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+---
+title: String.prototype.trimStart()
+slug: Web/JavaScript/Reference/Global_Objects/String/TrimLeft
+tags:
+  - JavaScript
+  - Method
+  - Prototype
+  - String
+  - 参考
+  - 字符串
+  - 方法
+translation_of: Web/JavaScript/Reference/Global_Objects/String/trimStart
+---
+<div>{{JSRef}}</div>
+
+<div><strong><code>trimStart()</code> </strong>方法从字符串的开头删除空格。<code>trimLeft()</code> 是此方法的别名。</div>
+
+<div>{{EmbedInteractiveExample("pages/js/string-trimstart.html")}}</div>
+
+
+
+<h2 id="语法">语法</h2>
+
+<pre class="syntaxbox"><var>str</var>.trimStart();
+<var>str</var>.trimLeft();</pre>
+
+<h3 id="返回值">返回值</h3>
+
+<p>一个新字符串，表示从其开头（左端）除去空格的调用字符串。</p>
+
+<h2 id="描述">描述</h2>
+
+<p><code>trimStart()</code> / <code>trimLeft()</code> 方法移除原字符串左端的连续空白符并返回一个新字符串，并不会直接修改原字符串本身。</p>
+
+<h3 id="别名">别名</h3>
+
+<p>为了与 {{jsxref("String.prototype.padStart")}} 等函数保持一致，标准方法名称为<code>trimStart</code>。 但是，出于 Web 兼容性原因，<code>trimLeft</code> 仍然是 <code>trimStart</code> 的别名。在某些引擎中，这意味着：</p>
+
+<pre class="brush: js">String.prototype.trimLeft.name === "trimStart";</pre>
+
+<h2 id="示例">示例</h2>
+
+<h3 id="使用_trimStart">使用 <code>trimStart()</code></h3>
+
+<p>下面的例子输出了小写的字符串 <code>"foo  "</code>：</p>
+
+<pre class="brush:js;highlight:[5]">var str = "   foo  ";
+
+console.log(str.length); // 8
+
+str = str.trimStart()    // 等同于 str = str.trimLeft();
+console.log(str.length); // 5
+console.log(str);        // "foo  "
+</pre>
+
+<h2 id="规范">规范</h2>
+
+<table>
+ <thead>
+  <tr>
+   <th scope="col">规范</th>
+   <th scope="col">状态</th>
+   <th scope="col">备注</th>
+  </tr>
+ </thead>
+ <tbody>
+  <tr>
+   <td><code><a href="https://github.com/tc39/proposal-string-left-right-trim/#stringprototypetrimstart--stringprototypetrimend">String.prototype.{trimStart,trimEnd}</a></code>proposal</td>
+   <td>Stage 4</td>
+   <td>Expected to be part of ES2019</td>
+  </tr>
+ </tbody>
+</table>
+
+<h2 id="浏览器兼容性">浏览器兼容性</h2>
+
+<p class="hidden">The compatibility table in this page is generated from structured data. If you'd like to contribute to the data, please check out <a href="https://github.com/mdn/browser-compat-data">https://github.com/mdn/browser-compat-data</a> and send us a pull request.</p>
+
+<p>{{Compat("javascript.builtins.String.trimStart")}}</p>
+
+<h2 id="Polyfill">Polyfill</h2>
+
+<pre class="brush: js">// https://github.com/FabioVergani/js-Polyfill_String-trimStart
+
+(function(w){
+    var String=w.String, Proto=String.prototype;
+
+    (function(o,p){
+        if(p in o?o[p]?false:true:true){
+            var r=/^\s+/;
+            o[p]=o.trimLeft||function(){
+                return this.replace(r,'')
+            }
+        }
+    })(Proto,'trimStart');
+
+})(window);
+
+
+/*
+ES6:
+(w=&gt;{
+    const String=w.String, Proto=String.prototype;
+
+    ((o,p)=&gt;{
+        if(p in o?o[p]?false:true:true){
+            const r=/^\s+/;
+            o[p]=o.trimLeft||function(){
+                return this.replace(r,'')
+            }
+        }
+    })(Proto,'trimStart');
+
+})(window);
+*/</pre>
+
+<h2 id="参见">参见</h2>
+
+<ul>
+ <li>{{jsxref("String.prototype.trim()")}}</li>
+ <li>{{jsxref("String.prototype.trimEnd()")}}</li>
+</ul>