Example 1: js create a promise
/*
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);
});
});
Example 2: javascript return promise
function doSomething() {
return new Promise((resolve, reject) => {
console.log("It is done.");
// Succeed half of the time.
if (Math.random() > .5) {
resolve("SUCCESS")
} else {
reject("FAILURE")
}
})
}
const promise = doSomething();
promise.then(successCallback, failureCallback);
Example 3: javascript promise
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 4: promise catch
//create a Promise
var p1 = new Promise(function(resolve, reject) {
resolve("Success");
});
//Execute the body of the promise which call resolve
//So it execute then, inside then there's a throw
//that get capture by catch
p1.then(function(value) {
console.log(value); // "Success!"
throw "oh, no!";
}).catch(function(e) {
console.log(e); // "oh, no!"
});
Example 5: promise js
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.
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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?
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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]
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let call asyncronus_function()!!!
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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!
Example 6: js promise examples
//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);
});
};