W4.1 Asynch & State - callbacks and promises

Question 1

Does JS have threads?

Answer 1

No

But runtime environment does have threads

Question 2

What are the 2 styles of asynchronous code in JS?

Answer 2

• Callbacks

- Promises

Question 3

What is a callback function?

Answer 3

A function passed by reference as a parameter - not executed immediately. It is “called back” asynchronously in the containing function’s body (usually in response to an event)

Question 4

Describe the use of callbacks in a JS runtime environment

Answer 4

• call a function that takes a callback function and it will run asynchronously
• when the callback completes (i.e event happens) the completed task is put on a queue
• when stack is empty another thread in runtime environment (event loop) takes tasks off the queue and pushes the tasks onto the stack where they are executed (See diagram)

Question 5

What is the event loop in a runtime environment?

Answer 5

a semi-infinite loop, polling (waiting) and blocking on the OS until something is ready - exits when there are no more events to wait on

Question 6

Give examples of things that are pollable (i.e held in event loop) and things that are not (implemented by thread pool).

Answer 6

Pollable:
- TCP/UDP sockets - http/https
- stdin, stdout, stderr
- timeouts
- asynch DNS operations
- signals
Not pollable: 
- filesystems
- DNS lookup

Question 7

What is a promise in JS?

Answer 7

• A promise is an object representing the eventual completion or failure of an asynch operation
• a promise is a returned object that callbacks are attached to (instead of passing them into a function)

Question 8

What three fields does a promise have?

Answer 8

• object that is promised
• function used to resolve/fulfil promise
• function used to reject promise

Question 9

What three states can a promise be in?

Answer 9

• fulfilled
• rejected
• pending

Question 10

What syntax is used for assigning fulfil/resolve and reject functions for a promise?

Answer 10

p.then(f, r)
where f is onFulfilled and r is onRejected.

Once promise fulfilled/rejected the respective handler function is called asynchronously.

Question 11

How does promise chaining work?

Answer 11

• when one is fulfilled move onto the next (fulfilled promise object is returned by handler and used as the next promise’s value)
• each then leads to more asynchronous actions and promises

Question 12

What is an asynchronous function?

Answer 12

• uses async function declaration (AsyncFunction object)

- function which executes asynchronously via the event loop, using implicit promise to return its result