Why is documentFragment no faster than repeated DOM access?

Question

I was always under the impression that rather than touching the DOM repeatedly, for performance reasons, we should use a documentFragment to append multiple element

Answer 1

Well actually your test code just insert nodes and do not alter their content or CSS which would in fact force the rendering engine to a reflow.

I have prepared 3 tests to demonstrate this dramatic difference.

1. Simple DOM modificiation resulting Layout Trashing
2. Simple DOM modificiation through window.requestAnimationFrame()
3. Virtual DOM modification through document.createDocumentFragment()

Notes:

• You might like to test on full screen.
• In Firefox I have obtained even more dramatic results.

// Resets the divs
function resetLayout() {
  divs = document.querySelectorAll('div');
  speed.textContent = "Resetting Layout...";
  setTimeout(function() {
               each.call(divs, function(div) {
                                 div.style.height = '';
                                 div.offsetTop;
                               });
               speed.textContent = "";
             }, 16);
}
// print the result
function renderSpeed(ms) {
  speed.textContent = ms + 'ms';
}

var divs        = document.querySelectorAll('div'),
    raf         = window.requestAnimationFrame,
    each        = Array.prototype.forEach,
    isAfterVdom = false,
    start       = 0;

// Reset the Layout

reset.onclick = resetLayout;

// Direct DOM Access

direct.onclick = function() {
                   isAfterVdom && (divs = document.querySelectorAll('div'), isAfterVdom = false);
                   start = performance.now();

                   each.call(divs, function(div) {
                   var width = div.clientWidth;
                   div.style.height = ~~(Math.random()*2*width+6) + 'px';
                   div.style.backgroundColor = '#' + Math.floor(Math.random() * 16777215).toString(16);
                 });

                   // Render result
                   renderSpeed(performance.now() - start);
                 };

// Access DOM at the next frame by requestAnimationFrame

rAF.onclick = function() {
                isAfterVdom && (divs = document.querySelectorAll('div'), isAfterVdom = false);
                start = performance.now();

                each.call(divs, function(div) {
                                  var width = div.clientWidth;
                                  // Schedule the write operation to be run in the next frame.
                                  raf(function() {
                                        div.style.height = ~~(Math.random()*2*width+6) + 'px';
                                        div.style.backgroundColor = '#' + Math.floor(Math.random() * 16777215).toString(16);
                                      });
                                });
               // Render result
               raf(function() {
                     renderSpeed(performance.now() - start);
                   });
             };

// Update the vDOM and access DOM just once by rAF

vdom.onclick = function() {
                 var sectCl = divCompartment.cloneNode(true),
                     divsCl = sectCl.querySelectorAll('div'),
                     dFrag  = document.createDocumentFragment(),
                     width  = divCompartment.querySelector('div').clientWidth;
                 
                 isAfterVdom = true;
                 end = 0;
                 start  = performance.now();
                 each.call(divsCl, function(div) {
                                     div.style.height = ~~(Math.random()*2*width+6) + 'px';
                                     div.style.backgroundColor = '#' + Math.floor(Math.random() * 16777215).toString(16);
                                   });
                 dFrag.appendChild(sectCl);
                 divCompartment.parentNode.replaceChild(dFrag, divCompartment);
                 // Render result
                 renderSpeed(performance.now() - start);
               };

html {
  font: 14px Helvetica, sans-serif;
  background: black;
  color: white;
}

* {
  box-sizing: border-box;
  margin-bottom: 1rem;
}

h1 {
  font-size: 2em;
  //word-break: break-word;
  -webkit-hyphens: auto;
}

button {
  background-color: white;
}

div {
  display: inline-block;
  width: 5%;
  margin: 3px;
  background: white;
  border: solid 2px white;
  border-radius: 10px
}

section {
  overflow: hidden;
}

#speed {
  font-size: 2.4em;
}

  
Updating 1000 DOM Nodes
  

    Reset Layout
    Update Directly
    Update by rAF
    Update by vDOM