Promises

Sources

A Promise is a JS object which represents the eventual completion or failure of an async operation and resulting value.

These run off the idea of error first callbacks, or just error handling.

Here is a basic example of a Promise object being created.

// promises
const myPromise = new Promise((resolve, reject) => {
    const rand = Math.floor(Math.random() * 2);
    if(rand == 0) {
        resolve();
    } else {
        reject();
    }
});

myPromise
    .then(() => console.log('Success'))
    .catch(() => console.log('Fail'));

The above is a simple random number checker looking for the number 0.

Promises, as they are created above, take in a resolve and a reject callback function and runs the resolve on a success and the reject on a fail.

We can then consume this Promise object by running the function and then calling the built in .then() and .catch() functions for a promise object.

`.then`

The .then is a built in promise object function used for consuming a success when a Promise is called.

`.catch`

The .catch is a built in promise object function used for consuming a fail when a promis is called.

Chaining

The way you see these written above:

myPromise
    .then(() => console.log('Success'))
    .catch(() => console.log('Fail'));

is called chaining, and it's a little cleaner to read. You can continually chain these together as well.

A better example:

import fetch from 'node-fetch';

fetch('https://pokeapi.co/api/v2/pokemon/ditto')
    .then((res) => res.json())
    .then((data) => console.log(data))
    .catch((err) => console.error(err));

In the above case we get the response promise object, res, convert it to JSON using the .json() function, then chain another .then which takes in the data and then logs that for us.

#JavaScript