JSFiddle - React, Tailwind, and code Playground

HTML

<input name="thumbnail" placeholder="Post image URL here" />
<br />
<img src="http://webspace.webring.com/people/jv/vladilyich/preview.gif" class="preview" />

JavaScript

$('[name="thumbnail"]').on('change', function() {
     $('img.preview').prop('src', this.value);
});