JS String Methods Reference

String.prototype.charCodeAt(index)

charCodeAt returns the 16-bit UTF-16 code unit at the given position, as a number between 0 and 65535. If the index is out of range it returns NaN — not an exception, and not undefined.

Basic usage

const s = "hello";
s.charCodeAt(0);   // 104  ('h')
s.charCodeAt(1);   // 101  ('e')
s.charCodeAt(99);  // NaN  (out of range)

UTF-16: two units for one character

JavaScript strings are sequences of UTF-16 code units, not Unicode code points. Characters outside the Basic Multilingual Plane (code points above U+FFFF) are stored as surrogate pairs and occupy two positions:

const s = "\u{1F600}"; // 😀
s.length;         // 2
s.charCodeAt(0);  // 55357 (0xD83D, high surrogate)
s.charCodeAt(1);  // 56832 (0xDE00, low surrogate)

If you need the actual character (code point 128512), use codePointAt instead.

Common pattern: iterating "characters"

for (let i = 0; i < s.length; i++) {
  console.log(s.charCodeAt(i).toString(16));
}

This loop prints code units. To iterate real code points, prefer the string iterator, which is surrogate-pair aware:

for (const ch of "\u{1F600}abc") {
  console.log(ch.codePointAt(0).toString(16));
  // 1f600, 61, 62, 63
}

Practical uses

• Character arithmetic: 'a'.charCodeAt(0) + 1 → 98 ('b').
• Sorting and comparison logic where locale collators are not wanted.
• Simple checksums and encoders where each unit must be inspected.

See also

• codePointAt — full code points.
• fromCodePoint — the inverse operation.