Promise: promise.catch()

catch() returns a Promise and deals with rejected cases only

prerequisites for understanding

return a fulfilled promise, to pass a test

Promise.resolve(); assert(false); // Don't touch! Make the test pass in the line above!

reminder: the test passes when a fulfilled promise is returned

return Promise.reject('I should fulfill.');

catch method basics

is an instance method

const p = Promise; assert.equal(typeof p.catch, 'function');

catches only promise rejections

//: {"jskatas": {"runnerOptions": {"topLevelAwait": true}}} const promise = Promise.resolve(); await assert.rejects(promise);

returns a new promise

const whatToReturn = () => Promise.reject(); const promise = Promise.reject(); return promise.catch(() => whatToReturn() );

converts it`s return value into a promise

const p = Promise.reject(); const p1 = p.catch(() => void 0); return p1.then(result => assert.equal(result, 'promise?'));

the first parameter is the rejection reason

const p = Promise.reject('rejection'); return p.catch(reason => { assert.equal(reason, 'oops'); });

multiple catches

only the first catch is called

const p = Promise.reject('1'); const p1 = p .catch(reason => void 0) .catch(reason => `${reason} AND 3`) ; return p1.then(result => assert.equal(result, '1 AND 2') );

if a catch throws, the next catch catches it

const p = Promise.reject('1'); const p1 = p .catch(reason => { throw Error(`${reason} AND 2`) }) .catch(err => { throw Error(`${err.message} AND 3`) }) .catch(err => `${err} but NOT THIS`) ; return p1.then(result => assert.equal(result, '1 AND 2 AND 3') );

Links

mdn docs A short description of how `catch` works.
spec The actual chapter about `catch`, you need to dive in from here.
spec The description of the actual flow of `catch`.

Required Knowledge

• basics (Promise)
• creation (Promise)

Related Katas

Promise

• basics
• creation
• chaining then()
• the API
• promise.catch()

Difficulty Level

INTERMEDIATE

First Published

15 March 2015

Stats

9 tests to solve