Using deferred to test async services

demo.js

function mockReadFile(mockPath, content) {
  // see https://gist.github.com/eschwartz/955a3f8925e24270550cb635dd49c12b
  const deferred = Deferred(); 
  
  sinon.stub(fs, 'readFile', _.wrap(fs.readFile, (origFn, path, encoding, cb) => {
    if (path !== mockPath) { 
      return origFn.readFile(path, encoding, cb);
    }
    
    // Only resolve `fs.readFile` when deferred is resolved
    deferred.promise.then(() => cb(null, content));
  });
  
  return deferred;
}


it('should wait to resolve readFile', () => {
  const deferred = mockReadFile('file.txt', 'fileContent');
  
  fs.readFile('file.txt', 'utf8', (err, content) => {
    console.log(`file.txt contains '${content}'`);
  });
  
  // ....
  
  // Resolve the deferred, allowing fs.readFile to complete
  deferred.resolve();
  // "file.txt contains 'file content'"
});