index

Zero-based index of the character to be returned. Converted to an integer — undefined is converted to 0.

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.

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 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.

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

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

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

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

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

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