JSFiddle - React, Tailwind, and code Playground

HTML

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

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

JavaScript

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

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

$('a').click(function() {
    $('textarea').val(function(i, val) {
        val = val.split('');
        
        $.each(val, function(i, e) {
            val[i] = replacements[e] ? replacements[e] : e;
        });
        
        return val.join('');
    });
    return false;
});