JSFiddle - React, Tailwind, and code Playground

HTML

<textarea></textarea>
<div></div><button type="button">Markdownify</button>

JavaScript

$(document.body).on('click', 'button', function() {

    var val=$('textarea').val();
    var bolded=val.replace(/\*\*[A-z0-9]+\*\*/gi, '<span style="font-weight: bold; color: blue;">' + val + '</span>');
    bolded = bolded.replace(/\*\*/gi, "");
    
    $('div').html(bolded);
});