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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: 'SyntaxError: test for equality (==) mistyped as assignment (=)?'
+slug: Web/JavaScript/Reference/Errors/Equal_as_assign
+tags:
+ - 语法错误
+translation_of: Web/JavaScript/Reference/Errors/Equal_as_assign
+---
+<div>{{jsSidebar("Errors")}}</div>
+
+<h2 id="消息">消息</h2>
+
+<pre class="syntaxbox">Warning: SyntaxError: test for equality (==) mistyped as assignment (=)?
+</pre>
+
+<h2 id="错误类型">错误类型</h2>
+
+<p>{{jsxref("SyntaxError")}} 只在<a href="/en-US/docs/Web/JavaScript/Reference/Strict_mode">严格模式</a>下会出现的警告。</p>
+
+<h2 id="什么地方出错了">什么地方出错了?</h2>
+
+<p>在通常期望进行相等判定(<code>==</code>)的地方出现了赋值<code>(=</code>)。 为了帮助调试,JavaScript(在开启严格模式的情况下)会对这种情况进行警告。</p>
+
+<h2 id="示例">示例</h2>
+
+<h3 id="条件表达式内的赋值">条件表达式内的赋值</h3>
+
+<p>不建议在条件表达式中 (例如 <code><a href="/en-US/docs/Web/JavaScript/Reference/Statements/if...else">if...else</a></code>) 使用简单赋值语句,因为在扫视代码的时候赋值操作与相等判定容易产生混淆。例如,不要使用以下写法:</p>
+
+<pre class="brush: js example-bad">if (x = y) {
+ // do the right thing
+}
+</pre>
+
+<p>如果你需要在条件表达式中使用赋值语句, 通常的做法是用一对括号把赋值语句包起来。 例如:</p>
+
+<pre class="brush: js">if ((x = y)) {
+ // do the right thing
+}</pre>
+
+<p>否则, 你的本意可能是想用比较操作符 (如 <code>==</code> 或 <code>===</code>):</p>
+
+<pre class="brush: js">if (x == y) {
+ // do the right thing
+}</pre>
+
+<h2 id="相关页面">相关页面</h2>
+
+<ul>
+ <li><a href="/en-US/docs/Web/JavaScript/Reference/Strict_mode">Strict mode</a></li>
+ <li><code><a href="/en-US/docs/Web/JavaScript/Reference/Statements/if...else">if...else</a></code></li>
+ <li><a href="/en-US/docs/Web/JavaScript/Reference/Operators/Comparison_Operators">Comparison operators</a></li>
+</ul>