modernizr/src/nativeTestProps.js

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

define(['injectElementWithStyles', 'domToCSS', 'computedStyle'], function(injectElementWithStyles, domToCSS, computedStyle) {
  /**
   * nativeTestProps allows for us to use native feature detection functionality if available.
   * some prefixed form, or false, in the case of an unsupported rule
   *
   * @access private
   * @function nativeTestProps
   * @param {Array} props - An array of property names
   * @param {string} value - A string representing the value we want to check via @supports
   * @returns {boolean|undefined} A boolean when @supports exists, undefined otherwise
   */
  // Accepts a list of property names and a single value
  // Returns `undefined` if native detection not available
  function nativeTestProps(props, value) {
    var i = props.length;
    // Start with the JS API: https://www.w3.org/TR/css3-conditional/#the-css-interface
    if ('CSS' in window && 'supports' in window.CSS) {
      // Try every prefixed variant of the property
      while (i--) {
        if (window.CSS.supports(domToCSS(props[i]), value)) {
          return true;
        }
      }
      return false;
    }
    // Otherwise fall back to at-rule (for Opera 12.x)
    else if ('CSSSupportsRule' in window) {
      // Build a condition string for every prefixed variant
      var conditionText = [];
      while (i--) {
        conditionText.push('(' + domToCSS(props[i]) + ':' + value + ')');
      }
      conditionText = conditionText.join(' or ');
      return injectElementWithStyles('@supports (' + conditionText + ') { #modernizr { position: absolute; } }', function(node) {
        return computedStyle(node, null, 'position') === 'absolute';
      });
    }
    return undefined;
  }
  return nativeTestProps;
});