Async Programming - 13%

Question 1

Promise

Answer 1

An obj representing the eventual completion or failure of an async operation and its resulting value.

Question 2

Promise creation

Answer 2

Use the new keyword, and pass resolve and reject.
Most often we consume already-created promises from APIs, rarely make our own.
ex:
Const promise1 = new Promise((resolve, reject) => { …}

Returns a Promise obj w/ 2 internal properties - state and result.

Question 3

Promise states

Answer 3

Pending, Fulfilled, Rejected
Fulfilled if resolved w/value.
Rejected if rejected w/error.
(resolved and settled are not states)

Question 4

other Promise descriptors (not states)

Answer 4

Resolved - cannot be resolved or rejected b/c it has either already been fulfilled or rejected OR it has been ‘locked in’ to follow another promise.
Settled - no longer pending - has been fulfilled or rejected.

Question 5

possible actions after a Promise is settled

Answer 5

.then, .catch, .finally

.finally - runs when it settles (either fulfills or rejects)
Can use to stop a loading indicator before .then / .catch
Has no arguments, the res or err passes through to next action.

Adding an action to a settled Promise will occur after the stack has been emptied (like setTimeout with 0 ms)

Question 6

Promise Concurrency

Answer 6

Not true concurrency, but control switches making it appear they are running at once.

Syntax ex: Promise.all(<iteratable>)</iteratable>

.all - fulfills when ALL fulfill, rejects when ANY reject
.allSettled - fulfills when ALL are settled
.any - fulfills when ANY fulfill, rejects when ALL reject
.race - fulfills when ANY fulfill, rejects when ANY reject

Question 7

Async Programming

Answer 7

Using the async keyword allows for the fn to contain await expressions, which make promise-returning fs behave like synchronous code.

The return of an async fn behaves like it is wrapped in a Promise.resolve, except the async return is a reference.