Implementation of utf8_encode in javascript

utf8_encode.js

//Javascript implementation of utf8_encode function from php.
function utf8_encode(argString) {
	if (argString === null || typeof argString === 'undefined') {
		return '';
	}

	var string = (argString + ''); // .replace(/\r\n/g, "\n").replace(/\r/g, "\n");
	var utftext = '',
		start, end, stringl = 0;

	start = end = 0;
	stringl = string.length;
	for (var n = 0; n < stringl; n++) {
		var c1 = string.charCodeAt(n);
		var enc = null;

		if (c1 < 128) {
			end++;
		} else if (c1 > 127 && c1 < 2048) {
			enc = String.fromCharCode(
				(c1 >> 6) | 192, (c1 & 63) | 128
			);
		} else if ((c1 & 0xF800) != 0xD800) {
			enc = String.fromCharCode(
				(c1 >> 12) | 224, ((c1 >> 6) & 63) | 128, (c1 & 63) | 128
			);
		} else { // surrogate pairs
			if ((c1 & 0xFC00) != 0xD800) {
				throw new RangeError('Unmatched trail surrogate at ' + n);
			}
			var c2 = string.charCodeAt(++n);
			if ((c2 & 0xFC00) != 0xDC00) {
				throw new RangeError('Unmatched lead surrogate at ' + (n - 1));
			}
			c1 = ((c1 & 0x3FF) << 10) + (c2 & 0x3FF) + 0x10000;
			enc = String.fromCharCode(
				(c1 >> 18) | 240, ((c1 >> 12) & 63) | 128, ((c1 >> 6) & 63) | 128, (c1 & 63) | 128
			);
		}
		if (enc !== null) {
			if (end > start) {
				utftext += string.slice(start, end);
			}
			utftext += enc;
			start = end = n + 1;
		}
	}

	if (end > start) {
		utftext += string.slice(start, stringl);
	}

	return utftext;
}