README.md

Promise - How to Resolve or Reject a Promise

Promise: An Object represents an eventual completion or failure of an asynchronous operations and its resulting value.

Example:

Resolved promise.

const promise1 = new Promise(function(resolve, reject){
   resolve('Water');
});

Rejected promise.

const promise2 = new Promise(function(resolve, reject){
   reject(new Error("Disaster!"));
});

Promise takes an exector function as parameter which can have deciding factor to resolve or reject it.

• Promise has 3 different states.

  1. pending: When the exection starts.
  2. fullfilled: When the promise gets resolved.
  3. rejected: When the promise gets rejected.

• Promise can have 3 different values based on the state.

  1. undefined: When promise is in pending state, not fulfilled or rejected.
  2. value: When the promise gets resolved.
  3. error: When the promise gets rejected.

• Promise provides 3 method handlers.

  1. .then()
  2. .catch()
  3. .finally()

Example:

const promise = new Promise(function(resolve, reject){
    resolve('Water');
    //reject(new Error("Jack fell down"));
});

const consumer = promise.then((result) => 
console.log(`Cooking with ${result}.`)
).catch((error) => 
console.log(`OMG ${error.message}.`));

consumer();

Output:
if promise gets resolved the output will be: Cooking with Water.
if promise gets rejected the output will be: OMG Jack fell down.