async.apply(exec, 'git rev-parse HEAD'),
  async.apply(exec, 'git symbolic-ref --short HEAD')
function (err, results) {

First, the async.parallel call executes all functions in the array concurrently. If you need the commands to be executed in order, use async.series instead. async.apply returns the function passed as a first argument with values already applied to its arguments and sets the async.parallel callback for us. Finally, the last functions receives the results in order when all functions have finished executing. It preserves the order of results as you would expect them.

For more info: