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<header>Updated: 2017-04-13 18:52:38 CST +08</header>
<main>
<h1 id="html-input-hidden-defaultvalue-vs-value-with-jquery-val-vs-attr">HTML Input hidden defaultValue vs. value with jQuery val() vs. attr()</h1>
<h2 id="problem">Problem</h2>
<p>承續上一篇 <a href="https://dinos80152.wordpress.com/2015/10/07/html-input-defaultvalue-vs-value-jquery-val-vs-attr/">HTML Input defaultValue vs. value, jQuery val() vs. attr()</a></p>
<p>這篇探討為什麼 type 使用 hidden，html 就成功的被改變了呢？</p>
<h2 id="example">Example</h2>
<p>範例如下</p>
<pre><code class="language-html"><form>
<input type="hidden" name="test" value="abc">
<input type="hidden" name="test2" value="abc">
</form>
</code></pre>
<p>Javascript</p>
<pre><code class="language-js">document.getElementsByName('test')[0].value = 123;
document.getElementsByName('test')[0].setAttribute('value', 123);
console.log(document.getElementsByTagName('form')[0].innerHTML);
</code></pre>
<p>jQuery</p>
<pre><code class="language-js">$('input[name="test"]').val(123);
$('input[name="test"]').attr('value', 123);
console.log($('form').html());
</code></pre>
<p>Output</p>
<pre><code class="language-html"><input type="text" name="test" value="123">
<input type="text" name="test2" value="123">
</code></pre>
<p>原因很簡單，一樣參考<a href="http://www.w3.org/TR/DOM-Level-2-HTML/html.html#ID-6043025">Input 的屬性</a>。</p>
<p>我們可以看到 defaultValue 是只有 <strong>text</strong>, <strong>file</strong>, <strong>password</strong> 才有的屬性，所以 hidden 這個 type 並沒有 defaultValue，只有 value，因為在制定時，hidden 的值是不會被 user 改變的。</p>
<p>因為沒有 defaultValue，所以 input type hidden 上原本的 value attribute 就是對到 value property，所以當你使用 js 改變 value 時，它的 html 也會一起被改變了。</p>
<p>但這邊最奇怪的地方，就算它沒有 defaultValue 這個屬性，但依舊可以使用</p>
<pre><code class="language-js">document.getElementsByName('test')[0].setAttribute('value', 123);
document.getElementsByName('test')[0].defaultValue = 123;
</code></pre>
<p>這兩個 method 則會去改 input type=hidden 元素的 value 了。</p>
<p>這該說方便還是讓人混淆呢..?</p>
<p>或許之後有機會看到更多討論再來做更多的釐清吧！</p>
<h3 id="reference">Reference</h3>
<ul>
<li><a href="http://stackoverflow.com/questions/5319678/javascript-and-defaultvalue-of-hidden-input-elements">Javascript and defaultValue of hidden input elements@stackoverflow</a></li>
<li><a href="http://forums.mozillazine.org/viewtopic.php?f=25&t=1787115">Firefox will not reset hidden fields@mozillazine</a></li>
<li><a href="http://www.w3.org/TR/DOM-Level-2-HTML/html.html#ID-6043025">Document Object Model HTML-Interface HTMLInputElement</a></li>
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