Promise

Created : August 04, 2020

use it to make a promise to do something, usually asynchronously. Promise is a constructor function, so you need to use the new keyword to create one. It takes a function, as its argument, with two parameters - resolve and reject. These are methods used to determine the outcome of the promise.

A promise has three states: pending, fulfilled, and rejected. resolve is used when you want your promise to succeed, and reject is used when you want it to fail.

const myPromise = new Promise((resolve, reject) => {
  if(condition here) {
    resolve("Promise was fulfilled");
  } else {
    reject("Promise was rejected");
  }
});

Handle a Fulfilled Promise with then

Promises are most useful when you have a process that takes an unknown amount of time in your code (i.e. something asynchronous), often a server request. When you make a server request it takes some amount of time, and after it completes you usually want to do something with the response from the server. This can be achieved by using the then method. The then method is executed immediately after your promise is fulfilled with resolve.

myPromise.then(result => {
  console.log(result);
});

result comes from the argument given to the resolve method.

Handle a Rejected Promise with catch

catch is the method used when your promise has been rejected. It is executed immediately after a promise's reject method is called.

myPromise.catch(error => {
  console.log(error);
});

the then and catch methods can be chained to the promise declaration if you choose.