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<header>Updated: 2017-04-13 18:52:38 CST +08</header>
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<h1 id="html-input-defaultvalue-vs-value-with-jquery-val-vs-attr">HTML Input defaultValue vs. value with jQuery val() vs. attr()</h1>
<h2 id="problem">Problem</h2>
<p>最近遇到有人想要把現有的 input tag 使用 .val(value) 改值以後，append 到另一個 form 上，結果發現怎麼 input 呈現的 value 沒變？！</p>
<p>但實驗過後 .attr(‘value’, value) 可以成功！</p>
<p>那這個 attribute 和 value 到底哪裡不一樣呢？</p>
<p>讓我們看看以下的例子</p>
<h2 id="example">Example</h2>
<pre><code class="language-html">
<form><input name="test" type="text" value="abc" />
<input name="test2" type="text" 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 name="test" type="text" value="abc" />
<input name="test2" type="text" value="123" />
</code></pre>
<p>我們可以看到改變 attribute 的話元素本身的 html 也會被改變。
但是改變 value 的話，元素本身的 html 是不會被改變的。</p>
<h2 id="explain">Explain</h2>
<p>為什麼呢？</p>
<p>這要從<a href="http://www.w3.org/TR/DOM-Level-2-HTML/html.html#ID-6043025">Input 的屬性</a>來看，可以看到裡面有 defaultValue 與 value 兩個關於 value 的值。</p>
<p>在 html 一開始 render 的時候，defaultValue 和 value 會是一樣的。</p>
<p>在 input 的欄位打上你想要的值，或是使用 javascript 改變 input 的 value 時，改變的是它的 value，defaultValue 並不會被改變，它依舊會是一開始被 render 時預設的那個值。</p>
<p>使用 value, val() 方法，改變的是蓋在 input 元素上面的東西，而不會真正改變它的本質。</p>
<p>而因為元素的 html 並沒有真正的被改變，所以使用 html 方法時，拿到的就會是它最原始的樣子。</p>
<p>要改變元素的本質，必須更改元素 value 的 attribute，也就是它的 defaultValue，則需要使用 attribute 相關 method: setAttribute, attr()。</p>
<p>如果想要更瞭解，可以查查 value attribute 與 value property。
以為這一切都沒事了嗎？</p>
<p>不，把 type 改成 hidden，value 和 val() 卻都成功改變了 html…</p>
<p>因為這實在是太 geek 了，所以寫另一篇探討。</p>
<p><a href="https://dinos80152.wordpress.com/2015/10/07/html-input-hidden-defaultvalue-vs-value-with-jquery-val-vs-attr/">HTML Input hidden defaultValue vs. value with jQuery val() vs. attr()</a></p>
<h3 id="reference">Reference</h3>
<ul>
<li><a href="http://www.w3schools.com/jsref/prop_text_defaultvalue.asp">Input Text defaultValue Property@W3Schools</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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