Promise

A promise is an asynchronous action with two main outcomes upon completion. In both cases, we notify any interested functions of the result’s value. A promise can either be successful (a resolve) or fail (a reject).

Why do we even have promises? These are cleaner than using callbacks. We also don’t need to always have a reject. We could have a promise that only resolves.

Promises are used primarily in JavaScript. We instantiate a new promise with:

let p = new Promise ((resolve, reject) => {
	// some operation
	let a = operation()
	if (a.failed()) {
		resolve()
	} else {
		reject()
	}
})

We call the promise with:

p.then((output) => {
	// code that executes for a successful promise
}).catch((output) => {
	// code that executes for an unsuccessful promise
})

where code after the .then method is executed.