#javascript #promise related examples

I would like to share some of my experience with javascript Promise. I will add on to this list when I have more promise examples to share.

Promise

Taken from MDN:
The Promise object represents the eventual completion (or failure) of an asynchronous operation, and its resulting value.

A Promise has 3 states:
i) pending: initial state when a promise has not been fulfilled nor rejected yet.
ii) fulfilled: the operation within a promise is executed successfully.
iii) rejected: the operation within a promise failed to complete.

MDN Promise handling and chaining

Promise related examples

1) Wrapping functions that return a callback with a promise

// a function that takes a callback function

function foo(type, callback) {

  callback(type);

}

function fooPromise(param) {

    return new Promise(function(resolve, reject) {

         foo(param, (err, res) => {

             if (err !== null) 

               reject(err);

             resolve(res);

         });

    });

}

2) Returning a promise within a promise:

var innerPromise = function() {

  return new Promise(function(resolve, reject) {

    setTimeout(() => resolve("innerPromise fulfilled"), 1000);

  });

}

var outerPromise = function() {

  // return a promise that contains another promise

  return new Promise(function(resolve, reject) {

    // calling innerPromise

    innerPromise().then(function(res) {

       // innerPromise is done

       console.log(res);

       // resolve with some value / object 

       setTimeout(() => resolve("outerPromise fulfilled"), 1000);

    })

  });

}

outerPromise()

  .then(res => console.log(res))

  .catch(err => console.log(err)); 

// on console

// innerPromise fulfilled

// outerPromise fulfilled

I will keep updating this list. Thanks for reading!

Jun
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