JSFiddle - React, Tailwind, and code Playground

HTML

<form name="justcuz">
    Convert to jUst Cuz: <br>
    <textarea id="chars">Can I has just cuz?</textarea> 
    <br><input type="button" value="Encode" id='encode'>
        <br><br>
    <textarea id="codebox" cols="50" rows="11" style="width:70%" wrap="virtual"> </textarea> 
    <br><input type="button" value="Decode" id='decode'>
</form>

JavaScript

var charCodes = {};

function binaryToJC (num, index) {
    var char = 'justcuz'[index];
    
    if (char === 'c') char = ' ' + char;
    if (num === '1') char = char.toUpperCase();
    
    return char;
};

Array.prototype.forEach.call('abcdefghijklmnopqrstuvwxyz1234567890ABCDEFGHIJKLMNOPQRSTUVWXYZ', function generateCharMap (char, index) {
    var binary = ('0000000'+(index >>> 0).toString(2)).slice(-7);
    
    var jc = Array.prototype.map.call(binary, binaryToJC).join('');
    
    charCodes[char] = jc;
    charCodes[jc] = char;
});

$('#encode').on('click', function encode () {
	var chars = Array.prototype.map.call($('#chars').val(), function charToJC (char) {
        if (charCodes[char]) {
			return charCodes[char];
        } else if (char !== ' ') {
            return char;
        }
    });
    
    $('#codebox').val(chars.join(' '));
});

$('#decode').on('click', function decode () {
    var temp;
    
	var chars = $('#codebox').val().split(' ').map(function jcToChar (jc) {
        var char;
        
        if (jc.toLowerCase() === 'just') {
            temp = jc + ' ';
        } else if (temp) {
            temp = temp + jc;
            
            char = charCodes[temp];
            temp = undefined;
        } else if (jc.length > 0) {
            char = jc;
        } else {
            char = ' ';
        }
        
        return char;
    });
    
    $('#chars').val(chars.join(''));
});