First cut at a simple capability testing

supports.js

(function (global) {
  // test: function to run OR value representing result
  // root: object where results are stored
  // name: key in storage object
  // success: function to run on success
  // failure: function to run on failure
  // retest: should the test be re-run after a failure
  //  (eg. failure applied a polyfill)
  //  test must be a function to retest
  function test(options)
  {
    if (!options.root) options.root = {};
    test.results = options.root;
    var run = function (options)
    {
      var result = (typeof options.test === 'function') ? options.test() : options.test;
      console.log('result', result)
      options.root[options.name] = result;
      ([options.failure, options.success][+result])();
      return result;
    };

    var result = run(options);
    if (!result && options.retest) result = run(options);
    return options.root[options.name] = result;
  }

  var results = {};
  var foo = test({
    name: 'global.foo',
    root: results,
    test: function () {
      return 'foo' in global;
    },
    success: function () {
      console.log('success', global.foo);
    },
    failure: function () {
      console.log('failure. apply polyfill', global.foo = 'bar')
    },
    retest: true
  });

  console.log('foo', foo, 'test.results', test.results);
})(this);