javascript promises
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 */ }
);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 {<pending>}
// 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 {<pending>}.
// This will happen because there is no data yet and we need to wait for it.promise javascript
let promise = new Promise(function(resolve, reject){
try{
resolve("works"); //if works
} catch(err){
reject(err); //doesn't work
}
}).then(alert, console.log); // if doesn't work, then console.log it, if it does, then alert "works"promise definition in javascript
A promise is an object that may produce a single value sometime in the future: either a resolved value, or a reason that it's not resolved (e.g., a network error occurred)promise javascript
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: 777Copyright © 2021 Codeinu
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