Measure time between click and expected result (defined by Xpath query)

script.js

// -- SETUP -----------
// demo:
//    1) go to https://ztlkl.cz/
//    2) open browser console, paste script (press Enter key to run it)
//    3) click on "#testování" link
// click on starter to start measure
var starter = '//a[@href="#f=stitek:testovani"]';
// define expected elements - XPath and count (0 means any)
var result = '//div[@class="project-description-meta"]';
var resultCount = 2;
var resultTimeout = 10;
// -- EXECUTION -------
var refreshInterval;
var startTime;
var starterXpath = document.evaluate(starter, document, null, XPathResult.ANY_TYPE, null );
var starterElement;
while(sEl = starterXpath.iterateNext()) {
    starterElement = sEl;
    starterElement.addEventListener('click', startStopwatch);
}
var nodesActual = 0;

if(refreshInterval) window.clearInterval(refreshInterval);
console.clear();
function startStopwatch() {
    refreshInterval = setInterval(checkFunction,100);
    starterElement.removeEventListener('click', startStopwatch);
    startTime = performance.now();
    console.log('%c0.000s: Action fired ('+startTime+')', 'color:blue');
}
 
function checkFunction() {
    var resultElements = document.evaluate(result, document, null, XPathResult.ANY_TYPE, null );
    var node, nodes = [];
    while(node = resultElements.iterateNext()) nodes.push(node);
    var timeElapsed = ((performance.now() - startTime) / 1000).toFixed(3);

    if(typeof nodes !== 'undefined') {
        
        if(nodesActual!=nodes.length) {
            nodesActual = nodes.length;
            console.log('%c'+timeElapsed + 's: Nodes:' + nodesActual + ' ('+performance.now()+')', 'color:gray');
        }

        if(nodes.length == resultCount || (resultCount == 0 && nodes.length>0)) {
            window.clearInterval(refreshInterval);
            console.log('%c' + timeElapsed + 's: Expected nodes ('+(resultCount>0 ? resultCount : '0+')+') presented ('+performance.now()+')', 'color:green');
        }
    }

    if(timeElapsed>resultTimeout) {console.log('%c' + timeElapsed + 's: Timeout expired', 'color:red');window.clearInterval(refreshInterval);}
}