Created
July 15, 2017 17:26
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Select all elements above the fold and append matched css rules in header
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(function () { | |
document.addEventListener('DOMContentLoaded', function(){ | |
var stylesheets = document.styleSheets; | |
var wHeight = window.innerHeight; | |
var criticalCSS = ''; | |
var uniqueRules = []; | |
var criticalRules = []; | |
console.log(stylesheets); | |
function matchCssRules(node) { | |
var matchedRules = []; | |
node.matches = node.matches || node.webkitMatchesSelector || node.mozMatchesSelector || node.msMatchesSelector || node.oMatchesSelector; | |
for (var i = 0; i < stylesheets.length; i++) { | |
var rules = stylesheets[i].rules || stylesheets[i].cssRules; | |
if(rules) { | |
for (var r = 0; r < rules.length; r++) { | |
if(rules[r] instanceof CSSMediaRule || rules[r] instanceof CSSKeyframesRule) { | |
var mediaRules = rules[r].cssRules; | |
for (var m = 0; m < mediaRules.length; m++) { | |
if (node.matches(mediaRules[m].selectorText)) { | |
matchedRules.push(rules[r].cssText); | |
} | |
} | |
} | |
else { | |
if (node.matches(rules[r].selectorText)) { | |
matchedRules.push(rules[r].cssText); | |
} | |
} | |
} | |
} | |
} | |
return matchedRules; | |
} | |
var treeWalker = document.createTreeWalker( | |
document, | |
NodeFilter.SHOW_ELEMENT, | |
{ acceptNode: function(node) { return NodeFilter.FILTER_ACCEPT; } }, | |
false | |
); | |
while(treeWalker.nextNode()) { | |
var node = treeWalker.currentNode; | |
var rect = node.getBoundingClientRect(); | |
if(rect.top < wHeight) { | |
var rules = matchCssRules(node); | |
if(rules) { | |
for(var i = 0; i < rules.length; i++) { | |
criticalRules.push(rules[i]); | |
} | |
} | |
} | |
} | |
uniqueRules = criticalRules.filter(function(e, i, s) { | |
return i == s.indexOf(e); | |
}); | |
var head = document.getElementsByTagName('head')[0]; | |
var s = document.createElement('style'); | |
s.setAttribute('type', 'text/css'); | |
s.setAttribute('id', 'nvl_critical_css'); | |
if (s.styleSheet) { // IE | |
s.styleSheet.cssText = uniqueRules; | |
} | |
else { // the world | |
s.appendChild(document.createTextNode(uniqueRules)); | |
} | |
head.appendChild(s); | |
}); | |
})(); |
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