1. Stack function calls form a stack of frames.
2. Heap objects are allocated in a heap (Denotes a large mostly unstructured region of memory).
3. Queue A JavaScript runtime uses a message queue, which is a list of messages to be processed.
   • Each message has an associated function which gets called in order to handle the message.
4. Event loop handling the messages on the queue, starting with the oldest one.
   • To do so, the message is removed from the queue and its corresponding function is called with the message as an input parameter.

A web worker or a cross-origin iframe has its own stack, heap, and message queue.
Two distinct runtimes can only communicate through sending messages via the postMessage method. This method adds a message to the other runtime if the latter listens to message events.

• a javascript runtime like V8 (heap/stack)
  • heap (memory allocation) and stack (execution contexts)
• Web APIs that the browser provides, like the DOM, Ajax, and setTimeout
• a callback queue for events with callbacks like onClick, onLoad, onDone
• an event loop*

To know more about internal working. Go through execution context of Javascript.

• https://developer.mozilla.org/en-US/docs/Web/JavaScript/EventLoop
• https://www.zeolearn.com/magazine/understanding-the-javascript-event-loop
• http://cek.io/blog/2015/12/03/event-loop/
• How Browsers Work: Behind the scenes of modern web browsers
  https://www.html5rocks.com/en/tutorials/internals/howbrowserswork/