modernizr/feature-detects/css/nthchild.js

Modernizr is a JavaScript library that detects HTML5 and CSS3 features in the user’s browser.

/*!
{
  "name": "CSS :nth-child pseudo-selector",
  "caniuse": "css-sel3",
  "property": "nthchild",
  "tags": ["css"],
  "notes": [
    {
      "name": "Related Github Issue",
      "href": "https://github.com/Modernizr/Modernizr/pull/685"
    },
    {
      "name": "Sitepoint :nth-child documentation",
      "href": "http://reference.sitepoint.com/css/pseudoclass-nthchild"
    }
  ],
  "authors": ["@emilchristensen"],
  "warnings": ["Known false negative in Safari 3.1 and Safari 3.2.2"]
}
!*/
/* DOC
Detects support for the ':nth-child()' CSS pseudo-selector.
*/
define(['Modernizr', 'testStyles'], function(Modernizr, testStyles) {
  // 5 `<div>` elements with `1px` width are created.
  // Then every other element has its `width` set to `2px`.
  // A Javascript loop then tests if the `<div>`s have the expected width
  // using the modulus operator.
  testStyles('#modernizr div {width:1px} #modernizr div:nth-child(2n) {width:2px;}', function(elem) {
    var elems = elem.getElementsByTagName('div');
    var correctWidths = true;

    for (var i = 0; i < 5; i++) {
      correctWidths = correctWidths && elems[i].offsetWidth === i % 2 + 1;
    }
    Modernizr.addTest('nthchild', correctWidths);
  }, 5);
});