Promises

Question 1

What is an asynchronous operation?

Answer 1

An asynchronous operation is one that allows the program to move on to other tasks while waiting for the asynchronous operation to complete or resolve.

Question 2

Name two situations where we might want to utilize asynchronous operations?

Answer 2

1. Requesting information or querying a database
2. Requesting information from a network or server

Question 3

What are JavaScript promises?

Answer 3

JavaScript promises are objects that represent the eventual outcome of an asynchronous operation.

Question 4

What are the three states of a JavaScript promise?

Answer 4

1. Pending - the initial state
2. Rejected - the operation has failed
3. Fulfilled - the operation has succeeded

Question 5

If a JavaScript promise has failed, what is returned?

Answer 5

An error is returned that we can access

Question 6

If a JavaScript promise is fulfilled what is returned?

Answer 6

A resolved value is returned. This resolved value can taken many forms and get be a JSON object or other data form.

Question 7

If a promise is not pending, what is it?

Answer 7

A promise would be considered settled, regardless of the outcome, if it is no longer pending.

Question 8

How would we create a promise?

Answer 8

We would use the new Promise constructor. The promise takes one parameter, the executorFunction.

Question 9

What role does the executorFunction play in defining a new promise.

Answer 9

The executorFunction determines what happens when a promise settles as either resolved or rejected.

Question 10

How many parameters does an executorFunction take?

Answer 10

Two, it takes a resolved parameter and a rejected parameter.

Question 11

What do we mean when we say we are consuming a promise?

Answer 11

We mean that we are using an already created promise and using the information returned upon either resolution or rejection.

Question 12

How do we consume a promise?

Answer 12

We use the .then( ) method on a promise to consume the returned data.

Question 13

How do we indicate what to do with the data returned from a settled promise?

Answer 13

The .then( ) method is a higher-order function that takes two callback functions to handle and manipulate either the resolved data or the rejected data.

Question 14

Describe the syntax involved with a .then( ) method

Answer 14

functionCallingPromise(parameter).then(handleSuccess, handleFailure)

Question 15

How can we pass the handler functions to the .then( ) method?

Answer 15

We can pass handler functions as either:

1. function definitions (i.e. const success = function(res){ })
2. anonymous functions (.then(function(success){ }, function(err){ }).

Question 16

How many handler functions will .then( ) accept?

Answer 16

It will accept either 0, 1, or 2 functions, but it is best to past 2 for more readable code.

Question 17

What does the .then( ) method return?

Answer 17

The .then( ) method returns a promise

Question 18

Can you chain .then( ) methods?

Answer 18

Yes, since .then( ) returns a promise, you can then further utilize the returned values in another .then( ) method with succes and failure handlers.

Question 19

How can we use the .catch( ) method to make our code cleaner and separate concerns?

Answer 19

The .catch( ) method will accept one callback function, the errorHandler.

We can pass our .then( ) the successHandler function and chain the .catch( ) method onto the end to handle failures.

Question 20

How does using .then( ) and .catch( ) separate our concerns?

Answer 20

By isolating our successHandler function and rejectHandler function into two methods we can make our code easier to read.

Question 21

What are two reasons we chain .then( ) methods onto a promise?

Answer 21

We chain .then( ) statements when:

1. we need to use the returned data from the original promise in other ways, perhaps another database request, or a different API call.
2. we need operations to occur in a specific order

Question 22

What is the process of chaining promises together called?

Answer 22

It is called composition.

Question 23

When we chain .then( ) methods, or promises, what must we keep in mind when utilizing returned values?

Answer 23

We must keep in mind that we should explicitly return the result of the manipulation of the previous promise value or we will simply get the former returned value again.

Question 24

Can we use anonymous functions as handler functions

Answer 24

Yes, we can use anonymous functions as handler functions for both .then( ) and .catch( )

Question 25

What are two mistakes made when chaining promises?

Answer 25

1. Nesting statements instead of chaining them
2. Forgetting to return a promise

Question 26

What happens if we forget to return a promise from

chained .then( ) methods?

Answer 26

The chained methods will return the original promise resolved value

Question 27

What do we mean by concurrency in regards to JavaScript promises?

Answer 27

We mean multiple asynchronous operations running at the same time.

Question 28

What function can we use to use concurrency in JavaScript?

Answer 28

We can use the Promise.all( ) funciton.

Question 29

How is the Promise.all( ) function used in JavaScript to run multiple asynchronous operations at once?

Answer 29

We pass an array of promises to the Promise.all( ) function.

1. If every promise in the passed array is resolved, Promise.all returns an array of the resolved values of all passed promises.
2. If any promise in the passed array rejects, the single promise returned from Promise.all will reject with the reason for rejection.

Question 30

const myPromise = new Promise((res,rej))

If we ran typeof myPromise what would be the result?

Answer 30

A Promise is an object representing the eventual completion or failure of an asynchronous operation.

Question 31

How many parameters does a Promise constructor take?

Answer 31

The Promise constructor takes one parameter - the executor function

Question 32

What are async …. await considered in regards to JavaScript promises?

Answer 32

async … await are considered syntactic sugar in regards to JavaScript promises.

They do not introduce new functionality, but make the syntax easier to write and read

Question 33

What is the await keyword considered to be?

Answer 33

It is considered to be an operator and can only be used within async functions.

Question 34

What does the await operator do for us?

Answer 34

It returns the resolved value of a promise, halting or pausing the code block until the promise is resolved.

Question 35

How do we typically use the async and await keywords?

Answer 35

Typically we declare an async function:

async function myFunction() {

then we set a variable equal to the returned promise value

let/const promiseValue = await myPromise( );

then we do something with the returned value

Question 36

What happens if we forget the await keyword when using async functions?

Answer 36

If we forget the await keyword the function will not be paused and we will get a Promise: pending as a result (or object: promise).

This can be difficult to debug as the function will still run, but the output will be different (and if they are chained will trickle down).

Question 37

What is one drawback when using .catch in a long promise chain?

Answer 37

In a long promise chain it can be difficult to deduce where the error was thrown when using .catch.

Question 38

How do we use catch in async … await blocks in our code?

Answer 38

We can catch errors with .catch in our async … await code by using the syntax try { } catch { }

Question 39

What is different when using the catch method in an async … await operation setting as opposed to a .then() setting?

Answer 39

In a try catch block we do not use the dot operator ( . ) to call try or catch. They are used much as an if … else statement

Question 40

Does the catch of a try … catch block catch synchronous errors as well as asynchronous errors?

Answer 40

Yes, catch will catch both asynchronous and synchronous errors in our code.

Question 41

Do we need to pass the catch method any parameters in a try .. catch block?

Answer 41

No. If we do not need to use the exception identifier then we do not need to pass any parameters. If, however, we want to log the error out, or do something else with it, we should pass a parameter representing the error.

Question 42

What is an exception identifier?

Answer 42

An exception identifier is an identifier that holds the exception value in a try … catch block (it is the parameter that we pass to the catch method, the e or err that holds the error value)

Question 43

How do we use the resolved value of an async … await function?

Answer 43

We typically save the resolved value to a variable:

let result = await myAsyncFunction( );

use result somehow.

Question 44

What can we do if we want our promise to run concurrently but need all values to resolve before taking the next step in our code block?

Answer 44

We can assign the promise values to variables, without the await keyword. These promises will settle as they are able.

Then we can await the result of doing the code action that utilizes all resolved promises. We still get the pause in code resolution, but the promises do not wait for one another to attempt resolution.

Question 45

Can we use Promise.all to run concurrent promises in JavaScript?

Answer 45

Yes, we can pass multiple promises, in an array, to Promise.all(). We prepend the keyword await to the Promise.all() to wait for the promises to all resolve before being able to access the resolves values as indices of the returned array.

Question 46

What do we pass to Promise.all()?

Answer 46

We pass an array of promises to Promise.all()

Question 47

What do we use the await keyword for in JavaScript?

Answer 47

We use the await keyword to pause execution of our function until an asynchronous action completes and the awaited promise is no longer pending

Question 48

What should we do if we have a string of promises that do not rely on or utilize the value of the preceding promise?

Answer 48

We should use concurrency to allow all promises to run in parallel.

Question 49

When do we use the finally( ) method?

Answer 49

The finally( ) method allows us to execute a code block regardless of how a promise settles. It follows the .then( ) and .catch( ) chain.