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    19. 

  19. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br></div></div><blockquote><p>参数<br></p></blockquote> <ol><li>object 要检测的对象.<br></li> <li>constructor  某个构造函数</li></ol> <h2 id="描述"><a href="#描述" class="header-anchor">#</a> 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> <div class="language- line-numbers-mode"><pre class="language-text"><code>// 定义构造函数
    20. 

  20. function C(){}
    21. 

  21. function D(){}
    22. 

  22. 

  23. var o = new C();
    24. 

  24. 

  25. // true，因为 Object.getPrototypeOf(o) === C.prototype
    26. 

  26. o instanceof C;
    27. 

  27. 

  28. // false，因为 D.prototype不在o的原型链上
    29. 

  29. o instanceof D;
    30. 

  30. 

  31. o instanceof Object; // true,因为Object.prototype.isPrototypeOf(o)返回true
    32. 

  32. C.prototype instanceof Object // true,同上
    33. 

  33. 

  34. C.prototype = {};
    35. 

  35. var o2 = new C();
    36. 

  36. 

  37. o2 instanceof C; // true
    38. 

  38. 

  39. 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.
    40. 

  40. 

  41. D.prototype = new C(); // 继承
    42. 

  42. var o3 = new D();
    43. 

  43. o3 instanceof D; // true
    44. 

  44. o3 instanceof C; // true
    45. 

  45. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br><span class="line-number">15</span><br><span class="line-number">16</span><br><span class="line-number">17</span><br><span class="line-number">18</span><br><span class="line-number">19</span><br><span class="line-number">20</span><br><span class="line-number">21</span><br><span class="line-number">22</span><br><span class="line-number">23</span><br><span class="line-number">24</span><br><span class="line-number">25</span><br><span class="line-number">26</span><br></div></div><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__proto__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) === &quot;[object Array]&quot;来判断myObj是否是数组。</p></blockquote> <h2 id="例子"><a href="#例子" class="header-anchor">#</a> 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> <div class="language- line-numbers-mode"><pre class="language-text"><code>var simpleStr = &quot;This is a simple string&quot;;
    46. 

  46. var myString  = new String();
    47. 

  47. var newStr    = new String(&quot;String created with constructor&quot;);
    48. 

  48. var myDate    = new Date();
    49. 

  49. var myObj     = {};
    50. 

  50. 

  51. simpleStr instanceof String; // returns false, 检查原型链会找到 undefined
    52. 

  52. myString  instanceof String; // returns true
    53. 

  53. newStr    instanceof String; // returns true
    54. 

  54. myString  instanceof Object; // returns true
    55. 

  55. 

  56. myObj instanceof Object;    // returns true, despite an undefined prototype
    57. 

  57. ({})  instanceof Object;    // returns true, 同上
    58. 

  58. 

  59. myString instanceof Date;   // returns false
    60. 

  60. 

  61. myDate instanceof Date;     // returns true
    62. 

  62. myDate instanceof Object;   // returns true
    63. 

  63. myDate instanceof String;   // returns false
    64. 

  64. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br><span class="line-number">15</span><br><span class="line-number">16</span><br><span class="line-number">17</span><br><span class="line-number">18</span><br><span class="line-number">19</span><br></div></div><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> <div class="language- line-numbers-mode"><pre class="language-text"><code>function Car(make, model, year) {
    65. 

  65.   this.make = make;
    66. 

  66.   this.model = model;
    67. 

  67.   this.year = year;
    68. 

  68. }
    69. 

  69. var mycar = new Car(&quot;Honda&quot;, &quot;Accord&quot;, 1998);
    70. 

  70. var a = mycar instanceof Car;    // 返回 true
    71. 

  71. var b = mycar instanceof Object; // 返回 true
    72. 

  72. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br></div></div><h3 id="instanceof运算符在数组中的使用"><a href="#instanceof运算符在数组中的使用" class="header-anchor">#</a> 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> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">var</span> colors<span class="token operator">=</span><span class="token punctuation">[</span><span class="token string">'aa'</span><span class="token punctuation">,</span><span class="token string">'bb'</span><span class="token punctuation">,</span><span class="token string">'cc'</span><span class="token punctuation">]</span><span class="token punctuation">;</span><span class="token comment">//创建包含三个字符串的数组</span>
    73. 

  73. <span class="token keyword">if</span> <span class="token punctuation">(</span>colors <span class="token keyword">instanceof</span> <span class="token class-name">Array</span><span class="token punctuation">)</span><span class="token punctuation">{</span>
    74. 

  74.   <span class="token function">alert</span><span class="token punctuation">(</span><span class="token number">1</span><span class="token punctuation">)</span>
    75. 

  75. <span class="token punctuation">}</span><span class="token keyword">else</span> <span class="token punctuation">{</span>
    76. 

  76.   <span class="token function">alert</span><span class="token punctuation">(</span><span class="token number">0</span><span class="token punctuation">)</span>
    77. 

  77. <span class="token punctuation">}</span>
    78. 

  78. <span class="token comment">//最后输出 1，说明该对象为数组</span>
    79. 

  79. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br></div></div></div> <footer class="page-edit"><!----> <div class="last-updated"><span class="prefix">Last Updated:</span> <span class="time">11/2/2018, 4:24:41 PM</span></div></footer> <div class="page-nav"><p class="inner"><span class="prev">
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