diff options
Diffstat (limited to 'files/zh-cn/web/javascript/reference/errors/read-only/index.html')
-rw-r--r-- | files/zh-cn/web/javascript/reference/errors/read-only/index.html | 76 |
1 files changed, 76 insertions, 0 deletions
diff --git a/files/zh-cn/web/javascript/reference/errors/read-only/index.html b/files/zh-cn/web/javascript/reference/errors/read-only/index.html new file mode 100644 index 0000000000..ecdc225497 --- /dev/null +++ b/files/zh-cn/web/javascript/reference/errors/read-only/index.html @@ -0,0 +1,76 @@ +--- +title: 'TypeError: "x" is read-only' +slug: Web/JavaScript/Reference/Errors/Read-only +translation_of: Web/JavaScript/Reference/Errors/Read-only +--- +<div>{{jsSidebar("Errors")}}</div> + +<h2 id="报错消息">报错消息</h2> + +<pre class="syntaxbox">TypeError: "x" is read-only (Firefox) //格式错误:"x"只读。(x可以代表任意值) +TypeError: 0 is read-only (Firefox) +TypeError: Cannot assign to read only property 'x' of #<Object> (Chrome) +//格式错误:对象的x属性是只读的不能设置 (chrome) +TypeError: Cannot assign to read only property '0' of [object Array] (Chrome) +</pre> + +<h2 id="错误格式">错误格式</h2> + +<p>{{jsxref("TypeError")}}</p> + +<h2 id="哪里出错了">哪里出错了?</h2> + +<p>全局变量或对象属性被设置为只读 (专业点讲,就是这条数据属性禁止写入.)</p> + +<p>这条错误值发生在<a href="/en-US/docs/Web/JavaScript/Reference/Strict_mode">strict mode code</a>(俗称严格模式). 正常情况下,是没有报错的。</p> + +<h2 id="例如">例如</h2> + +<h3 id="无效例子(也就是下面这么做会导致报这种错)">无效例子(也就是下面这么做会导致报这种错)</h3> + +<p>只读属性不能直接创建, 但我们可以通过{{jsxref("Object.defineProperty()")}} 或 {{jsxref("Object.freeze()")}}设置.</p> + +<pre class="brush: js example-bad">"use strict"; +var obj = Object.freeze({name: "Elsa", score: 157}); +obj.score = 0; // TypeError + +"use strict"; +Object.defineProperty(this, "LUNG_COUNT", {value: 2, writable: false}); +LUNG_COUNT = 3; // TypeError + +"use strict"; +var frozenArray = Object.freeze([0, 1, 2]); +frozenArray[0]++; // TypeError + +还有几个JavaScript内置属性. 如果你尝试修改一个常量. + +"use strict"; +Math.PI = 4; // TypeError</pre> + +<p>傻了吧,报错了</p> + +<p><code>全局变量undefined也是只读的</code>, 所以你不能忽视臭名昭著的"undefined is not a function"错误:</p> + +<pre class="brush: js example-bad">"use strict"; +undefined = function () {}; // TypeError: "undefined" is read-only +</pre> + +<h3 id="下面这样都是有效,不报错的">下面这样都是有效,不报错的</h3> + +<pre class="brush: js example-good">"use strict"; +var obj = Object.freeze({name: "Score", points: 157}); +obj = {name: obj.name, points: 0}; // 用一个新对象替换原来的对象(其实就是更改了对象的指针) + +"use strict"; +var LUNG_COUNT = 2; // +LUNG_COUNT = 3; // +</pre> + +<h2 id="参见">参见</h2> + +<ul> + <li>{{jsxref("Object.defineProperty()")}}</li> + <li>{{jsxref("Object.freeze()")}}</li> + <li><a href="https://www.answers.com/Q/Which_animals_have_three_lungs">"Which animals have three lungs?" on answers.com</a></li> + <li><a href="https://aliens.wikia.com/wiki/Klingon">Klingons</a> (another answer to that query)</li> +</ul> |