waitForElement.js

function waitForElement(selector, target, callback) {
    // observer target node
    var target = document.querySelector(target);
     
    // create new observer instance
    var observer = new MutationObserver(function(mutations) {
      mutations.forEach(function(mutation) {
        var addedNodes = mutation.addedNodes;
            for (var i=0; i < addedNodes.length; i++) {
                var node = addedNodes[i];
                if (node.matches && node.matches(selector)) {
                    callback();
                    observer.disconnect();
                }
            }
        });    
    });
     
    // observer config
    var config = {  childList: true, subtree: true };
     
    // start with observervation
    if (target) {
        observer.observe(target, config);
    }
}