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- <h1 id="instanceof运算符">instanceof运算符</h1>
- <h4 id="原文"><a href="https://developer.mozilla.org/zh-CN/docs/Web/JavaScript/Reference/Operators/instanceof" target="_blank">原文</a></h4>
- <hr>
- <h2 id="语法">语法</h2>
- <pre><code>object instanceof constructor
- </code></pre><blockquote>
- <p>参数<br></p>
- <ol>
- <li>object 要检测的对象.<br></li>
- <li>constructor 某个构造函数</li>
- </ol>
- </blockquote>
- <h2 id="描述">描述</h2>
- <blockquote>
- <p>instanceof 运算符用来检测 constructor.prototype 是否存在于参数 object 的原型链上。</p>
- </blockquote>
- <pre><code>// 定义构造函数
- function C(){}
- function D(){}
- var o = new C();
- // true，因为 Object.getPrototypeOf(o) === C.prototype
- o instanceof C;
- // false，因为 D.prototype不在o的原型链上
- o instanceof D;
- o instanceof Object; // true,因为Object.prototype.isPrototypeOf(o)返回true
- C.prototype instanceof Object // true,同上
- C.prototype = {};
- var o2 = new C();
- o2 instanceof C; // true
- o instanceof C; // false,C.prototype指向了一个空对象,这个空对象不在o的原型链上.
- D.prototype = new C(); // 继承
- var o3 = new D();
- o3 instanceof D; // true
- o3 instanceof C; // true
- </code></pre><blockquote>
- <p>需要注意的是，如果表达式 obj instanceof Foo 返回true，则并不意味着该表达式会永远返回true，因为Foo.prototype属性的值有可能会改变，改变之后的值很有可能不存在于obj的原型链上，这时原表达式的值就会成为false。另外一种情况下，原表达式的值也会改变，就是改变对象obj的原型链的情况，虽然在目前的ES规范中，我们只能读取对象的原型而不能改变它，但借助于非标准的<strong>proto</strong>魔法属性，是可以实现的。比如执行obj.<strong>proto</strong> = {}之后，obj instanceof Foo就会返回false了。</p>
- </blockquote>
- <p>instanceof和多全局对象(多个frame或多个window之间的交互)</p>
- <blockquote>
- <p>在浏览器中，我们的脚本可能需要在多个窗口之间进行交互。多个窗口意味着多个全局环境，不同的全局环境拥有不同的全局对象，从而拥有不同的内置类型构造函数。这可能会引发一些问题。比如，表达式 [] instanceof window.frames[0].Array 会返回false，因为 Array.prototype !== window.frames[0].Array.prototype，因此你必须使用 Array.isArray(myObj) 或者 Object.prototype.toString.call(myObj) === "[object Array]"来判断myObj是否是数组。</p>
- </blockquote>
- <h2 id="例子">例子</h2>
- <p>例子: 表明String对象和Date对象都属于Object类型</p>
- <blockquote>
- <p>下面的代码使用了instanceof来证明：String和Date对象同时也属于Object类型。</p>
- </blockquote>
- <pre><code>var simpleStr = "This is a simple string";
- var myString = new String();
- var newStr = new String("String created with constructor");
- var myDate = new Date();
- var myObj = {};
- simpleStr instanceof String; // returns false, 检查原型链会找到 undefined
- myString instanceof String; // returns true
- newStr instanceof String; // returns true
- myString instanceof Object; // returns true
- myObj instanceof Object; // returns true, despite an undefined prototype
- ({}) instanceof Object; // returns true, 同上
- myString instanceof Date; // returns false
- myDate instanceof Date; // returns true
- myDate instanceof Object; // returns true
- myDate instanceof String; // returns false
- </code></pre><p>演示mycar属于Car类型的同时又属于Object类型</p>
- <blockquote>
- <p>下面的代码创建了一个类型Car，以及该类型的对象实例mycar. instanceof运算符表明了这个mycar对象既属于Car类型，又属于Object类型。</p>
- </blockquote>
- <pre><code>function Car(make, model, year) {
- this.make = make;
- this.model = model;
- this.year = year;
- }
- var mycar = new Car("Honda", "Accord", 1998);
- var a = mycar instanceof Car; // 返回 true
- var b = mycar instanceof Object; // 返回 true
- </code></pre><h3 id="instanceof运算符在数组中的使用">instanceof运算符在数组中的使用</h3>
- <p>确定一个对象在全局中是否为数组</p>
- <pre><code class="lang-js"><span class="hljs-keyword">var</span> colors=[<span class="hljs-string">'aa'</span>,<span class="hljs-string">'bb'</span>,<span class="hljs-string">'cc'</span>];<span class="hljs-comment">//创建包含三个字符串的数组</span>
- <span class="hljs-keyword">if</span> (colors <span class="hljs-keyword">instanceof</span> <span class="hljs-built_in">Array</span>){
- alert(<span class="hljs-number">1</span>)
- }<span class="hljs-keyword">else</span> {
- alert(<span class="hljs-number">0</span>)
- }
- <span class="hljs-comment">//最后输出 1，说明该对象为数组</span>
- </code></pre>
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