observe_completion_of_p5js.js

var MutationObserver = window.MutationObserver || window.WebKitMutationObserver || window.MozMutationObserver;
var observer = new MutationObserver(function(mutations) {
    mutations.forEach(function(mutation) {
        if(mutation.addedNodes.length > 0 && mutation.addedNodes[0].nodeName == "CANVAS"){
          // do stuff after the sketch loaded 
        }
    });
});
var config = { attributes: false, childList: true, characterData: false }
target = document.getElementById("myDiv")
observer.observe(target, config);
new p5(mySketch, target);