btoaUTF8.src.js

var btoaUTF8 = (function(btoa, replacer){"use strict";
	return function(inputString, BOMit){
		return btoa((BOMit ? "\xEF\xBB\xBF" : "") + inputString.replace(
			/[\x80-\uD7ff\uDC00-\uFFFF]|[\uD800-\uDBFF][\uDC00-\uDFFF]?/g, replacer
		));
	}
})(btoa, function(fromCharCode){
	return function(nonAsciiChars){"use strict";
		// make the UTF string into a binary UTF-8 encoded string
		var point = nonAsciiChars.charCodeAt(0);
		if (point >= 0xD800 && point <= 0xDBFF) {
			var nextcode = nonAsciiChars.charCodeAt(1);
			if (nextcode !== nextcode) // NaN because string is 1 code point long
				return fromCharCode(0xef/*11101111*/, 0xbf/*10111111*/, 0xbd/*10111101*/);
			// https://mathiasbynens.be/notes/javascript-encoding#surrogate-formulae
			if (nextcode >= 0xDC00 && nextcode <= 0xDFFF) {
				point = (point - 0xD800) * 0x400 + nextcode - 0xDC00 + 0x10000;
				if (point > 0xffff)
					return fromCharCode(
						(0x1e/*0b11110*/<<3) | (point>>>18),
						(0x2/*0b10*/<<6) | ((point>>>12)&0x3f/*0b00111111*/),
						(0x2/*0b10*/<<6) | ((point>>>6)&0x3f/*0b00111111*/),
						(0x2/*0b10*/<<6) | (point&0x3f/*0b00111111*/)
					);
			} else return fromCharCode(0xef, 0xbf, 0xbd);
		}
		if (point <= 0x007f) return nonAsciiChars;
		else if (point <= 0x07ff) {
			return fromCharCode((0x6<<5)|(point>>>6), (0x2<<6)|(point&0x3f));
		} else return fromCharCode(
			(0xe/*0b1110*/<<4) | (point>>>12),
			(0x2/*0b10*/<<6) | ((point>>>6)&0x3f/*0b00111111*/),
			(0x2/*0b10*/<<6) | (point&0x3f/*0b00111111*/)
		);
	}
}(String.fromCharCode));