法

  • 它可以确保更好的数据质量

  • 对于在幕后做事情很有用


对象.defineProperty()

Object.defineProperty() 方法也可用于添加 Getter 和 二传手:

反例

// Define object
const obj = {counter : 0};

// Define setters and getters
Object.defineProperty(obj, "reset", {
  get : function () {this.counter = 0;}
});
Object.defineProperty(obj, "increment", {
  get : function () {this.counter++;}
});
Object.defineProperty(obj, "decrement", {
  get : function () {this.counter--;}
});
Object.defineProperty(obj, "add", {
  set : function (value) {this.counter += value;}
});
Object.defineProperty(obj, "subtract", {
  set : function (value) {this.counter -= value;}
});

// Play with the counter:
obj.reset;
obj.add = 5;
obj.subtract = 1;
obj.increment;
obj.decrement;

自己尝试一下 →

<!DOCTYPE html>
<html>
<body>

<h2>JavaScript Getters and Setters</h2>

<p>Perfect for creating counters:</p>

<p id="demo"></p>

<script>
// Define an object
const obj = {counter : 0};

// Define Setters and Getters
Object.defineProperty(obj, "reset", {
  get : function () {this.counter = 0;}
});
Object.defineProperty(obj, "increment", {
  get : function () {this.counter++;}
});
Object.defineProperty(obj, "decrement", {
  get : function () {this.counter--;}
});
Object.defineProperty(obj, "add", {
  set : function (value) {this.counter += value;}
});
Object.defineProperty(obj, "subtract", {
  set : function (value) {this.counter -= value;}
});

// Play with counter:
obj.reset;
obj.add = 5;
obj.subtract = 1;
obj.increment;
obj.decrement;
document.getElementById("demo").innerHTML = obj.counter;
</script>

</body>
</html>