quartz/content/notes/06-async-javascript.md

title	aliases	tags
06-async-javascript		lecture cosc203

Async programming allows you to start a potentially long running task have still be able to interact while it it running

general process

• start the task
• return immediately so other tasks can run
• notify us with result when the task is finished

promises

• an object returned by an async function
• represents the current state of the operation
• when the promise is returned to the caller it not always finished
• the promise object has methods to handle the eventual success or failure of the operation

const fetchPromise = fetch('https://url.url.json')

fetchPromise
	.then((reponse) => {
		return response.json();
	})
	.then((data) => {
		console.log(data[0].name)
	})

each .then itself returns another promise which also has a .then method. This is called promise chaining

promise statuses

pending: the promise is created but the task has not finished yet fulfilled: the task has finished successfully -> then() is called rejected: the task has failed -> catch() is called settled: fulfilled or rejected

combining promises

Promise.all([fetchPromsise1, fetchPromsise2, fetchPromsise3]) //all promises need to be fulfilled but they dont depend of each other
Promise.any([fetchPromsise1, fetchPromsise2, fetchPromsise3]) //any 

error handling

the fetch API can throw errors

to handle these promise objects have a catch method. this is similar to a .then but only trigger when there is an error.

const fetchPromise = fetch('https://url.url.json')

fetchPromise
	.then((reponse) => {
		return response.json();
	})
	.then((data) => {
		console.log(data[0].name)
	})
	.catch((error) => {
		console.error('could not get products: ${error}')
	})