Promises

Question 1

What are the 3 states of the Promise object

Answer 1

Fulfilled, Rejected and Pending

Question 2

Can a Promise be in more than 1 state at a time?

Answer 2

No

Question 3

Picture an example of the Promise Constructor

Answer 3

var promise1 = new Promise(function(resolve, reject) {
  setTimeout(resolve, 100, 'foo');
});

console.log(promise1);
// expected output: [object Promise]

Question 4

What is the name of the function passed to the Promise constructor?

Answer 4

The executor

Question 5

What arguments are passed to the executor

Answer 5

Resolve, Reject

Question 6

What are 2 ways to handle exceptions in a Promise?

Answer 6

1. ) By using the .catch() method

2. ) By passing a second function to the .then() method

Question 7

Name 3 benefits of Promises

Answer 7

1. ) Improved error handling
2. ) Logical syntax
3. ) Chaining

Question 8

What happens if we return a value from the .catch() method?

Answer 8

We can chain after a catch. The value is passed back and can be handled with further .then / .catch chaining.

Question 9

What happens when an exception is thrown but there are 5 .then() methods chained after it?

Answer 9

It doesn’t matter, the exception falls to the next .catch() method. A promise chain stops if there’s an exception.
* Keeping in mind that if you decide to continue execution you can by returning a value.

Question 10

What is Promise.all() for?

Answer 10

For resolving an array of Promises once each one has completed

Question 11

What happens if 1 promise fails in the Promise.all([]) array?

Answer 11

We exit the execution and hit the .catch error handler.

Question 12

When is .then() call synchronously?

Answer 12

Never - functions passed to .then() will never be called synchronously, even with an already-resolved promise.

Question 13

Which errors does a nested .catch() statement handle?

Answer 13

Only failures within its scope and below, not higher up in the chain outside its scope.

Question 14

What are the only 3 things you can do inside a .then() function

Answer 14

1. ) Return another promise
2. ) Return a synchronous value, or undefined
3. ) Throw a synchronous error

Question 15

What are 3 things to ask yourself when using a promise?

Answer 15

1. ) Are you returning another promise?
2. ) Are you returning a value?
3. ) Are you returning an error?

Question 16

What happens inside the .catch() method if an exception is never thrown?

Answer 16

Nothing, the .catch() method is skipped entirely.

Question 17

How are synchronous return values treated in the chain?

Answer 17

A synchronous value is automatically converted into a resolved Promise.

A synchronous error is automatically converted into a rejected Promise.