Promises Basics, Promise.then() & Promise.catch() in Javascript

 

     Promises Basics, Promise.then() Promise.catch():

 Promises are the ideal choice for handling asynchronous operations in the simplest-      manner.

   

• Benefits of Promises 
  1. Improves Code Readability
  2. Better handling of asynchronous operations
  3. Better flow of control definition in asynchronous logic
  4. Better Error Handling

• A Promise has four states: 
  1. fulfilled: Action related to the promise succeeded
  2. rejected: Action related to the promise failed
  3. pending: Promise is still pending i.e. not fulfilled or rejected yet
  4. settled: Promise has been fulfilled or rejected

   ---A promise can be created using the Promise constructor. --

  Here is the syntax: 

     let promise= new Promise(function(resolve, reject){

     //do something
});

• Parameters 
  • Promise constructor takes only one argument which is a callback function (and that callback function is also referred as anonymous function too).
  • Callback function takes two arguments, resolve and reject
  • Perform operations inside the callback function and if everything went well then call resolve.
  • If desired operations do not go well then call reject.

    Promise Consumers

   Promises can be consumed by registering functions using .then and .catch methods.

   1. then() : then() is invoked when a promise is either resolved or rejected.

   Parameters: then() method takes two functions as parameters. 

   1. First function is executed if promise is resolved and a result is received.

  2. Second function is executed if promise is rejected and an error is received.

   

   Syntax:

    .then(function(result){

        //handle success
    }, function(error){
        //handle error
    })

   2. catch() : catch() is invoked when a promise is either rejected or some error has occurred in execution.

   Parameters: 

   catch() method takes one function as parameter. 

  

1. Function to handle errors or promise rejections.(.catch() method internally calls .then(null, errorHandler), i.e. .catch() is just a shorthand for .then(null, errorHandler) )

 Syntax: 

  

   .catch(function(error){

        //handle error
    })

  Here is all code:

console.log("hello console");

    //promise

    //resolve

    //reject

    //pending

    //promise constructor

    function func1() {

    return new Promise(function (resolve, reject) {

        const error = true;

        if (!error) {

        console.log("your promise is sucessfully resolve");

        resolve();

        } else {

        console.log("your promise is failed");

        reject("sorry not fulfil");

        }

        setTimeout(() => {

        console.log("your promise is return");

        resolve();

        }, 2000);

    });

    }

    func1()

    .then(function () {

        console.log("Hasan: thanks for resolving");

    })

    .catch(function () {

        console.log("failed");

    });