JSFiddle - React, Tailwind, and code Playground

HTML

<input id="datepicker" readonly="true" />

<button id="addBtn">Add Second Handler</button>

JavaScript

var firstHandler = function(dateText, inst) {
    console.log('Original Handler', dateText, inst);
};

var secondHandler = function(dateText, inst) {
    console.log('Second Handler', dateText, inst);
};


$('#datepicker').datepicker({
    onSelect: firstHandler
});

$('#addBtn').click(function(event) {
    $('#datepicker').datepicker('option', 'onSelect', secondHandler);
});