Edit in JSFiddle

var brackify = (function(){
	var quirkyDictionary = {
		"a": "(![]+[])[!![]+![]]",
		"b": "({}+[])[+(!+[]+!+[])]",
		"c": "({}+[])[+(!+[]+!+[]+!+[]+!+[]+!+[])]",
		"d": "([][[]]+[])[!+[]+!+[]]",
		"e": "(!![]+[])[!+[]+!+[]+!+[]]",
		"f": "(![]+[])[+[]]",
		"g": "([]+([]+[])[([][(![]+[])[+[]]+(![]+[]+[][[]])[+!+[]+[+[]]]+(![]+[])[!+[]+!+[]]+(!![]+[])[+[]]+(!![]+[])[!+[]+!+[]+!+[]]+(!![]+[])[+!+[]]]+[])[!+[]+!+[]+!+[]]+(!![]+[][(![]+[])[+[]]+(![]+[]+[][[]])[+!+[]+[+[]]]+(![]+[])[!+[]+!+[]]+(!![]+[])[+[]]+(!![]+[])[!+[]+!+[]+!+[]]+(!![]+[])[+!+[]]])[+!+[]+[+[]]]+([][[]]+[])[+!+[]]+(![]+[])[!+[]+!+[]+!+[]]+(!![]+[])[+[]]+(!![]+[])[+!+[]]+([][[]]+[])[+[]]+([][(![]+[])[+[]]+(![]+[]+[][[]])[+!+[]+[+[]]]+(![]+[])[!+[]+!+[]]+(!![]+[])[+[]]+(!![]+[])[!+[]+!+[]+!+[]]+(!![]+[])[+!+[]]]+[])[!+[]+!+[]+!+[]]+(!![]+[])[+[]]+(!![]+[][(![]+[])[+[]]+(![]+[]+[][[]])[+!+[]+[+[]]]+(![]+[])[!+[]+!+[]]+(!![]+[])[+[]]+(!![]+[])[!+[]+!+[]+!+[]]+(!![]+[])[+!+[]]])[+!+[]+[+[]]]+(!![]+[])[+!+[]]])[+!+[]+[!+[]+!+[]+!+[]+!+[]]]",
		"i": "(![]+[]+[][[]])[+!+[]+[+[]]]",
		"j": "({}+[])[!+[]+!+[]+!+[]]",	
		"l": "(![]+[])[!+[]+!+[]]",
		"m": "((+[])[([][(![]+[])[+[]]+(![]+[]+[][[]])[+!+[]+[+[]]]+(![]+[])[!+[]+!+[]]+(!![]+[])[+[]]+(!![]+[])[!+[]+!+[]+!+[]]+(!![]+[])[+!+[]]]+[])[!+[]+!+[]+!+[]]+(!![]+[][(![]+[])[+[]]+(![]+[]+[][[]])[+!+[]+[+[]]]+(![]+[])[!+[]+!+[]]+(!![]+[])[+[]]+(!![]+[])[!+[]+!+[]+!+[]]+(!![]+[])[+!+[]]])[+!+[]+[+[]]]+([][[]]+[])[+!+[]]+(![]+[])[!+[]+!+[]+!+[]]+(!![]+[])[+[]]+(!![]+[])[+!+[]]+([][[]]+[])[+[]]+([][(![]+[])[+[]]+(![]+[]+[][[]])[+!+[]+[+[]]]+(![]+[])[!+[]+!+[]]+(!![]+[])[+[]]+(!![]+[])[!+[]+!+[]+!+[]]+(!![]+[])[+!+[]]]+[])[!+[]+!+[]+!+[]]+(!![]+[])[+[]]+(!![]+[][(![]+[])[+[]]+(![]+[]+[][[]])[+!+[]+[+[]]]+(![]+[])[!+[]+!+[]]+(!![]+[])[+[]]+(!![]+[])[!+[]+!+[]+!+[]]+(!![]+[])[+!+[]]])[+!+[]+[+[]]]+(!![]+[])[+!+[]]]+[])[+!+[]+[+!+[]]]",
		"n": "([][[]]+[])[+!+[]]",
		"o": "(!![]+[][(![]+[])[+[]]+(![]+[]+[][[]])[+!+[]+[+[]]]+(![]+[])[!+[]+!+[]]+(!![]+[])[+[]]+(!![]+[])[!+[]+!+[]+!+[]]+(!![]+[])[+!+[]]])[+!+[]+[+[]]]",
		"r": "(!![]+[])[+!+[]]",
		"s": "(![]+[])[!+[]+!+[]+!+[]]",
		"t": "(!![]+[])[+[]]",
		"u": "([][[]]+[])[+[]]",
		"v": "(+[]+[][(![]+[])[+[]]+(![]+[]+[][[]])[+!+[]+[+[]]]+(![]+[])[!+[]+!+[]]+(!![]+[])[+[]]+(!![]+[])[!+[]+!+[]+!+[]]+(!![]+[])[+!+[]]])[!+[]+!+[]+!+[]+[+[]]]",
		"y": "(+[![]]+[+(+!+[]+(!+[]+[])[!+[]+!+[]+!+[]]+[+!+[]]+[+[]]+[+[]]+[+[]])])[+!+[]+[+[]]]",
	};

	var convert = function(inputString){
		//we only want characters from a-z, except the ones we have missing
		//i'll leave the second regex restriction apart, as it's supposed to be temporal 
		if(/^[a-z]*/.test(inputString) && /^[^hkpwxz]+$/.test(inputString)){
			var res = "";
			for(var i = 0; i < inputString.length; i++){
				res = res + quirkyDictionary[ inputString[i] ] +"+";
			}
			res = res.substring(0, res.length - 1);
			return res;
		}
		else {
			return "Sorry, I don't have enough letters, but you can improve me!";
		}
	};
	
	return {
		"convertString": convert
	};
	
})();

var convertButton = document.getElementById('convertor');
convertButton.addEventListener("click", function(){
    var inputBox = document.getElementById('myInput');
    var textArea = document.getElementById('result');
    var result = brackify.convertString(inputBox.value);
    textArea.value = result;
});
<p><input type="text" id="myInput" /><button id="convertor">Convert to []()+!</button>
<p>(Missing letters: h,k,p, w,x,z)</p>
<textarea id="result"></textarea>