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---
title: >-
SyntaxError: a declaration in the head of a for-of loop can't have an
initializer
slug: Web/JavaScript/Reference/Errors/Invalid_for-of_initializer
tags:
- JavaScript
- 语法错误
- 错误提示
translation_of: Web/JavaScript/Reference/Errors/Invalid_for-of_initializer
---
<div>{{jsSidebar("Errors")}}</div>
<h2 id="错误信息">错误信息</h2>
<pre class="syntaxbox">SyntaxError: a declaration in the head of a for-of loop can't have an initializer (Firefox)
SyntaxError: for-of loop variable declaration may not have an initializer. (Chrome)
</pre>
<h2 id="错误类型">错误类型</h2>
<p>{{jsxref("SyntaxError")}}</p>
<h2 id="哪里出错了？">哪里出错了？</h2>
<p><a href="/en-US/docs/Web/JavaScript/Reference/Statements/for...of">for...of</a> 循环的头部包含有初始化表达式。也就是对一个变量进行声明并赋值|<code>for (var i = 0 of iterable)</code>|。这在 for-of 循环中是被禁止的。你想要的可能是允许包含初始化器的 <code><a href="/en-US/docs/Web/JavaScript/Reference/Statements/for">for</a></code> 循环形式。</p>
<h2 id="示例">示例</h2>
<h3 id="非法的_for-of_循环形式">非法的 <code>for-of</code> 循环形式</h3>
<pre class="brush: js example-bad">let iterable = [10, 20, 30];
for (let value = 50 of iterable) {
console.log(value);
}
// SyntaxError: a declaration in the head of a for-of loop can't
// have an initializer</pre>
<h3 id="合法的_for-of_循环形式">合法的 <code>for-of</code> 循环形式</h3>
<p>需要将初始化器 (<code>value = 50</code>) 从<code>for-of</code> 循环的头部移除。或许你的本意是给每个值添加 50 的偏移量，在这种情况下，可以在循环体中进行添加。</p>
<pre class="brush: js example-good">let iterable = [10, 20, 30];
for (let value of iterable) {
value += 50;
console.log(value);
}
// 60
// 70
// 80
</pre>
<h2 id="相关内容">相关内容</h2>
<ul>
<li><code><a href="/en-US/docs/Web/JavaScript/Reference/Statements/for...of">for...of</a></code></li>
<li><code><a href="/en-US/docs/Web/JavaScript/Reference/Statements/for...in">for...in</a></code> – 在严格模式下也同样禁止使用初始化器 (<a href="/en-US/docs/Web/JavaScript/Reference/Errors/Invalid_for-in_initializer">SyntaxError: for-in loop head declarations may not have initializers</a>)</li>
<li><code><a href="/en-US/docs/Web/JavaScript/Reference/Statements/for">for</a></code> – 在迭代时允许定义初始化器</li>
</ul>
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