Benchmark: <div id="foo" class="demo"></div>

Tests:

className
var element = document.getElementById("foo"); var i = 1000; while(i > 0) { element.className += " bar bar2 bar3"; i--; element.className = "demo" }
setAttribute
var element = document.getElementById("foo"); var i = 1000; while(i > 0) { element.setAttribute("class", "demo bar bar2 bar3"); i--; element.className = "demo" }
classList
var element = document.getElementById("foo"); var i = 1000; while(i > 0) { element.classList.add(["bar", "bar2", "bar3"]); i--; element.className = "demo" }

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Previous results

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<div id="foo" class="demo"></div> (version: 0)