There is no way to do it in JavaScript natively. (See Riccardo Galli's answer for a modern approach.)
For historical reference or where TextEncoder APIs are still unavailable.
If you know the character encoding, you can calculate it yourself though.
encodeURIComponent assumes UTF-8 as the character encoding, so if you need that encoding, you can do,
function lengthInUtf8Bytes(str) { // Matches only the 10.. bytes that are non-initial characters in a multi-byte sequence. var m = encodeURIComponent(str).match(/%[89ABab]/g); return str.length + (m ? m.length : 0);}This should work because of the way UTF-8 encodes multi-byte sequences. The first encoded byte always starts with either a high bit of zero for a single byte sequence, or a byte whose first hex digit is C, D, E, or F. The second and subsequent bytes are the ones whose first two bits are 10. Those are the extra bytes you want to count in UTF-8.
The table in wikipedia makes it clearer
Bits Last code point Byte 1 Byte 2 Byte 3 7 U+007F 0xxxxxxx 11 U+07FF 110xxxxx 10xxxxxx 16 U+FFFF 1110xxxx 10xxxxxx 10xxxxxx...If instead you need to understand the page encoding, you can use this trick:
function lengthInPageEncoding(s) { var a = document.createElement('A'); a.href = '#'+ s; var sEncoded = a.href; sEncoded = sEncoded.substring(sEncoded.indexOf('#') + 1); var m = sEncoded.match(/%[0-9a-f]{2}/g); return sEncoded.length - (m ? m.length * 2 : 0);}