Promise

Promise States:

Pending - Operation is still running

Fulfilled - Operation has finished successfully

Rejected - Operation has failed (normally an error)

const executorFunction = (resolve, reject) => {
	if (someCondition) {
      resolve('I resolved!');
  } else {
      reject('I rejected!'); 
  }
};

const promiseName = new Promise(executorFunction);

What is the resole and reject functions?

resolve is a function with one argument. Under the hood, if invoked, resolve() will change the promise’s status from pending to fulfilled, and the promise’s resolved value will be set to the argument passed into resolve().

reject is a function that takes a reason or error as an argument. Under the hood, if invoked, reject() will change the promise’s status from pending to rejected, and the promise’s rejection reason will be set to the argument passed into reject().

.then - Promise handlers

Method 1

const prom = new Promise((resolve, reject) => {
	if (condition) {
		resolve('went well');
	} else {
		reject('went bad');
	}
});

// what happens after a resolve... resolvedValue will = 'went well'
const handleSuccess = (resolvedValue) => {
  console.log(resolvedValue);
};

// what happends after a reject... rejectionReason will = 'went bad'
const handleFailure = (rejectionReason) => {
  console.log(rejectionReason);
};

prom.then(handleSuccess, handleFailure);

Method 2

// Note, this works because, the first then will,
// return the setteled value from the resolved,
// thus we can then run the second then/catch for the rejection
prom(order)
  .then(handleSuccess)
  .catch(handleFailure);

Chaining Promises (DONT NEST THEM)

firstPromiseFunction()
.then((firstResolveVal) => {
  return secondPromiseFunction(firstResolveVal);
})
.then((secondResolveVal) => {
  return thirdPromiseFunction(secondResolveVal);
})
.catch((error) => {
	// ... do whatever with the error message
});

Concurrent Promises ALL

let myPromises = Promise.all([returnsPromOne(), returnsPromTwo(), returnsPromThree()]);
 
myPromises
  .then((arrayOfValues) => {
    console.log(arrayOfValues);
  })
  .catch((rejectionReason) => {
    console.log(rejectionReason);
  });

setTimeout

Will delay the execution of a function... after n time, the function will be added to the execution stack (I presume)

Note: rest of the program still executes

setTimeout(funcName, 2000); // 2 sec timeout