JSFiddle - React, Tailwind, and code Playground

HTML

<textarea></textarea><br />
<a href="#">Generate</a>

<span id="output"></span>

JavaScript

$('textarea').bind('keypress', function(event) {     
    return /[a-zA-Z ]/.test(String.fromCharCode(event.which)); 
});

var replacements = {
    'a': '1',
    'b': '2',
    'c': '3'
}

$('a').on('click', function(e) {
    e.preventDefault();
   val =  $('textarea').val().split('');    
    $.each(val, function(i, e) {
        val[i] = replacements[e] ? replacements[e] : e;
    });
    $('#output').html(val.join(''))  

});