promise js

1/*
2	A Promise is a proxy for a value not necessarily known when the promise is created. 
3    It allows you to associate handlers with an asynchronous action's eventual success 
4    value or failure reason.
5*/            
6let promise = new Promise((resolve , reject) => {
7  fetch("https://myAPI")
8    .then((res) => {
9      // successfully got data
10      resolve(res);
11    })
12    .catch((err) => {
13      // an error occured
14      reject(err);
15    });          
16});

1function doSomething() {
2  return new Promise((resolve, reject) => {
3    console.log("It is done.");
4    // Succeed half of the time.
5    if (Math.random() > .5) {
6      resolve("SUCCESS")
7    } else {
8      reject("FAILURE")
9    }
10  })
11}
12
13const promise = doSomething(); 
14promise.then(successCallback, failureCallback);

1We use promise to make a  AsyncFunction, cose simetime we have to wait that function give 
2us some result.
3Example, if we use ajax we have await ajax data or statament.
4_____________________________________
5Make a simple example.
6_____________________________________
7var asyncronus_function= (number)=>
8		{
9			return new Promise( (accept, reject)=>
10			{
11			})
12		}                 
13_____________________________________
14this function return a promise Object, thet required a function executor
15this functions (accept, reject) are defined in the executor 
16function, that was needed in promise constructor.
17Syntax: new Promise (executor)
18executor= (accept, reject) =>{}
19
20if function end well we return a accept(), otherwise reject()
21_____________________________________
22Let complete asyncronus_function
23_____________________________________
24var asyncronus_function= (number)=>
25		{
26			return new Promise( (accept, reject)=>
27			{
28				if(number>10)
29				return accept("my first async");
30				return reject("my first async error")
31			})
32
33		}
34if it dont return any of this 2 function, Promise state is [PENDING] ,
35if return accept is [RESOLVED]  end if return reject is [REJECTED]
36_____________________________________
37how we can retrieve accept or reject?
38_____________________________________
39there is two methods really important, that we have to consider afther we call this function
401) .then(function(error){}) is call when promise state is [RESOLVED]
412) .error(function(error){}) is call when promise state is [REJECTED]
423) do nothing if [PENDING]
43_____________________________________
44let call asyncronus_function()!!! 
45_____________________________________
46	asyncronus_function(MY_NUMBER).then(function(data)
47        {
48			console.log(data)
49    	}).catch(error => 
50        {
51      			console.log(error)
52    	});
53		
54if  MY_NUMBER> 10 ,asyncronus_function print data : OUTPUT my first async
55if MY_NUMBER<10 , asyncronus_function print error : OUTPUT my first async error
56HOPE it halp and have a nice day! 
57
58
59

1A promise is an object that may produce a single value some time in the future: 
2either a resolved value, or a reason that it’s not resolved 

1A Promise is in one of these states:
2
3pending: initial state, neither fulfilled nor rejected.
4fulfilled: meaning that the operation was completed successfully.
5rejected: meaning that the operation failed.

1//example
2function tetheredGetNumber(resolve, reject) {
3  try {
4    setTimeout( 
5      function() {
6        const randomInt = Date.now();
7        const value = randomInt % 10;
8        try { 
9          if(value >= THRESHOLD_A) {
10            throw new Error(`Too large: ${value}`);
11          } 
12        } catch(msg) {
13            reject(`Error in callback ${msg}`); 
14        }
15      resolve(value);
16      return;
17    }, 500);
18  } catch(err) {
19    reject(`Error during setup: ${err}`);
20  }
21  return;
22}