Truncate a string at a given length in UTF-8 encoded bytes with JavaScript. Many APIs have lengths limitations given in these kind of numbers.

truncateUtf8Bytes.js

// Truncate a string at a given length in UTF-8 encoded bytes.
// Many APIs have lengths limitations given in these kind of numbers.
function truncateUtf8Bytes(string, byteLength) {
    const charLength = string.length;
    let curByteLength = 0;

    for (let i = 0; i < charLength; ++ i) {
        const start = i;
        const w1 = string.charCodeAt(i);
        let cp = w1;

        if ((w1 & 0xfc00) === 0xd800) {
            const w2 = string.charCodeAt(i + 1);
            if ((w2 & 0xfc00) === 0xdc00) {
                const hi = w1 & 0x3ff;
                const lo = w2 & 0x3ff;
                cp = (hi << 10) | lo | 0x10000;
                ++ i;
            }
        }

        curByteLength += (
            cp >= 0x10000 ? 4 :
            cp >= 0x800   ? 3 :
            cp >= 0x80    ? 2 :
                            1);

        if (curByteLength === byteLength) {
            return string.slice(0, i + 1);
        }
        else if (curByteLength > byteLength) {
            return string.slice(0, start + 1);
        }
    }

    return string;
}