概述
charAt() 方法从一个字符串中返回指定的字符。
语法
str.charAt(index)
参数
-
index
- 一个介于0 和字符串长度减1之间的整数。 (0~length-1)
- 如果没有提供索引,charAt() 将使用0。
描述
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 stringName.length - 1
。 如果指定的 index 值超出了该范围,则返回一个空字符串。
示例
例子:输出字符串中不同位置的字符
下例输出字符串 "Brave new world
" 不同位置处的字符:
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) + "'");
上面代码的输出为:
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 ''
例子:获取所有字符
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基本多文种平面(BMP)中的字符。
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; }
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)。
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]; }
示例:修复charAt以支持非基本多文种平面(BMP)字符
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:
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; }
规范
Specification | Status | Comment |
---|---|---|
ECMAScript 1st Edition. | Standard | Initial definition. |
ECMAScript 5.1 (ECMA-262) String.prototype.charAt |
Standard | |
ECMAScript 2015 (6th Edition, ECMA-262) String.prototype.charAt |
Standard |
浏览器兼容性
We're converting our compatibility data into a machine-readable JSON format. This compatibility table still uses the old format, because we haven't yet converted the data it contains.
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Feature | Chrome | Firefox (Gecko) | Internet Explorer | Opera | Safari |
---|---|---|---|---|---|
Basic support | (Yes) | (Yes) | (Yes) | (Yes) | (Yes) |
Feature | Android | Chrome for Android | Firefox Mobile (Gecko) | IE Mobile | Opera Mobile | Safari Mobile |
---|---|---|---|---|---|---|
Basic support | (Yes) | (Yes) | (Yes) | (Yes) | (Yes) | (Yes) |