function encode(s) {
    // Calculate result length and allocate output array.
    // encodedLength() also validates string and throws errors,
    // so we don't need repeat validation here.
    var arr = new Uint8Array(encodedLength(s));
    var pos = 0;
    for (var i = 0; i < s.length; i++) {
        var c = s.charCodeAt(i);
        if (c < 0x80) {
            arr[pos++] = c;
        }
        else if (c < 0x800) {
            arr[pos++] = 0xc0 | c >> 6;
            arr[pos++] = 0x80 | c & 0x3f;
        }
        else if (c < 0xd800) {
            arr[pos++] = 0xe0 | c >> 12;
            arr[pos++] = 0x80 | (c >> 6) & 0x3f;
            arr[pos++] = 0x80 | c & 0x3f;
        }
        else {
            i++; // get one more character
            c = (c & 0x3ff) << 10;
            c |= s.charCodeAt(i) & 0x3ff;
            c += 0x10000;
            arr[pos++] = 0xf0 | c >> 18;
            arr[pos++] = 0x80 | (c >> 12) & 0x3f;
            arr[pos++] = 0x80 | (c >> 6) & 0x3f;
            arr[pos++] = 0x80 | c & 0x3f;
        }
    }
    return arr;
}
Encodes the given string into UTF-8 byte array.
Throws if the source string has invalid UTF-16 encoding.