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.

In practice

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. We also don’t need to always have a reject. We could have a promise that only resolves.

Another option is to wrap it in an async function, then use await within a try catch block:

try {
	output = await p();
} catch(e) {
	// err
}