JSFiddle - React, Tailwind, and code Playground

HTML

<textarea id="code" placeholder="code here"></textarea>
<br/>
<textarea id="bookmarklet" placeholder="bookmarklet generated here"></textarea>

CSS

textarea {
    height: 200px;
    width: 100%;
}

JavaScript

var a = document.getElementById('code'),
    b = document.getElementById('bookmarklet');

function gen() {
    b.value = 'javascript:'
    + encodeURIComponent('(function () {' + a.value + '}())')
    + ';';
}

a.addEventListener('keyup', gen);
a.addEventListener('keypress', gen);
a.addEventListener('change', gen);