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DOM Performance (Reflow & Repaint) (Summary)

DOM Performance

Rendering

  • How the browser renders the document
    • Receives the data (bytes) from the server.
    • Parses and converts into tokens (<, TagName, Attribute, AttributeValue, >).
    • Turns tokens into nodes.
    • Turns nodes into the DOM tree.
    • Builds CSSOM tree from the css rules.
    • CSSOM and DOM trees are combined into a RenderTree.
      • Computes which elements are visible and their computed styles.
      • Starting from the root of the dom tree.
      • Not visible elements like (meta, script, link) and display: none are ommited from the render tree.
      • For each visible node, find the appropriate matching CSSOM rules and apply them.
    • Reflow: compute the layout of each visible element (position and size).
    • Repaint: renders the pixels to screen.
  • Repaint
    • Occurs when changes affect the visibility.
    • Triggers examples: opacity, color, background-color, visibility.
  • Reflow (Layout, LayoutFlush, LayoutThrashing)
    • Occurs when the changes affect the layout.
    • Triggers examples: width, position, float.
    • Recalculate of positions and dimensions.
    • Has a bigger impact, changing a single element can affect all children, ancestors, and siblings or the whole document.
    • Triggers (chagne dom or css, scrolling, user actions like focus).
    • Reflow only has a cost if the document has changed and invalidated the layout.
    • Something Invalidates + Something Triggers = Costly Reflow.

Minimizing Repaints And Reflows

  • Don't change styles by multiple statements, instead:
    • Add class.
    • Change the cssText.
  • Batch DOM changes
    • Options to batch DOM changes
      • Use a documentFragment to hold temp changes.
      • Clone, update, replace the node.
      • Hide the element with display: none (1 reflow, 1 repaint), add 100 changes, restore the display (total 2 reflow, 2 repaint).
  • Don't ask for computed styles repeatedly, cache them into variable.
    • Multiple reads/writes (like for the height property of an element)
      • Writes, then reads, from the DOM, multiple times causing document reflows.
      • Read(cached), write(invalidate layout), read(trigger layout).
      • To fix: read everything first then write everything.
  • Resources

Chrome DevTools Performance

Chrome provide a really great tool that helps us to figure out what is going on with our code, how many reflows (layout) and repaint do we have and more details about the memory, events, etc.

Bad code with 6 costly reflows (layout):

Bad Code

var box1Height = document.getElementById('box1').clientHeight;
document.getElementById('box1').style.height = box1Height + 10 + 'px';

var box2Height = document.getElementById('box2').clientHeight;
document.getElementById('box2').style.height = box2Height + 10 + 'px';

var box3Height = document.getElementById('box3').clientHeight;
document.getElementById('box3').style.height = box3Height + 10 + 'px';

var box4Height = document.getElementById('box4').clientHeight;
document.getElementById('box4').style.height = box4Height + 10 + 'px';

var box5Height = document.getElementById('box5').clientHeight;
document.getElementById('box5').style.height = box5Height + 10 + 'px';

var box6Height = document.getElementById('box6').clientHeight;
document.getElementById('box6').style.height = box6Height + 10 + 'px';

Optimized to have 1 reflow:

Optimized Code

var box1Height = document.getElementById('box1').clientHeight;
var box2Height = document.getElementById('box2').clientHeight;
var box3Height = document.getElementById('box3').clientHeight;
var box4Height = document.getElementById('box4').clientHeight;
var box5Height = document.getElementById('box5').clientHeight;
var box6Height = document.getElementById('box6').clientHeight;

document.getElementById('box1').style.height = box1Height + 10 + 'px';
document.getElementById('box2').style.height = box2Height + 10 + 'px';
document.getElementById('box3').style.height = box3Height + 10 + 'px';
document.getElementById('box4').style.height = box4Height + 10 + 'px';
document.getElementById('box5').style.height = box5Height + 10 + 'px';
document.getElementById('box6').style.height = box6Height + 10 + 'px';

Performance Tips (VanillaJS, jQuery)

  • Optimize Selectors
    • jQuery uses:
      • document.querySelectorAll().
      • document.getElementById() faster.
      • jQuery selector extensions.
    • Avoid jQuery selector extensions (:even, :has, :gt, :eq).
    • Avoid complex specificity.
    • ID-Based Selectors the fastest.
  • Add style element for chaning > 20 instead of .css.
  • Cache Length During Loops.
  • Avoid inspecting large numbers of nodes
    • document.getElementById('id').getElementsByTagName('*') better than document.getElementsByTagName('*').
  • Cache DOM values in script variables
    • var sample = document.getElementById('test')
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