path: root/files/zh-tw/web/javascript/reference/global_objects/string/index.html

---
title: 字串
slug: Web/JavaScript/Reference/Global_Objects/String
tags:
  - ECMAScript 2015
  - JavaScript
  - Reference
  - String
translation_of: Web/JavaScript/Reference/Global_Objects/String
---
<p>{{JSRef("Global_Objects", "String")}}</p>

<p><strong><code>String</code></strong> 全域物件為字串的構造函數，或是一個字符序列。</p>

<h2 id="Syntax" name="Syntax">語法</h2>

<p>字串文字採用下列形式：</p>

<pre>'string text' "string text" "中文 español deutsch English हिन्दी العربية português বাংলা русский 日本語 ਪੰਜਾਬੀ 한국어 தமிழ் עברית"</pre>

<p>字串也可以被全域的 <code>String</code> 物件建立：</p>

<pre>String(thing)</pre>

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

<dl>
 <dt><code>thing</code></dt>
 <dd>任何要轉換成字串的物件。</dd>
</dl>

<h3 id="樣板字面值">樣板字面值</h3>

<p>自 ECMAScript 2015，字串文字也可以是<a href="https://developer.mozilla.org/zh-TW/docs/Web/JavaScript/Reference/Template_literals">樣板字面值（Template literals）</a>：</p>

<pre>`hello world` `hello! world!` `hello ${who}` escape `&lt;a&gt;${who}&lt;/a&gt;`</pre>

<h3 id="跳脫符號">跳脫符號</h3>

<p>除了常規的、可印出來的字元，特殊字元也可以被跳脫符號來表示編碼。</p>

<table>
 <thead>
  <tr>
   <th scope="col">代碼</th>
   <th scope="col">輸出</th>
  </tr>
 </thead>
 <tbody>
  <tr>
   <td><code>\0</code></td>
   <td>空字元</td>
  </tr>
  <tr>
   <td><code>\'</code></td>
   <td>單引號</td>
  </tr>
  <tr>
   <td><code>\"</code></td>
   <td>雙引號</td>
  </tr>
  <tr>
   <td><code>\\</code></td>
   <td>反斜線</td>
  </tr>
  <tr>
   <td><code>\n</code></td>
   <td>斷行</td>
  </tr>
  <tr>
   <td><code>\r</code></td>
   <td>回車</td>
  </tr>
  <tr>
   <td><code>\v</code></td>
   <td>垂直制表</td>
  </tr>
  <tr>
   <td><code>\t</code></td>
   <td>制表</td>
  </tr>
  <tr>
   <td><code>\b</code></td>
   <td>退格</td>
  </tr>
  <tr>
   <td><code>\f</code></td>
   <td>饋頁</td>
  </tr>
  <tr>
   <td><code>\uXXXX</code></td>
   <td>unicode 代碼</td>
  </tr>
  <tr>
   <td><code>\u{X}</code> ... <code>\u{XXXXXX}</code></td>
   <td>unicode 代碼 {{experimental_inline}}</td>
  </tr>
  <tr>
   <td><code>\xXX</code></td>
   <td>Latin-1 字元</td>
  </tr>
 </tbody>
</table>

<dl>
</dl>

<div class="note">
<p>和其他語言不同，JavaScript 將單引號字串和雙引號字串是做相同；因此，上述的序列可以在單引號或雙引號中作用。</p>
</div>

<dl>
</dl>

<h3 id="長字面值字串">長字面值字串</h3>

<p>有些時候，你的程式碼會包含非常長的字串。 為了不讓長字串無止盡地往下長，抑或是在你心血來潮的時候，你可能希望將這樣長的字串能夠斷成多行卻不影響到實際的內容。</p>

<p>你可以用 <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Arithmetic_Operators#Addition_()">+</a> 運算子附加多個字串在一起，像是這樣：</p>

<pre><code>let longString = "This is a very long string which needs " +
                 "to wrap across multiple lines because " +
                 "otherwise my code is unreadable.";</code></pre>

<p>或者，你可以在每一行尾端用反斜線字元("\")表示字串會繼續被顯示在下一列。 你必須要確定在反斜線後面沒有任何空白或其他字元，甚至是縮排；否則這個方法將失效。 這個形式看起來像這樣：</p>

<pre><code>let longString = "This is a very long string which needs \
to wrap across multiple lines because \
otherwise my code is unreadable.";</code></pre>

<p>這兩個方法都會建立相同的字串內容。</p>

<h2 id="Description" name="Description">說明</h2>

<p>字串對於能保留以文字形式表達的資料這件事來說，是有用的。在字串上，一些最常被使用的運算即確認字串長度 {{jsxref("String.length", "length")}} ，用 <a href="/en-US/docs/Web/JavaScript/Reference/Operators/String_Operators">+ 或 += 字串運算元</a> 建造或者串接字串，用 {{jsxref("String.indexOf", "indexOf")}} 方法檢查?子字串是否存在或子字串的位置，或者是用 {{jsxref("String.substring", "substring")}} 方法將子字串抽取出來。</p>

<h3 id="Character_access" name="Character_access">存取字元</h3>

<p>有兩個方法可以存取字串中個別的字元。第一個是用 {{jsxref("String.charAt", "charAt")}} 方法：</p>

<pre class="brush: js">return 'cat'.charAt(1); // 回傳 "a"
</pre>

<p>另一個(在ECMAScript 5中被提到)方法是將字串當作一個類似陣列的物件，直接存取字串中對應的數值索引。</p>

<pre class="brush: js">return 'cat'[1]; // 回傳 "a"
</pre>

<p>對於存取字元使用的括號表達式，沒辦法去刪除或指派一個值給這些屬性。 這些屬性既非可寫的，也非可設定的。(參見 {{jsxref("Object.defineProperty")}})</p>

<h3 id="Comparing_strings" name="Comparing_strings">比較字串</h3>

<p>C 語言的開發者有 <code>strcmp()</code> 函式可以用來比較字串。 在 JavaScript 中，你只能用<a href="/en-US/docs/Web/JavaScript/Reference/Operators/Comparison_Operators">小於和大於運算子</a>：</p>

<pre class="brush: js">var a = "a";
var b = "b";
if (a &lt; b) // true
  print(a + " 小於 " + b);
else if (a &gt; b)
  print(a + " 大於 " + b);
else
  print(a + " 和 " + b + " 相等");
</pre>

<p>這樣類似的結果，也能使用繼承 <code>String</code> 實體的 {{jsxref("String.localeCompare", "localeCompare")}} 方法來實現。</p>

<h3 id="分辨_string_原始型別和_String_物件">分辨 string 原始型別和 <code>String</code> 物件</h3>

<p>請注意，JavaScript 會區別 <code>String</code> 物件和原始字串({{jsxref("Global_Objects/Boolean", "Boolean")}} 和 {{jsxref("Global_Objects/Number", "Numbers")}} 也是如此)。</p>

<p>String literals (denoted by double or single quotes) and strings returned from <code>String</code> calls in a non-constructor context (i.e., without using the <a href="/en-US/docs/Web/JavaScript/Reference/Operators/new"><code>new</code> keyword</a>) are primitive strings. JavaScript automatically converts primitives to <code>String</code> objects, so that it's possible to use <code>String</code> object methods for primitive strings. In contexts where a method is to be invoked on a primitive string or a property lookup occurs, JavaScript will automatically wrap the string primitive and call the method or perform the property lookup.</p>

<pre class="brush: js">var s_prim = "foo";
var s_obj = new String(s_prim);

console.log(typeof s_prim); // 印出 "string"
console.log(typeof s_obj);  // 印出 "object"
</pre>

<p>字串原始型別和 <code>String</code> 物件也會在使用 {{jsxref("Global_Objects/eval", "eval")}} 時給出不同的結果。 原始型別傳進 <code>eval</code> 會被視為原始代碼；<code>String</code> 物件則會回傳，且被視作是其他物件。舉個例子：</p>

<pre class="brush: js">s1 = "2 + 2";               // 建立一個字串原始型別
s2 = new String("2 + 2");   // 建立一個字串物件
console.log(eval(s1));      // 回傳數字 4
console.log(eval(s2));      // 回傳字串 "2 + 2"
</pre>

<p>因為一些原因，程式碼也許在遇到 <code>String</code> 物件時，但需要的卻是字串原始型別；儘管如此，通常作者們不需要擔心它的差異。</p>

<p>一個 <code>String</code> 物件總能夠使用 {{jsxref("String.valueOf", "valueOf")}} 方法被轉換成自身的原始副本。</p>

<pre class="brush: js">console.log(eval(s2.valueOf())); // 回傳數字 4
</pre>

<div class="note"><strong>注意：</strong> 對於在 JavaScript 中其他可用的字串方法，請參閱這篇文章<a href="/en-US/docs/Web/JavaScript/Typed_arrays/StringView" title="/en-US/docs/Web/JavaScript/Typed_arrays/StringView"><code>StringView</code> – a C-like representation of strings based on typed arrays</a>。</div>

<h2 id="Properties" name="Properties">屬性</h2>

<dl>
 <dt>{{jsxref("String.prototype")}}</dt>
 <dd>能讓字串物件增加屬性。</dd>
</dl>

<div>{{jsOverrides("Function", "Properties", "prototype")}}</div>

<h2 id="Methods" name="Methods">方法</h2>

<dl>
 <dt>{{jsxref("String.fromCharCode()")}}</dt>
 <dd>利用 Unicode 值的序列建立並回傳一個字串。</dd>
 <dt>{{jsxref("String.fromCodePoint()")}} {{experimental_inline}}</dt>
 <dd>利用編碼位置的序列建立並回傳一個字串。</dd>
</dl>

<div>{{jsOverrides("Function", "Methods", "fromCharCode", "fromCodePoint")}}</div>

<h2 id="String_通用方法"><code>String</code> 通用方法</h2>

<div class="warning">
<p>字串通用方法是非標準化的、被棄用的，也有近期將被刪除的。</p>
</div>

<p>The <code>String </code>instance methods are also available in Firefox as of JavaScript 1.6 (though not part of the ECMAScript standard) on the String object for applying String methods to any object:</p>

<pre class="brush: js">var num = 15;
alert(String.replace(num, /5/, '2'));
</pre>

<p class="brush: js"><a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array#Array_generic_methods">Generics</a> are also available on {{jsxref("Global_Objects/Array", "Array")}} methods.</p>

<p class="brush: js">The following is a shim to provide support to non-supporting browsers:</p>

<pre class="brush: js">/*globals define*/
// Assumes all supplied String instance methods already present
// (one may use shims for these if not available)
(function () {
    'use strict';

    var i,
        // We could also build the array of methods with the following, but the
        //   getOwnPropertyNames() method is non-shimable:
        // Object.getOwnPropertyNames(String).filter(function (methodName)
        //  {return typeof String[methodName] === 'function'});
        methods = [
            'quote', 'substring', 'toLowerCase', 'toUpperCase', 'charAt',
            'charCodeAt', 'indexOf', 'lastIndexOf', 'startsWith', 'endsWith',
            'trim', 'trimLeft', 'trimRight', 'toLocaleLowerCase',
            'toLocaleUpperCase', 'localeCompare', 'match', 'search',
            'replace', 'split', 'substr', 'concat', 'slice'
        ],
        methodCount = methods.length,
        assignStringGeneric = function (methodName) {
            var method = String.prototype[methodName];
            String[methodName] = function (arg1) {
                return method.apply(arg1, Array.prototype.slice.call(arguments, 1));
            };
        };

    for (i = 0; i &lt; methodCount; i++) {
        assignStringGeneric(methods[i]);
    }
}());</pre>

<h2 id="String_instances" name="String_instances"><code>String</code> instances</h2>

<h3 id="Properties_2">Properties</h3>

<p>{{page('en-US/docs/Web/JavaScript/Reference/Global_Objects/String/prototype', 'Properties')}}</p>

<p>{{page('en-US/docs/Web/JavaScript/Reference/Global_Objects/String/prototype', 'Methods')}}</p>

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

<h3 id="String_conversion">String conversion</h3>

<p>It's possible to use <code>String</code> as a "safer" {{jsxref("String.toString", "toString")}} alternative, as although it still normally calls the underlying <code>toString</code>, it also works for <code>null</code> and <code>undefined</code>. For example:</p>

<pre class="brush: js">var outputStrings = [];
for (let i = 0, n = inputValues.length; i &lt; n; ++i) {
  outputStrings.push(String(inputValues[i]));
}
</pre>

<h2 id="規範">規範</h2>

<table class="standard-table">
 <tbody>
  <tr>
   <th scope="col">Specification</th>
   <th scope="col">Status</th>
   <th scope="col">Comment</th>
  </tr>
  <tr>
   <td>{{SpecName('ES1')}}</td>
   <td>{{Spec2('ES1')}}</td>
   <td>Initial definition.</td>
  </tr>
  <tr>
   <td>{{SpecName('ES5.1', '#sec-15.5', 'String')}}</td>
   <td>{{Spec2('ES5.1')}}</td>
   <td> </td>
  </tr>
  <tr>
   <td>{{SpecName('ES2015', '#sec-string-objects', 'String')}}</td>
   <td>{{Spec2('ES2015')}}</td>
   <td> </td>
  </tr>
  <tr>
   <td>{{SpecName('ESDraft', '#sec-string-objects', 'String')}}</td>
   <td>{{Spec2('ESDraft')}}</td>
   <td> </td>
  </tr>
 </tbody>
</table>

<h2 id="瀏覽器相容性">瀏覽器相容性</h2>

<p>{{Compat("javascript.builtins.String.String")}}</p>

<h2 id="參見">參見</h2>

<ul>
 <li>{{domxref("DOMString")}}</li>
 <li><a href="/en-US/Add-ons/Code_snippets/StringView"><code>StringView</code> — a C-like representation of strings based on typed arrays</a></li>
 <li><a href="/en-US/docs/Web/API/DOMString/Binary">Binary strings</a></li>
</ul>