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authorPeter Bengtsson <mail@peterbe.com>2020-12-08 14:40:17 -0500
committerPeter Bengtsson <mail@peterbe.com>2020-12-08 14:40:17 -0500
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+---
+title: 'null'
+slug: Web/JavaScript/Reference/Global_Objects/null
+tags:
+ - JavaScript
+ - Literal
+ - Primitive
+translation_of: Web/JavaScript/Reference/Global_Objects/null
+---
+<div>{{jsSidebar("Objects")}}</div>
+
+<p>值 <code>null</code> 特指对象的值未设置。它是 JavaScript {{Glossary("Primitive", "基本类型")}} 之一,在布尔运算中被认为是<a href="https://developer.mozilla.org/en-US/docs/Glossary/Falsy">falsy</a>。</p>
+
+<div>{{EmbedInteractiveExample("pages/js/globalprops-null.html")}}</div>
+
+
+
+<h2 id="语法">语法</h2>
+
+<pre class="syntaxbox">null</pre>
+
+<h2 id="描述">描述</h2>
+
+<p>值 <code>null</code> 是一个字面量,不像 {{jsxref("Global_Objects/undefined","undefined")}},它不是全局对象的一个属性。<code>null</code> 是表示缺少的标识,指示变量未指向任何对象。把 <code>null</code> 作为尚未创建的对象,也许更好理解。在 API 中,<code>null</code> 常在返回类型应是一个对象,但没有关联的值的地方使用。</p>
+
+<pre class="brush: js">// foo 不存在,它从来没有被定义过或者是初始化过:
+foo;
+"ReferenceError: foo is not defined"
+
+// foo 现在已经是知存在的,但是它没有类型或者是值:
+var foo = null;
+foo;
+null</pre>
+
+<h3 id="null_与_undefined_的不同点:"><code>null</code> 与 <code>undefined</code> 的不同点:</h3>
+
+<p>当检测 <code>null</code> 或 <code>undefined</code> 时,注意<a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Comparison_Operators">相等(==)与全等(===)两个操作符的区别</a> ,前者会执行类型转换:</p>
+
+<pre class="brush: js">typeof null        // "object" (因为一些以前的原因而不是'null')
+typeof undefined   // "undefined"
+null === undefined // false
+null  == undefined // true
+null === null // true
+null == null // true
+!null //true
+isNaN(1 + null) // false
+isNaN(1 + undefined) // true
+</pre>
+
+<h2 id="规范">规范</h2>
+
+<table class="standard-table">
+ <tbody>
+ <tr>
+ <th scope="col">规范版本</th>
+ <th scope="col">规范状态</th>
+ <th scope="col">注解</th>
+ </tr>
+ <tr>
+ <td>{{SpecName('ES1')}}</td>
+ <td>{{Spec2('ES1')}}</td>
+ <td>Initial definition.</td>
+ </tr>
+ <tr>
+ <td>{{SpecName('ES5.1', '#sec-4.3.11', 'null value')}}</td>
+ <td>{{Spec2('ES5.1')}}</td>
+ <td></td>
+ </tr>
+ <tr>
+ <td>{{SpecName('ES6', '#sec-null-value', 'null value')}}</td>
+ <td>{{Spec2('ES6')}}</td>
+ <td></td>
+ </tr>
+ <tr>
+ <td>{{SpecName('ESDraft', '#sec-null-value', 'null value')}}</td>
+ <td>{{Spec2('ESDraft')}}</td>
+ <td></td>
+ </tr>
+ </tbody>
+</table>
+
+<h2 id="浏览器兼容性">浏览器兼容性</h2>
+
+
+
+<p>{{Compat("javascript.builtins.null")}}</p>
+
+<h2 id="参见">参见</h2>
+
+<ul>
+ <li>{{jsxref("undefined")}}</li>
+ <li>{{jsxref("NaN")}}</li>
+</ul>