getElementById vs. querySelector

JavaScript performance comparison

Revision 254 of this test case created by till t

Preparation code

<p id="foo">Foo</p>
<p id="bar">Bar</p>
<p id="baz">Baz</p>
      
<script>
Benchmark.prototype.setup = function() {
  var idSelectorRegexp = /^#[a-zA-Z]+[a-zA-Z0-9_\-]*$/;
  
  var select = function(query) {
    // pure ID selector?
    if (idSelectorRegexp.test(query))
      return document.getElementById(query.slice(1));
    else
      return document.querySelector(query);
  };
  
  var select2 = function(query) {
    // pure ID selector?
    if (query.startsWith("#"))
      return document.getElementById(query.slice(1));
    else
      return document.querySelector(query);
  };
  
  var select3 = function(query) {
    // pure ID selector?
    if (query[0] === "#")
      return document.getElementById(query.slice(1));
    else
      return document.querySelector(query);
  };
  
  var select4Cache = {};
  
  var select4 = function(id) {
    var r = select4Cache[id];
    if (!r) {
      r = document.getElementById(id);
      select4Cache[id] = r;
    }
    return r;
  };

};
</script>

Preparation code output

<p id="foo">Foo</p> <p id="bar">Bar</p> <p id="baz">Baz</p>

Test runner

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Testing in CCBot 2.0.0 / Other 0.0.0
Test Ops/sec
getElementById
var bar = document.getElementById("bar");
pending…
querySelector
var bar = document.querySelector("#bar");
pending…
mapping to getElementById
var bar = select("#bar");
pending…
mapping with startsWith to getElementById
var bar = select2("#bar");
pending…
mapping with [0] === "#" to getElementById
var bar = select3("#bar");
pending…
cached getElementById
var bar = select4("bar");
pending…

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Revisions

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Foo

Bar

Baz