modernizr/src/testProps.js

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

define(['contains', 'mStyle', 'createElement', 'nativeTestProps', 'is', 'cssToDOM'], function(contains, mStyle, createElement, nativeTestProps, is, cssToDOM) {
  // testProps is a generic CSS / DOM property test.

  // In testing support for a given CSS property, it's legit to test:
  //    `elem.style[styleName] !== undefined`
  // If the property is supported it will return an empty string,
  // if unsupported it will return undefined.

  // We'll take advantage of this quick test and skip setting a style
  // on our modernizr element, but instead just testing undefined vs
  // empty string.

  // Property names can be provided in either camelCase or hyphen-case.

  function testProps(props, prefixed, value, skipValueTest) {
    skipValueTest = is(skipValueTest, 'undefined') ? false : skipValueTest;

    // Try native detect first
    if (!is(value, 'undefined')) {
      var result = nativeTestProps(props, value);
      if (!is(result, 'undefined')) {
        return result;
      }
    }

    // Otherwise do it properly
    var afterInit, i, propsLength, prop, before;

    // If we don't have a style element, that means we're running async or after
    // the core tests, so we'll need to create our own elements to use.

    // Inside of an SVG element, in certain browsers, the `style` element is only
    // defined for valid tags. Therefore, if `modernizr` does not have one, we
    // fall back to a less used element and hope for the best.
    // For strict XHTML browsers the hardly used samp element is used.
    var elems = ['modernizr', 'tspan', 'samp'];
    while (!mStyle.style && elems.length) {
      afterInit = true;
      mStyle.modElem = createElement(elems.shift());
      mStyle.style = mStyle.modElem.style;
    }

    // Delete the objects if we created them.
    function cleanElems() {
      if (afterInit) {
        delete mStyle.style;
        delete mStyle.modElem;
      }
    }

    propsLength = props.length;
    for (i = 0; i < propsLength; i++) {
      prop = props[i];
      before = mStyle.style[prop];

      if (contains(prop, '-')) {
        prop = cssToDOM(prop);
      }

      if (mStyle.style[prop] !== undefined) {

        // If value to test has been passed in, do a set-and-check test.
        // 0 (integer) is a valid property value, so check that `value` isn't
        // undefined, rather than just checking it's truthy.
        if (!skipValueTest && !is(value, 'undefined')) {

          // Needs a try catch block because of old IE. This is slow, but will
          // be avoided in most cases because `skipValueTest` will be used.
          try {
            mStyle.style[prop] = value;
          } catch (e) {}

          // If the property value has changed, we assume the value used is
          // supported. If `value` is empty string, it'll fail here (because
          // it hasn't changed), which matches how browsers have implemented
          // CSS.supports()
          if (mStyle.style[prop] !== before) {
            cleanElems();
            return prefixed === 'pfx' ? prop : true;
          }
        }
        // Otherwise just return true, or the property name if this is a
        // `prefixed()` call
        else {
          cleanElems();
          return prefixed === 'pfx' ? prop : true;
        }
      }
    }
    cleanElems();
    return false;
  }

  return testProps;
});