promise javascript

JavaScript
var promise = new Promise(function(resolve, reject) {
  // do some long running async thing…
  
  if (/* everything turned out fine */) {
    resolve("Stuff worked!");
  }
  else {
    reject(Error("It broke"));
  }
});

//usage
promise.then(
  function(result) { /* handle a successful result */ },
  function(error) { /* handle an error */ }
);/*
	A Promise is a proxy for a value not necessarily known when the promise is created. 
    It allows you to associate handlers with an asynchronous action's eventual success 
    value or failure reason.
*/            
let promise = new Promise((resolve , reject) => {
  fetch("https://myAPI")
    .then((res) => {
      // successfully got data
      resolve(res);
    })
    .catch((err) => {
      // an error occured
      reject(err);
    });          
});We use promise to make a  AsyncFunction, cose simetime we have to wait that function give 
us some result.
Example, if we use ajax we have await ajax data or statament.
_____________________________________
Make a simple example.
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var asyncronus_function= (number)=>
		{
			return new Promise( (accept, reject)=>
			{
			})
		}                 
_____________________________________
this function return a promise Object, thet required a function executor
this functions (accept, reject) are defined in the executor 
function, that was needed in promise constructor.
Syntax: new Promise (executor)
executor= (accept, reject) =>{}

if function end well we return a accept(), otherwise reject()
_____________________________________
Let complete asyncronus_function
_____________________________________
var asyncronus_function= (number)=>
		{
			return new Promise( (accept, reject)=>
			{
				if(number>10)
				return accept("my first async");
				return reject("my first async error")
			})

		}
if it dont return any of this 2 function, Promise state is [PENDING] ,
if return accept is [RESOLVED]  end if return reject is [REJECTED]
_____________________________________
how we can retrieve accept or reject?
_____________________________________
there is two methods really important, that we have to consider afther we call this function
1) .then(function(error){}) is call when promise state is [RESOLVED]
2) .error(function(error){}) is call when promise state is [REJECTED]
3) do nothing if [PENDING]
_____________________________________
let call asyncronus_function()!!! 
_____________________________________
	asyncronus_function(MY_NUMBER).then(function(data)
        {
			console.log(data)
    	}).catch(error => 
        {
      			console.log(error)
    	});
		
if  MY_NUMBER> 10 ,asyncronus_function print data : OUTPUT my first async
if MY_NUMBER<10 , asyncronus_function print error : OUTPUT my first async error
HOPE it halp and have a nice day! 


//Promise to load a file...
//use with openFile(url, fCallback);
//fCallback(fileContents){ //do something with fileContents}
const loadFile = url => {
    return new Promise(function(resolve, reject) {
        //Open a new XHR
        var request = new XMLHttpRequest();
        request.open('GET', url);
        // When the request loads, check whether it was successful
        request.onload = function() {
            if (request.status === 200) {
                // If successful, resolve the promise
                resolve(request.response);
            } else {
                // Otherwise, reject the promise
                reject(Error('An error occurred while loading image. error code:' + request.statusText));
            }
        };
        request.send();
    });
};
const openFile = (url, processor) => {
    loadFile(url).then(function(result) {
            processor(result);
        },
        function(err) {
            console.log(err);
        });
};const promiseA = new Promise( (resolutionFunc,rejectionFunc) => {
    resolutionFunc(777);
});
// At this point, "promiseA" is already settled.
promiseA.then( (val) => console.log("asynchronous logging has val:",val) );
console.log("immediate logging");

// produces output in this order:
// immediate logging
// asynchronous logging has val: 777
/*
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. 
The syntax looks like this:
*/

const myPromise = new Promise((resolve, reject) => {

});


Source

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