diff options
author | Peter Bengtsson <mail@peterbe.com> | 2020-12-08 14:40:17 -0500 |
---|---|---|
committer | Peter Bengtsson <mail@peterbe.com> | 2020-12-08 14:40:17 -0500 |
commit | 33058f2b292b3a581333bdfb21b8f671898c5060 (patch) | |
tree | 51c3e392513ec574331b2d3f85c394445ea803c6 /files/zh-cn/web/javascript/reference/errors/invalid_const_assignment | |
parent | 8b66d724f7caf0157093fb09cfec8fbd0c6ad50a (diff) | |
download | translated-content-33058f2b292b3a581333bdfb21b8f671898c5060.tar.gz translated-content-33058f2b292b3a581333bdfb21b8f671898c5060.tar.bz2 translated-content-33058f2b292b3a581333bdfb21b8f671898c5060.zip |
initial commit
Diffstat (limited to 'files/zh-cn/web/javascript/reference/errors/invalid_const_assignment')
-rw-r--r-- | files/zh-cn/web/javascript/reference/errors/invalid_const_assignment/index.html | 90 |
1 files changed, 90 insertions, 0 deletions
diff --git a/files/zh-cn/web/javascript/reference/errors/invalid_const_assignment/index.html b/files/zh-cn/web/javascript/reference/errors/invalid_const_assignment/index.html new file mode 100644 index 0000000000..330bab78c8 --- /dev/null +++ b/files/zh-cn/web/javascript/reference/errors/invalid_const_assignment/index.html @@ -0,0 +1,90 @@ +--- +title: 'TypeError: invalid assignment to const "x"' +slug: Web/JavaScript/Reference/Errors/Invalid_const_assignment +tags: + - JavaScript + - 类型错误 + - 错误提示 +translation_of: Web/JavaScript/Reference/Errors/Invalid_const_assignment +--- +<div>{{jsSidebar("Errors")}}</div> + +<h2 id="错误提示">错误提示</h2> + +<pre class="syntaxbox">TypeError: invalid assignment to const "x" (Firefox) +TypeError: Assignment to constant variable. (Chrome) +TypeError: Redeclaration of const 'x' (IE/Edge) +</pre> + +<h2 id="错误类型">错误类型</h2> + +<p>{{jsxref("TypeError")}}</p> + +<h2 id="哪里出错了?">哪里出错了?</h2> + +<p>常量指的是无法在程序正常运行过程中进行修改的值。一方面无法通过重新赋值进行修改,另外一方面也无法进行重新声明。在 JavaScript 中,常量通过关键字 <code><a href="/en-US/docs/Web/JavaScript/Reference/Statements/const">const</a></code> 来声明。</p> + +<h2 id="示例">示例</h2> + +<h3 id="不合法的二次声明">不合法的二次声明</h3> + +<p>在同一作用域内为相同的常量名进行赋值会报错。</p> + +<pre class="brush: js example-bad">const COLUMNS = 80; + +// ... + +COLUMNS = 120; // TypeError: invalid assignment to const `COLUMNS'</pre> + +<h3 id="问题修复">问题修复</h3> + +<p>修复的方式有很多种。可以根据你想要达到的目的来有针对性地对其进行处理。</p> + +<h4 id="重新命名">重新命名</h4> + +<p>如果想要声明另一个变量,那么请选择其他名称对其重新命名。原来的常量名在该作用域中已经被占有。</p> + +<pre class="brush: js example-good">const COLUMNS = 80; +const WIDE_COLUMNS = 120;</pre> + +<h4 id="const_let_or_var"><code>const</code>, <code>let</code> or <code>var</code>?</h4> + +<p>如果你的目的不是为了创建一个常量的话,那么就不要使用 const 关键字。可以使用 <code><a href="/en-US/docs/Web/JavaScript/Reference/Statements/let">let</a></code> 关键字来声明一个拥有块作用域的变量,或者使用 <code><a href="/en-US/docs/Web/JavaScript/Reference/Statements/var">var</a> </code>来声明一个全局变量。</p> + +<pre class="brush: js example-good">let columns = 80; + +// ... + +let columns = 120; +</pre> + +<h4 id="作用域">作用域</h4> + +<p>检查一下作用域是否正确。例如这个常量是否应该出现在当前作用域,还是应该出现在函数内部?</p> + +<pre class="brush: js example-good">const COLUMNS = 80; + +function setupBigScreenEnvironment() { + const COLUMNS = 120; +}</pre> + +<h3 id="const与不可变性"><code>const与不可变性</code></h3> + +<p>const 声明语句创建了一个对值的只读引用。这并<strong>不</strong>意味着它指向的值是不可变的,而是说这个变量标记符不能被再次分配。例如,在值是对象的情况下,引用的对象自身内容依然是可以改变的。不能改变该变量的引用:</p> + +<pre class="brush: js example-bad">const obj = {foo: 'bar'}; +obj = {foo: 'baz'}; // TypeError: invalid assignment to const `obj' +</pre> + +<p>但是可以改变它引用的值的属性: </p> + +<pre class="brush: js example-good">obj.foo = 'baz'; +obj; // Object { foo: "baz" }</pre> + +<h2 id="相关内容">相关内容</h2> + +<ul> + <li><code><a href="/en-US/docs/Web/JavaScript/Reference/Statements/const">const</a></code></li> + <li><code><a href="/en-US/docs/Web/JavaScript/Reference/Statements/let">let</a></code></li> + <li><code><a href="/en-US/docs/Web/JavaScript/Reference/Statements/var">var</a></code></li> +</ul> |