Run results for: getElementById and getElementsByClassName [0] vs querySelector

Run details:

User agent: Mozilla/5.0 (X11; FreeBSD amd64; rv:80.0) Gecko/20100101 Firefox/80.0

Browser: Firefox 80

Operating system: FreeBSD

Device Platform: Desktop

Date tested: 3 years ago

Test name	Executions per second
getElementById	5396236.0 Ops/sec
querySelector	2334668.0 Ops/sec
getElementById and getElementsByClassName	2951803.0 Ops/sec

HTML Preparation code:

<div id="testdiv">
  <div id="unique1" class="unique" name="unique" data-unique="1">test1</div>
  <div id="unique2" class="unique" name="unique" data-unique="2">test2</div>
  <div id="unique3" class="unique" name="unique" data-unique="3">test3</div>
  <div id="unique4" class="unique" name="unique" data-unique="4">test4</div>
  <div id="unique5" class="unique" name="unique" data-unique="5">test5</div>
</div>

Script Preparation code:

var i, imax;
var doc = document;

Tests:

getElementById
var test = doc.getElementById('testdiv').childNodes[0].innerHTML;
querySelector
var test = doc.querySelector('#testdiv .unique').innerHTML;
getElementById and getElementsByClassName
var test = doc.getElementById('testdiv').getElementsByClassName('unique')[0].innerHTML;

Run results for: getElementById and getElementsByClassName [0] vs querySelector

Test performance of different ways of get just one particular DOM element

getElementById

querySelector

getElementById and getElementsByClassName