prop65-updates.js

// Notes
// A first recommendation is to provide some closure surrounding the introduced JS code to protect globl scope.
// Secondly, leverage load/onload detection so that we don't clobber window.onload
// Lastly, let's get this in a file and load asynchourlys through admin content area.
/*
    (function(d, t) {
        var g = d.createElement(t),
            s = d.getElementsByTagName(t)[0];
        g.src = 'https://third-party.com/resource.js';
        s.parentNode.insertBefore(g, s);
    }(document, 'script'));
*/
!function Prop65() {
    function init() {
        window.addEventListener ? 
        window.addEventListener('load', checkDOM, false) : 
        window.attachEvent && window.attachEvent('onload', checkDOM);
    }
    
    function checkDOM(){
        // TODO: I would recommend an interval here
        if (document.getElementById("address-province") && document.getElementById("address-province").length > 0){
            var stateSelect = document.getElementById("address-province");
            _bindStateSelectEvent(stateSelect);
            return;
        }
        setTimeout( checkDOM, 100);
    }
    function _bindStateSelectEvent(stateSelect){
      // We can movie this stateValue check within the insertMessage() function and pass in state value
        if(stateSelect.value === "California") {
            _insertMessage();
        };
        stateSelect.onchange = function(){
            if(stateSelect.value === "California") {
               _insertMessage();
            }
        };
    }
  
    function _insertMessage(){
        var submitButton = document.querySelector('#checkout-shipping-continue');
        var span = document.createElement('div');
        span.className = 'prop65-message';
        span.setAttribute('style', 'display: block; padding: 10px 0; border: 1px solid #989898; text-align: center; margin-bottom: 30px;');
        span.innerHTML = 'Prop 65 Warning Message lorem ipsem ';
        submitButton.parentNode.insertBefore(span, submitButton);
    }
    
    // Initialize
    init()
}()

prop65.js

window.onload=function(){
    if(document.getElementsByClassName("optimizedCheckout-header").length > 0){
        checkDOM();
        return;
    }
};
function checkDOM(){
    if (document.getElementById("address-province") && document.getElementById("address-province").length > 0){
        var stateSelect = document.getElementById("address-province");
        _bindStateSelectEvent(stateSelect);
        return;
    }
    setTimeout( checkDOM, 100);
}
function _bindStateSelectEvent(stateSelect){
    if(stateSelect.value === "California") {
        _insertMessage();
    };
    stateSelect.onchange = function(){
        if(stateSelect.value === "California") {
           _insertMessage();
        }
    };
}
function _insertMessage(){
     var submitButton = document.querySelector('#checkout-shipping-continue');
            var span = document.createElement('div');
            span.className = 'prop65-message';
            span.setAttribute('style', 'display: block; padding: 10px 0; border: 1px solid #989898; text-align: center; margin-bottom: 30px;');
            span.innerHTML = 'Prop 65 Warning Message lorem ipsem ';
            submitButton.parentNode.insertBefore(span, submitButton);
}