how to use promise code example
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 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 3: what is a promise
// Promise is a special type of object that helps you work with asynchronous operations.
// Many functions will return a promise to you in situations where the value cannot be retrieved immediately.
const userCount = getUserCount();
console.log(userCount); // Promise {}
// In this case, getUserCount is the function that returns a Promise. If we try to immediately display the value of the userCount variable, we get something like Promise {}.
// This will happen because there is no data yet and we need to wait for it.
Example 4: js return a promise
function myAsyncFunction(url) {
return new Promise((resolve, reject) => {
const xhr = new XMLHttpRequest();
xhr.open("GET", url);
xhr.onload = () => resolve(xhr.responseText);
xhr.onerror = () => reject(xhr.statusText);
xhr.send();
});
}
Example 5: promises in javascript
myPromise.then(
function(value) { /* code if successful */ },
function(error) { /* code if some error */ }
);
Example 6: making promises in js
getData()
.then(data => console.log(data))
.catch(error => console.log(error));