promise 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 */ }
);
//example
function tetheredGetNumber(resolve, reject) {
try {
setTimeout(
function() {
const randomInt = Date.now();
const value = randomInt % 10;
try {
if(value >= THRESHOLD_A) {
throw new Error(`Too large: ${value}`);
}
} catch(msg) {
reject(`Error in callback ${msg}`);
}
resolve(value);
return;
}, 500);
} catch(err) {
reject(`Error during setup: ${err}`);
}
return;
}
const p1 = new Promise((resolve, reject) => {
console.log("Promise has started");
const lifeIsEasy = false;
if (lifeIsEasy) {
//Value to be returned if the promise is resolved
//return value is optional
resolve("No it's not");
}
else {
//Value to be returned if the promise is rejected
//return value is also optional
reject("welcome to the real world");
}
});
//then is executed when the promise is resolved
p1.then(value => {
console.log(value);
})
//catch is executed when the promise is rejected
.catch (err => {
console.log(err);
});
/*expected output
Promise has started
welcome to the real world*/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)=>
{
})
}
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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()
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Let complete asyncronus_function
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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]
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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!
let myFirstPromise = new Promise((resolve, reject) => {
// We call resolve(...) when what we were doing asynchronously was successful, and reject(...) when it failed.
// In this example, we use setTimeout(...) to simulate async code.
// In reality, you will probably be using something like XHR or an HTML5 API.
setTimeout( function() {
resolve("Success!") // Yay! Everything went well!
}, 250)
})
myFirstPromise.then((successMessage) => {
// successMessage is whatever we passed in the resolve(...) function above.
// It doesn't have to be a string, but if it is only a succeed message, it probably will be.
console.log("Yay! " + successMessage)
});
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
