String.prototype.charCodeAt()

¥Syntax

charCodeAt(index)

¥Parameters

index

要返回的字符的从零开始的索引。转换为整数 — undefined 转换为 0。

¥Return value

0 和 65535 之间的整数，表示指定 index 处字符的 UTF-16 代码单元值。如果 index 超出 0 – str.length - 1 的范围，则 charCodeAt() 返回 NaN。

¥An integer between 0 and 65535 representing the UTF-16 code unit value of the character at the specified index. If index is out of range of 0 – str.length - 1, charCodeAt() returns NaN.

¥Description

字符串中的字符从左到右进行索引。名为 str 的字符串中第一个字符的索引是 0，最后一个字符的索引是 str.length - 1。

¥Characters in a string are indexed from left to right. The index of the first character is 0, and the index of the last character in a string called str is str.length - 1.

Unicode 代码点范围从 0 到 1114111 (0x10FFFF)。charCodeAt() 始终返回小于 65536 的值，因为较高的代码点由一对 16 位代理伪字符表示。因此，为了获得值大于 65535 的完整字符，不仅需要检索 charCodeAt(i)，还需要检索 charCodeAt(i + 1)（就像操作两个字符的字符串一样），或者使用 codePointAt(i) 代替。有关 Unicode 的信息，请参阅 UTF-16 字符、Unicode 代码点和字素簇。

¥Unicode code points range from 0 to 1114111 (0x10FFFF). charCodeAt() always returns a value that is less than 65536, because the higher code points are represented by a pair of 16-bit surrogate pseudo-characters. Therefore, in order to get a full character with value greater than 65535, it is necessary to retrieve not only charCodeAt(i), but also charCodeAt(i + 1) (as if manipulating a string with two characters), or to use codePointAt(i) instead. For information on Unicode, see UTF-16 characters, Unicode code points, and grapheme clusters.

¥Examples

¥Using charCodeAt()

以下示例返回 65，即 A 的 Unicode 值。

¥The following example returns 65, the Unicode value for A.

"ABC".charCodeAt(0); // returns 65

charCodeAt() 可能会返回单独的代理，它们不是有效的 Unicode 字符。

¥charCodeAt() may return lone surrogates, which are not valid Unicode characters.

const str = "𠮷𠮾";
console.log(str.charCodeAt(0)); // 55362, or d842, which is not a valid Unicode character
console.log(str.charCodeAt(1)); // 57271, or dfb7, which is not a valid Unicode character

要获取给定索引处的完整 Unicode 代码点，请使用 String.prototype.codePointAt()。

¥To get the full Unicode code point at the given index, use String.prototype.codePointAt().

const str = "𠮷𠮾";
console.log(str.codePointAt(0)); // 134071

注意：避免使用 charCodeAt() 重新实现 codePointAt()。从 UTF-16 代理项到 Unicode 代码点的转换很复杂，而 codePointAt() 可能性能更高，因为它直接使用字符串的内部表示形式。如有必要，请为 codePointAt() 安装 Polyfill。

¥Note: Avoid re-implementing codePointAt() using charCodeAt(). The translation from UTF-16 surrogates to Unicode code points is complex, and codePointAt() may be more performant as it directly uses the internal representation of the string. Install a polyfill for codePointAt() if necessary.

下面是将一对 UTF-16 代码单元转换为 Unicode 代码点的可能算法，改编自 统一码常见问题解答：

¥Below is a possible algorithm to convert a pair of UTF-16 code units into a Unicode code point, adapted from the Unicode FAQ:

// constants
const LEAD_OFFSET = 0xd800 - (0x10000 >> 10);
const SURROGATE_OFFSET = 0x10000 - (0xd800 << 10) - 0xdc00;

function utf16ToUnicode(lead, trail) {
  return (lead << 10) + trail + SURROGATE_OFFSET;
}
function unicodeToUTF16(codePoint) {
  const lead = LEAD_OFFSET + (codePoint >> 10);
  const trail = 0xdc00 + (codePoint & 0x3ff);
  return [lead, trail];
}

const str = "𠮷";
console.log(utf16ToUnicode(str.charCodeAt(0), str.charCodeAt(1))); // 134071
console.log(str.codePointAt(0)); // 134071

¥See also

• String.fromCharCode()
• String.prototype.charAt()
• String.fromCodePoint()
• String.prototype.codePointAt()