JSFiddle - React, Tailwind, and code Playground

HTML

<link rel="stylesheet" href="http://ajax.googleapis.com/ajax/libs/jqueryui/1.8.4/themes/ui-lightness/jquery-ui.css">
<input value="ciao" size="4">

CSS

input {
    font-family:Courier;
}

JavaScript

var input = document.getElementsByTagName('input')[0];

input.onkeyup = function() {
    this.size = Math.max(this.value.length, 1);
};