jsDom React Testing - getMarkupWrap - Markup wrapping node not initialized. A friendly module-monkey-patch and module-state reminder courtesy of React.

gistfile1.js

/*
 While debugging using jsdom with react tests, I reminded myself of "when" module code executes.
 In doing so, I also reminded myself how to monkey patch loaded modules.
 Happy remembering!
*/

beforeEach(function() {
  // setup jsdom
  testDom();
  // tell react DOM is usable
  require('react/lib/ExecutionEnvironment').canUseDOM = !!document;
});

/**
 * Test body debugging. 
 * It reminded me that even though I set canUseDOM in the beforeEach,
 * dummyNode (the value in getMarkupWrap module state) had already been taken in getMarkupWrap at load time.
 */
it('some test with debug code in it', function() {
    // @@@ debug-start

    var getMarkupWrapOrig = require('react/lib/getMarkupWrap');

    function getMarkupWrap(nodeName) {
      // canUseDOM was set to true in beforeEach after jsdom setup success
      console.log('@@@ canUseDOM = '+require('react/lib/ExecutionEnvironment').canUseDOM);
      // this is a real HTMLDivElement
      console.log('@@@ div result = '+document.createElement('div'));
      getMarkupWrapOrig(nodeName);
    }

    // replace cached, loaded module exports, could've monkey patched individuals...
    require.cache[require.resolve('react/lib/getMarkupWrap')] = {
      exports: getMarkupWrap
    };

    // This still gets dummyNode === null, even though canUseDOM was corrected after the fact.
    // (to fix, you'll have to reload getMarkupWrap entirely with the new canUseDOM to get dummyNode set correctly)
    var wrap = require('react/lib/getMarkupWrap')('*');

    // never executes:
    console.log('@@@ wrapped = '+wrap);

    // @@@ debug-end
});