---
title: String.prototype.padStart()
slug: Web/JavaScript/Reference/Global_Objects/String/padStart
translation_of: Web/JavaScript/Reference/Global_Objects/String/padStart
---
<div>{{JSRef}}{{SeeCompatTable}}</div>

<p><strong><code>padStart()</code></strong><code> 會將用給定用於填充的字串，以重複的方式，插入到目標字串的起頭(左側)，直到目標字串到達指定長度。</code></p>

<h2 id="Syntax">Syntax</h2>

<pre class="syntaxbox"><var>str</var>.padStart(<var>targetLength</var> [, <var>padString</var>])</pre>

<h3 id="Parameters">Parameters</h3>

<dl>
 <dt><code>targetLength</code></dt>
 <dd>目標字串被填充後的長度。如果此參數小於原字串的長度，那將直接返回原字串。</dd>
 <dt><code>padString</code> {{optional_inline}}</dt>
 <dd>用來填充的字串。如果填充字串太長，則由左側開始，擷取所需要的長度。此參數預設值是空白 " " (U+0020).</dd>
</dl>

<h3 id="Return_value">Return value</h3>

<p>目標字串被填充到指定長度後，所得的新字串。</p>

<h2 id="Examples">Examples</h2>

<pre class="brush: js">'abc'.padStart(10);         // "       abc"
'abc'.padStart(10, "foo");  // "foofoofabc"
'abc'.padStart(6,"123465"); // "123abc"
</pre>

<h2 id="Specifications">Specifications</h2>

<p>這個方法還沒有被納入 ECMAScript 標準。現在還只是個<a href="https://github.com/tc39/proposal-string-pad-start-end">提案</a>。</p>

<h2 id="Browser_compatibility">Browser compatibility</h2>

<div>{{CompatibilityTable}}</div>

<div id="compat-desktop">
<table class="compat-table">
 <tbody>
  <tr>
   <th>Feature</th>
   <th>Chrome</th>
   <th>Edge</th>
   <th>Firefox (Gecko)</th>
   <th>Internet Explorer</th>
   <th>Opera</th>
   <th>Safari</th>
  </tr>
  <tr>
   <td>Basic support</td>
   <td>{{CompatChrome(57)}} </td>
   <td>15</td>
   <td>{{CompatGeckoDesktop(51)}}</td>
   <td>{{CompatNo}}</td>
   <td>{{CompatOpera(44)}}</td>
   <td>{{CompatSafari(10)}}</td>
  </tr>
 </tbody>
</table>
</div>

<div id="compat-mobile">
<table class="compat-table">
 <tbody>
  <tr>
   <th>Feature</th>
   <th>Android</th>
   <th>Chrome for Android</th>
   <th>Firefox Mobile (Gecko)</th>
   <th>IE Mobile</th>
   <th>Opera Mobile</th>
   <th>Safari Mobile</th>
  </tr>
  <tr>
   <td>Basic support</td>
   <td>{{CompatUnknown}}</td>
   <td>{{CompatChrome(57)}}</td>
   <td>{{CompatGeckoMobile(51)}}</td>
   <td>{{CompatUnknown}}</td>
   <td>{{CompatUnknown}}</td>
   <td>{{CompatSafari(10)}}</td>
  </tr>
 </tbody>
</table>
</div>

<h2 id="See_also">See also</h2>

<ul>
 <li>{{jsxref("String.padEnd()")}}</li>
</ul>