This whole Promise work-out idea and explanation comes from Roger Melo video on Promises as it's in pure Javascript.
Click below to be redirected to the video in portuguese-brazilian - there's no english version.
A promise is an object that represents the success or the error of a asynchronous operation.
When executing a asynchronous operation - a request, for example - Javascript pulls out a new thread to resolve the request separatedely while the main thread flows on with it's functioning.
Promises normally receives two functions for it's callbacks, a resolve and a reject function. These functions are invokeds as it's needed to resolve or reject some operation/value.
A promise usually is chained by other promises inside of it, as it happens on the example below:
const aPromise = new Promise((resolve, reject) => {
const aNUmber = 37
//resolve(aNumber) // resolve returns the value
reject(aNumber)
})
aPromise
.then(value => value)
.then(value => {
console.log(value)
})
.catch(rejectValue => {
// catch is only invoked as an
// error handler for the promise
console.log(rejectValue)
})The example above is not an asynchronous operation, but shows off how does a Promise works by it's syntax.
If you want to check out a asynchronous operation working with a Promise, go to the promise.js file.
There you'll see how to we do a request to an API, where each request returns a random dog image for us, and we handle this operation with a Promise.