JSFiddle - React, Tailwind, and code Playground
HTML
<input type="month" id="incidentDate" name="incidentDate" class="incidentDate" />
<input id="incidentMonth" class="incidentMonth" value="05" />
<input id="incidentYear" class="incidentYear" value="2011" />
JavaScript
var newdate = $(".incidentYear").val() + "-" + $(".incidentMonth").val()
var converdate = new Date(newdate);
alert(converdate)
$("#incidentDate").val(converdate);