Promise

Question 1

A Promise

Answer 1

An object representing the eventual completion or failure of an asynchronous operation.

Question 2

Guarantees that a promise comes with

Answer 2

• Callbacks will never be called before the completion of the current run of the JavaScript event loop
• Callbacks added with then(), even after a promise has been resolved or rejected, will still be called
• Multiple callbacks may be added by calling then() several times.
  
  • each callback is executed one after another, in the order they were inserted

Question 3

createAudioFileAsync(audioSettings).then(successCallback, failureCallback)

is shorthand for what?

Answer 3

const promise = createAudioFileAsync(audioSettings)

promise.then(successCallback, failureCallback)

Question 4

Promise Chain

Answer 4

• Ability to execute two or more asynchronous tasks back-to-back
• Each subsequent operation starts when the previous succeeds, with the result from the previous step

Question 5

Why must you always return results?

Answer 5

So that other callbacks will receive the results of a previous promise.

Question 6

What is chaining after a catch?

Answer 6

When you place a then() after a catch() to accomplish some actions even after there was a failure.

Question 7

What is error propogation?

Answer 7

• When a promise chain stops if there’s an exception.
• It looks down the chain for catch handlers instead
  
  • modeling try/catch in synchronous code

Question 8

What are the only native asychronous functions in JavaScript?

Answer 8

setTimeout and setInterval

Question 9

What are three forms of asynchronous execution?

Answer 9

• Http requests
• Any I/O operation
• Dealing with a websocket

Question 10

When does any function passed to setTimeout execute?

Answer 10

• Executes asynchronously
• No less than the delay amount, but possibly more
• When the main thread is not busy anymore

Question 11

Why is asynchronous code needed?

Answer 11

The main thread can’t execute the code and refresh the user interface at the same time.

Question 12

What does the then function do?

Answer 12

It binds one or two callbacks to the promise, and can use the data given to the resolve or reject function.

Question 13

How many times can you call resolve or reject in a promise?

Answer 13

Just one. Calling resolve or reject more than once is useless as only the first one will be recognized.

Question 14

When should you use Promise.resolve()?

Answer 14

When you’re in a conditional and could either return a promise or a simple value. Use Promise.resolve(simpleValue) so that a promise is always being returned.

function test() {

` if (something) {`

` return aNewPromise()`

` } else {`

` return Promise.resolve(42)`

` }`

}

Question 15

How do you handle logging and recover from the error in a promise?

Answer 15

Simply use a catch block to recover and send errors to logging, and then rethrow the error at the end of the catch block using throw err.

https://developers.google.com/web/fundamentals/primers/promises