Coding diary to reflect concepts that I am learning and revisiting

Promises

console.log("A");
wait(1000).then(()=> {
    console.log("B");
});
console.log("C");

/* OUTPUT
A
C
B (after 1 second)
*/

• You can only can then() on functions that returns a new promise
• Several Web APIs (which are functions in the browswer) return promises. Examples of such Web APIs include fetch and getUserMedia (which is for accessing camera and audio)

A promise has 3 different states:

1. Pending
   • Creating a promise starts the pending state
2. fulfiled
   • When the promise is completed the callback gets passed to .then(callback) which will be executed
3. Rejected

getWeatherConditions(city)
    .then(data => {
        console.log(data);
    })
    .catch(error => {
        console.error(error);
    });

• When a promise is successfully completed it becomes fulfilled

Promise finally()

• The .finally() callback executed whenever the state of a promise changes from pending to fulfilled or from pending to rejected
• finally() executes when the promise fails or succeeds

Manually rejecting a promise

A promise can be manually rejected by throwing an error

E.g.

getWeatherCondition(city)
	.then(data => {
		throw new Error("Stop!!!.")
		console.log(`Info ${data}`); // not executed
	})
	.catch(error => {
		console.error(error); // executed
	})

Creating Promises

const waitOneSecond = () => {
	return new Promise((resolve) => {
		resolve();
	})
}

Topics Reviewed:

• reduce()
• nullish coalescing and optional chaining
• Using Object.keys() to get an array of all the keys in an object
• Using Object.values() to get an array of all the values in an object
• Using Object.entries() to get a 2D array for an object