Promises

Question 1

promises

Answer 1

• object that represents the outcome of an asynchronous call
• consists of a status and a value
• value gets updated when async call is completed

Question 2

promises

states

Answer 2

3 states

1. pending
2. fulfilled (resolved)
3. rejected

• a promise that is fulfilled or rejected is said to be settled
• a settled promise cannot be unsettled
• a settled promise has a value associated with it, which cannot change

Question 3

promises

Promise constructor

Answer 3

• used to create the promise object
• constructor is where all the async logic takes place
• promise object will automatically be rejected if error is raised anywhere in constructor
• constructor takes a function which take two parameters, resolve and reject
• resolve and reject are built-in functions that will update promie’s status and value
• the parameter of resolve will be passed to promise’s success handler
• the parameter of reject will be passed to the promise’s failure handler

Syntax

var myPromise = new Promise(
     function(resolve, reject) {
        // Async logic
        resolve("Some value");
            //-and/or-
        reject("Error object or value");
     });

Question 4

promises

then

Answer 4

• built-in method used to handle success and failure
• then method takes two functions as parameters: success callback and failure callback
• the success callback receive parameter that was passed to resolve (in constructor)
• failure callback receives parameter that was passed to reject (in constructor)
• both are optional
• then always returns another promise
• this makes them chainable
• user can also return a promise himself
• if user returns value (or error) instead, new promise will automatically be fulfilled (or rejected) with that value (or error)
• a promise can be ‘thenned’ more than once (can handle success/failure on same promise multiple times
• a use then on a promise even after it is settled (overcomes race condition)

Syntax

promise.then(function success(result) {}
}, function failure(err) { });

Question 5

promises

catch

Answer 5

• built-in function available to promise objects
• returns another promise (like then)
• can be used to handle errors that arose anyway along promise chain
• syntactc sugar; equivalent to then’s failure handler [.then(undefined, failure())]
• considered good practice to end all promises with catch

Question 6

Promise.all

Answer 6

• built-in method on Promise constructor
• takes an array (or iterable) of promises
• returns a promise that resolves when all promises resolve
• or rejects when first promise rejects
• value of resolve promise is array of values from indiviudal promises
• value of reject promise is the value of first promise to reject

Question 7

Promise.race

Answer 7

• built-in method on Promise constructor
• takes an array (or iterable) of promises
• returns a single promise that resolves when first promise resolves
• or rejects when first promise rejects
• value is value of promise that resolved or rejected first