title: Is the global Execution Context pushed onto Execution Context Stack(ECS) before Execution Context of a Job is initialized?
body: <p>I have read these:</p>
  
  <ul>
  <li><a href="https://stackoverflow.com/questions/29879525/is-the-initial-global-execution-context-ever-popped-off-the-call-stack-in-javasc">Is the initial global execution context ever popped off the call stack in JavaScript?</a></li>
  <li><a href="https://stackoverflow.com/questions/33869145/is-it-possible-for-global-execution-context-to-pop-off-the-execution-stack">Is it possible for Global Execution Context to pop off the execution stack?</a></li>
  </ul>
  
  <p>But I still confused about it.</p>
  
  <p>When the stack if empty, a Job will be executed.It will create a Execution Context for the Job.</p>
  
  <p>So the question is, will the Execution Context of the Job be put at the bottom of the stack or firstly create a global Execution Context and put  the Execution Context of the Job onto it?</p>