---
title: 條件運算子
slug: Web/JavaScript/Reference/Operators/Conditional_Operator
translation_of: Web/JavaScript/Reference/Operators/Conditional_Operator
---
<div>{{jsSidebar("Operators")}}</div>

<p><strong>條件 (三元) 運算子</strong> 是 JavaScript 唯一用到三個運算元的運算子：在一個條件後面會跟著一個問號 (<code>?</code>)，如果條件是 <a href="/en-US/docs/Glossary/truthy">truthy</a>，在冒號(<code>:</code>)前的表達式會被執行，如果條件是 <a href="/en-US/docs/Glossary/falsy">falsy</a>，在冒號後面的表達式會被執行，這個運算子常常被用來當作 <a href="/en-US/docs/Web/JavaScript/Reference/Statements/if...else"><code>if</code></a> 的簡潔寫法.</p>

<div>{{EmbedInteractiveExample("pages/js/expressions-conditionaloperators.html")}}</div>

<h2 id="語法">語法</h2>

<pre class="syntaxbox"><var>condition</var> ? <var>exprIfTrue</var> : <var>exprIfFalse</var></pre>

<h3 id="參數">參數</h3>

<dl>
 <dt><code><var>condition</var></code></dt>
 <dd>值用來做為條件的表達式</dd>
 <dt><code><var>exprIfTrue</var></code></dt>
 <dd>如果 <code><var>condition</var></code> 的值是 <a href="/en-US/docs/Glossary/truthy">truthy</a> (等於或是可轉換為 <code>true</code>) , <code><var>exprIfTrue</var></code>  會被執行</dd>
 <dt><code><var>exprIfFalse</var></code></dt>
 <dd>如果 <code><var>condition</var></code> 的值是 <a href="/en-US/docs/Glossary/falsy">falsy</a> (等於或是可轉換為 <code>false</code>) , <code><var>exprIfFalse</var></code>  會被執行</dd>
</dl>

<h2 id="描述">描述</h2>

<p>除了 <code>false</code>, 可能是 falsy 的表達式有 <code>null</code>, <code>NaN</code>, <code>0</code>, 空字串 (<code>""</code>) 和 <code>undefined</code>. 如果<code><var>condition</var></code> 是他們其中之一 , 那麼條件表達式的結果會是 <code>exprIfFalse</code> 的執行結果. </p>

<p>一個簡單的範例:</p>

<pre class="brush: js">var age = 26;
var beverage = (age &gt;= 21) ? "Beer" : "Juice";
console.log(beverage); // "Beer"
</pre>

<p>一個常用來處理 <code>null</code> 的用法 : </p>

<pre class="brush: js">function greeting(person) {
    var name = person ? person.name : "stranger";
    return "Howdy, " + name;
}

console.log(greeting({name: 'Alice'}));  // "Howdy, Alice"
console.log(greeting(null));             // "Howdy, stranger"
</pre>

<h3 id="條件鏈">條件鏈</h3>

<p>條件 (三元) 運算子是右相依性的 (right-associative), 代表他可以以下面的方式鏈結 , 類似於 <code>if … else if … else if … else</code> 的鏈結方法 :</p>

<pre class="brush: js">function example(…) {
    return condition1 ? value1
         : condition2 ? value2
         : condition3 ? value3
         : value4;
}

// Equivalent to:

function example(…) {
    if (condition1) { return value1; }
    else if (condition2) { return value2; }
    else if (condition3) { return value3; }
    else { return value4; }
}
</pre>

<h2 id="規範">規範</h2>

<table class="standard-table">
 <thead>
  <tr>
   <th scope="col">Specification</th>
  </tr>
 </thead>
 <tbody>
  <tr>
   <td>{{SpecName('ESDraft', '#sec-conditional-operator', 'Conditional Operator')}}</td>
  </tr>
 </tbody>
</table>

<h2 id="瀏覽器相容性">瀏覽器相容性</h2>

<p>{{Compat("javascript.operators.conditional")}}</p>

<h2 id="參見">參見</h2>

<ul>
 <li><a href="/en-US/docs/Web/JavaScript/Reference/Statements/if...else">if statement</a></li>
 <li><a href="/en-US/docs/Web/JavaScript/Reference/Operators/Nullish_coalescing_operator">Nullish coalescing operator</a></li>
 <li><a href="/en-US/docs/Web/JavaScript/Reference/Operators/Optional_chaining">Optional chaining</a></li>
 <li><a href="/en-US/docs/Learn/JavaScript/Building_blocks/conditionals">Making decisions in your code — conditionals</a></li>
 <li><a href="/en-US/docs/Web/JavaScript/Guide/Expressions_and_Operators">Expressions and operators</a></li>
</ul>