JSFiddle - React, Tailwind, and code Playground

HTML

<label for="priceInput">Price:</label><input id="priceInput" type=text name=price><br>
<label for="taxInput">Tax:</label><input id="taxInput" type=text name=tax>

JavaScript

//Test if the falu of the input is < 2000
if ($( "#priceInput" ).val()<2000) {
    //Set the value of the tax input to 20
    $("#taxInput").val(20)
};