2020-05-14 21:41:53 +08:00
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## 对象的基本操作
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### 创建对象
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使用new关键字调用的函数，是构造函数constructor。**构造函数是专门用来创建对象的函数**。
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例如：
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```javascript
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var obj = new Object();
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```
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记住，使用`typeof`检查一个对象时，会返回`object`。
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关于常见对象的更多方式，可以看上一篇文章《对象的创建&构造函数》。
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### 向对象中添加属性
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在对象中保存的值称为属性。
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向对象添加属性的语法：
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```javascript
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对象.属性名 = 属性值;
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```
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举例：
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```javascript
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var obj = new Object();
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//向obj中添加一个name属性
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obj.name = "孙悟空";
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//向obj中添加一个gender属性
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obj.gender = "男";
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//向obj中添加一个age属性
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obj.age = 18;
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console.log(JSON.stringify(obj)); // 将 obj 以字符串的形式打印出来
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```
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打印结果：
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```
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{
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"name":"孙悟空",
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"gender":"男",
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"age":18
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}
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```
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### 获取对象中的属性
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**方式1**：
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语法：
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```javascript
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对象.属性名
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```
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如果获取对象中没有的属性，不会报错而是返回`undefined`。
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举例：
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```javascript
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var obj = new Object();
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//向obj中添加一个name属性
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obj.name = "孙悟空";
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//向obj中添加一个gender属性
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obj.gender = "男";
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//向obj中添加一个age属性
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obj.age = 18;
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// 获取对象中的属性，并打印出来
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console.log(obj.gender); // 打印结果：男
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console.log(obj.color); // 打印结果：undefined
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```
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**方式2**：可以使用`[]`这种形式去操作属性
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对象的属性名不强制要求遵守标识符的规范，不过我们尽量要按照标识符的规范去做。
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但如果确实要使用特殊的属性名，就不能采用`.`的方式来操作对象的属性。比如说，`123`这种属性名，如果我们直接写成`obj.123 = 789`来操作属性，是会报错的。那怎么办呢？办法如下：
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语法格式如下：（读取时，也是采用这种方式）
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```javascript
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// 注意，括号里的属性名，必须要加引号
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对象['属性名'] = 属性值
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```
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上面这种语法格式，举例如下：
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```javascript
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obj['123'] = 789;
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```
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**重要**：使用`[]`这种形式去操作属性，更加的灵活，因为，我们可以在`[]`中直接传递一个**变量**。
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### 修改对象的属性值
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语法：
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```javascript
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对象.属性名 = 新值
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```
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```javascript
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obj.name = "tom";
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```
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### 删除对象的属性
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语法：
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```javascript
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delete obj.name;
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```
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### in 运算符
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通过该运算符可以检查一个对象中是否含有指定的属性。如果有则返回true，没有则返回false。
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语法：
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```javascript
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"属性名" in 对象
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```
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举例：
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```javascript
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//检查obj中是否含有name属性
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console.log("name" in obj);
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```
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我们平时使用的对象不一定是自己创建的，可能是别人提供的，这个时候，in 运算符可以派上用场。
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2020-05-23 17:44:13 +08:00
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## 遍历对象：for in
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2020-05-14 21:41:53 +08:00
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语法：
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```javascript
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for (var 变量 in 对象名) {
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}
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```
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解释：对象中有几个属性，循环体就会执行几次。每次执行时，会将对象中的**每个属性的 属性名 赋值给变量**。
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语法举例：
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```javascript
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for (var key in obj) {
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2020-05-23 17:44:13 +08:00
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console.log(key); // 这里的 key 是：对象属性的键（也就是属性名）
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console.log(obj[key]); // 这里的 obj[key] 是：对象属性的值（也就是属性值）
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2020-05-14 21:41:53 +08:00
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}
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```
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举例：
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```html
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<!DOCTYPE html>
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<html>
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<head>
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<meta charset="UTF-8">
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<title></title>
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<script type="text/javascript">
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var obj = {
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name: "smyhvae",
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age: 26,
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gender: "男",
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address: "shenzhen",
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sayHi: function() {
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console.log(this.name);
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}
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};
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// 遍历对象中的属性
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for (var key in obj) {
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console.log("属性名:" + key);
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console.log("属性值:" + obj[key]); // 注意，因为这里的属性名 keu 是变量，所以，如果想获取属性值，不能写成 obj.key，而是要写成 obj[key]
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}
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</script>
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</head>
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<body>
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</body>
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</html>
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```
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打印结果：
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```
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属性名:name
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属性值:smyhvae
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属性名:age
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属性值:26
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属性名:gender
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属性值:男
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属性名:address
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属性值:shenzhen
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属性名:sayHi
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属性值:function() {
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console.log(this.name);
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}
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```
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