Here is a simple example of a promise in js

basicPromiseExample.js

let addPromise = (a, b) => new Promise((resolve, reject) => {
    if (isNaN(a) === false && isNaN(b) === false) {
        resolve(a + b)
    } else {
        reject('a or b, is not a number')
    }
})
addPromise(1, 2).then(resolved => {
    console.log(`promise resolved with ${resolved}`)
    return addPromise(1, 3)
}).then(resolved => {
    console.log(`promise resolved with ${resolved}`)
}).catch(err => {
    console.log(`promise rejected with ${err}`)
})
addPromise(1, 'cookie monster').then(resolved => {
    console.log(`promise resolved with ${resolved}`)
}).catch(err => {
    console.log(`promise rejected with ${err}`)
})