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diff --git a/files/zh-cn/web/javascript/reference/global_objects/string/charat/index.html b/files/zh-cn/web/javascript/reference/global_objects/string/charat/index.html new file mode 100644 index 0000000000..3ed7dd0e7b --- /dev/null +++ b/files/zh-cn/web/javascript/reference/global_objects/string/charat/index.html @@ -0,0 +1,280 @@ +--- +title: String.prototype.charAt() +slug: Web/JavaScript/Reference/Global_Objects/String/charAt +tags: + - String.prototype.charAt() +translation_of: Web/JavaScript/Reference/Global_Objects/String/charAt +--- +<p>{{JSRef}}</p> + +<h2 id="Summary" name="Summary">概述</h2> + +<p><strong>charAt()</strong> 方法从一个字符串中返回指定的字符。</p> + +<h2 id="Syntax" name="Syntax">语法</h2> + +<pre class="syntaxbox"><em>str</em>.charAt(<em>index</em>)</pre> + +<h3 id="Parameters" name="Parameters">参数</h3> + +<dl> + <dt><code>index</code></dt> + <dd>一个介于0 和字符串长度减1之间的整数。 (0~length-1)</dd> + <dd>如果没有提供索引，charAt() 将使用0。</dd> + <dt> </dt> +</dl> + +<h2 id="Description" name="Description">描述</h2> + +<p>字符串中的字符从左向右索引，第一个字符的索引值为 0，最后一个字符（假设该字符位于字符串 stringName 中）的索引值为 <code>stringName.length - 1</code>。 如果指定的 index 值超出了该范围，则返回一个空字符串。</p> + +<h2 id="Examples" name="Examples">示例</h2> + +<h3 id="Example:_Displaying_characters_at_different_locations_in_a_string" name="Example:_Displaying_characters_at_different_locations_in_a_string">例子：输出字符串中不同位置的字符</h3> + +<p>下例输出字符串 "<code>Brave new world</code>" 不同位置处的字符：</p> + +<pre class="brush:js">var anyString = "Brave new world"; + +console.log("The character at index 0 is '" + anyString.charAt(0) + "'"); +console.log("The character at index 1 is '" + anyString.charAt(1) + "'"); +console.log("The character at index 2 is '" + anyString.charAt(2) + "'"); +console.log("The character at index 3 is '" + anyString.charAt(3) + "'"); +console.log("The character at index 4 is '" + anyString.charAt(4) + "'"); +console.log("The character at index 999 is '" + anyString.charAt(999) + "'"); +</pre> + +<p>上面代码的输出为：</p> + +<pre>The character at index 0 is 'B' +The character at index 1 is 'r' +The character at index 2 is 'a' +The character at index 3 is 'v' +The character at index 4 is 'e' +The character at index 999 is '' +</pre> + +<h3 id="例子：获取所有字符">例子：获取所有字符</h3> + +<p>以下提供了一种确保通过字符串循环总是提供整个字符的方法，即使该字符串包含不在<a href="https://zh.wikipedia.org/wiki/Unicode%E5%AD%97%E7%AC%A6%E5%B9%B3%E9%9D%A2%E6%98%A0%E5%B0%84#.E5.9F.BA.E6.9C.AC.E5.A4.9A.E6.96.87.E7.A7.8D.E5.B9.B3.E9.9D.A2">基本多文种平面</a>（BMP）中的字符。</p> + +<pre class="brush:js">var str = 'A \uD87E\uDC04 Z'; // We could also use a non-BMP character directly +for (var i=0, chr; i < str.length; i++) { + if ((chr = getWholeChar(str, i)) === false) { + continue; + } // Adapt this line at the top of each loop, passing in the whole string and + // the current iteration and returning a variable to represent the + // individual character + + alert(chr); +} + +function getWholeChar (str, i) { + var code = str.charCodeAt(i); + + if (isNaN(code)) { + return ''; // Position not found + } + if (code < 0xD800 || code > 0xDFFF) { + return str.charAt(i); + } + + // High surrogate (could change last hex to 0xDB7F to treat high private + // surrogates as single characters) + if (0xD800 <= code && code <= 0xDBFF) { + if (str.length <= (i+1)) { + throw 'High surrogate without following low surrogate'; + } + var next = str.charCodeAt(i+1); + if (0xDC00 > next || next > 0xDFFF) { + throw 'High surrogate without following low surrogate'; + } + return str.charAt(i)+str.charAt(i+1); + } + // Low surrogate (0xDC00 <= code && code <= 0xDFFF) + if (i === 0) { + throw 'Low surrogate without preceding high surrogate'; + } + var prev = str.charCodeAt(i-1); + + // (could change last hex to 0xDB7F to treat high private + // surrogates as single characters) + if (0xD800 > prev || prev > 0xDBFF) { + throw 'Low surrogate without preceding high surrogate'; + } + // We can pass over low surrogates now as the second component + // in a pair which we have already processed + return false; +} + +</pre> + +<p>在允许解构分配的独占JavaScript 1.7+环境（如Firefox）中，以下是一个更简洁和更灵活的替代方法，它会自动递增一个递增变量（如果字符保证它是一个替代对）。</p> + +<pre class="brush: js">var str = 'A\uD87E\uDC04Z'; // We could also use a non-BMP character directly +for (var i=0, chr; i < str.length; i++) { + [chr, i] = getWholeCharAndI(str, i); + // Adapt this line at the top of each loop, passing in the whole string and + // the current iteration and returning an array with the individual character + // and 'i' value (only changed if a surrogate pair) + + alert(chr); +} + +function getWholeCharAndI (str, i) { + var code = str.charCodeAt(i); + + if (isNaN(code)) { + return ''; // Position not found + } + if (code < 0xD800 || code > 0xDFFF) { + return [str.charAt(i), i]; // Normal character, keeping 'i' the same + } + + // High surrogate (could change last hex to 0xDB7F to treat high private + // surrogates as single characters) + if (0xD800 <= code && code <= 0xDBFF) { + if (str.length <= (i+1)) { + throw 'High surrogate without following low surrogate'; + } + var next = str.charCodeAt(i+1); + if (0xDC00 > next || next > 0xDFFF) { + throw 'High surrogate without following low surrogate'; + } + return [str.charAt(i)+str.charAt(i+1), i+1]; + } + // Low surrogate (0xDC00 <= code && code <= 0xDFFF) + if (i === 0) { + throw 'Low surrogate without preceding high surrogate'; + } + var prev = str.charCodeAt(i-1); + + // (could change last hex to 0xDB7F to treat high private surrogates + // as single characters) + if (0xD800 > prev || prev > 0xDBFF) { + throw 'Low surrogate without preceding high surrogate'; + } + // Return the next character instead (and increment) + return [str.charAt(i+1), i+1]; +}</pre> + +<h3 id="示例：修复charAt以支持非基本多文种平面（BMP）字符">示例：修复charAt以支持非基本多文种平面（BMP）字符</h3> + +<p>虽然上面的例子对于那些希望支持非BMP字符的用户可能更有用（因为它不要求调用者知道任何非BMP字符可能出现在哪里），在人们希望的情况下，在选择字符 通过索引，将字符串中的替代对作为它们表示的单个字符，可以使用以下：</p> + +<pre class="brush: js">function fixedCharAt (str, idx) { + var ret = ''; + str += ''; + var end = str.length; + + var surrogatePairs = /[\uD800-\uDBFF][\uDC00-\uDFFF]/g; + while ((surrogatePairs.exec(str)) != null) { + var li = surrogatePairs.lastIndex; + if (li - 2 < idx) { + idx++; + } else { + break; + } + } + + if (idx >= end || idx < 0) { + return ''; + } + + ret += str.charAt(idx); + + if (/[\uD800-\uDBFF]/.test(ret) && /[\uDC00-\uDFFF]/.test(str.charAt(idx+1))) { + // Go one further, since one of the "characters" is part of a surrogate pair + ret += str.charAt(idx+1); + } + return ret; +}</pre> + +<h2 id="规范">规范</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>ECMAScript 1st Edition.</td> + <td>Standard</td> + <td>Initial definition.</td> + </tr> + <tr> + <td>{{SpecName('ES5.1', '#sec-15.5.4.4', 'String.prototype.charAt')}}</td> + <td>{{Spec2('ES5.1')}}</td> + <td> </td> + </tr> + <tr> + <td>{{SpecName('ES6', '#sec-string.prototype.charat', 'String.prototype.charAt')}}</td> + <td>{{Spec2('ES6')}}</td> + <td> </td> + </tr> + </tbody> +</table> + +<h2 id="浏览器兼容性">浏览器兼容性</h2> + +<p>{{ CompatibilityTable() }}</p> + +<div id="compat-desktop"> +<table class="compat-table"> + <tbody> + <tr> + <th>Feature</th> + <th>Chrome</th> + <th>Firefox (Gecko)</th> + <th>Internet Explorer</th> + <th>Opera</th> + <th>Safari</th> + </tr> + <tr> + <td>Basic support</td> + <td>{{ CompatVersionUnknown() }}</td> + <td>{{ CompatVersionUnknown() }}</td> + <td>{{ CompatVersionUnknown() }}</td> + <td>{{ CompatVersionUnknown() }}</td> + <td>{{ CompatVersionUnknown() }}</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>{{ CompatVersionUnknown() }}</td> + <td>{{ CompatVersionUnknown() }}</td> + <td>{{ CompatVersionUnknown() }}</td> + <td>{{ CompatVersionUnknown() }}</td> + <td>{{ CompatVersionUnknown() }}</td> + <td>{{ CompatVersionUnknown() }}</td> + </tr> + </tbody> +</table> +</div> + +<h2 id="See_also" name="See_also">相关链接</h2> + +<ul> + <li>{{jsxref("String.prototype.indexOf()")}}, {{jsxref("String.prototype.lastIndexOf()")}}</li> + <li>{{jsxref("String.prototype.split()")}}</li> + <li>{{jsxref("String.prototype.charCodeAt()")}}</li> + <li>{{jsxref("String.fromCodePoint()")}}</li> + <li>{{jsxref("String.prototype.codePointAt()")}}</li> +</ul> |