String.prototype.charCodeAt() - JavaScript 编辑

The charCodeAt() method returns an integer between 0 and 65535 representing the UTF-16 code unit at the given index.

The source for this interactive example is stored in a GitHub repository. If you'd like to contribute to the interactive examples project, please clone https://github.com/mdn/interactive-examples and send us a pull request.

The source for this interactive example is stored in a GitHub repository. If you'd like to contribute to the interactive examples project, please clone https://github.com/mdn/interactive-examples and send us a pull request.

The UTF-16 code unit matches the Unicode code point for code points which can be represented in a single UTF-16 code unit. If the Unicode code point cannot be represented in a single UTF-16 code unit (because its value is greater than 0xFFFF) then the code unit returned will be the first part of a surrogate pair for the code point. If you want the entire code point value, use codePointAt().

Syntax

str.charCodeAt(index)

Parameters

index

An integer greater than or equal to 0 and less than the length of the string. If index is not a number, it defaults to 0.

Return value

A number representing the UTF-16 code unit value of the character at the given index. If index is out of range, charCodeAt() returns NaN.

Description

Unicode code points range from 0 to 1114111 (0x10FFFF). The first 128 Unicode code points are a direct match of the ASCII character encoding. (For information on Unicode, see the JavaScript Guide.)

Note: charCodeAt() will always return a value that is less than 65536. This is because the higher code points are represented by a pair of (lower valued) "surrogate" pseudo-characters which are used to comprise the real character.

Because of this, in order to examine (or reproduce) the full character for individual character values of 65536 or greater, for such characters, it is necessary to retrieve not only charCodeAt(i), but also charCodeAt(i+1) (as if manipulating a string with two letters), or to use codePointAt(i) instead. See examples 2 and 3 (below).

charCodeAt() returns NaN if the given index is less than 0, or if it is equal to or greater than the length of the string.

Backward compatibility: In historic versions (like JavaScript 1.2) the charCodeAt() method returns a number indicating the ISO-Latin-1 codeset value of the character at the given index. The ISO-Latin-1 codeset ranges from 0 to 255. The first 0 to 127 are a direct match of the ASCII character set.

Examples

Using charCodeAt()

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

'ABC'.charCodeAt(0)  // returns 65

Fixing charCodeAt() to handle non-Basic-Multilingual-Plane characters if their presence earlier in the string is unknown

This version might be used in for loops and the like when it is unknown whether non-BMP characters exist before the specified index position.

function fixedCharCodeAt(str, idx) {
  // ex. fixedCharCodeAt('\uD800\uDC00', 0); // 65536
  // ex. fixedCharCodeAt('\uD800\uDC00', 1); // false
  idx = idx || 0;
  var code = str.charCodeAt(idx);
  var hi, low;

  // High surrogate (could change last hex to 0xDB7F
  // to treat high private surrogates
  // as single characters)
  if (0xD800 <= code && code <= 0xDBFF) {
    hi = code;
    low = str.charCodeAt(idx + 1);
    if (isNaN(low)) {
      throw 'High surrogate not followed by ' +
        'low surrogate in fixedCharCodeAt()';
    }
    return (
      (hi - 0xD800) * 0x400) +
      (low - 0xDC00) + 0x10000;
  }
  if (0xDC00 <= code && code <= 0xDFFF) { // Low surrogate
    // We return false to allow loops to skip
    // this iteration since should have already handled
    // high surrogate above in the previous iteration
    return false;
    // hi = str.charCodeAt(idx - 1);
    // low = code;
    // return ((hi - 0xD800) * 0x400) +
    //   (low - 0xDC00) + 0x10000;
  }
  return code;
}

Fixing charCodeAt() to handle non-Basic-Multilingual-Plane characters if their presence earlier in the string is known

function knownCharCodeAt(str, idx) {
  str += '';
  var code,
      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 NaN;
  }

  code = str.charCodeAt(idx);

  var hi, low;
  if (0xD800 <= code && code <= 0xDBFF) {
    hi = code;
    low = str.charCodeAt(idx + 1);
    // Go one further, since one of the "characters"
    // is part of a surrogate pair
    return ((hi - 0xD800) * 0x400) +
      (low - 0xDC00) + 0x10000;
  }
  return code;
}

Specifications

Specification
ECMAScript (ECMA-262) The definition of 'String.prototype.charCodeAt' in that specification.

Browser compatibility

BCD tables only load in the browser

See also

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