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 */ }
);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"let num = 10;
//call back function
const promis = new Promise(function (resolve, reject) {
if (num > 5) {
//this resolve method will send data to resoleveData variable
resolve(" Problem resolved successfully")
}
else {
//this reject method will send data to rejectData variable
reject("sorry problem couldn't solve")
}
})
//resoleveData variable
promis.then(function (resolveData) {
console.log(resolveData)
//rejectData variable
}).catch(function (rejectData) {
console.log(rejectData)
})
Promises are used to handle asynchronous operations in JavaScript.
They are easy to manage when dealing with multiple asynchronous operations
where callbacks can create callback hell leading to unmanageable code.var posts = [
{name:"Mark42",message:"Nice to meet you"},
{name:"Veronica",message:"I'm everywhere"}
];
function Create_Post(){
setTimeout(() => {
posts.forEach((item) => {
console.log(`${item.name} --- ${item.message}`);
});
},1000);
}
function New_Post(add_new_data){
return new Promise((resolve, reject) => {
setTimeout(() => {
posts.push(add_new_data);
var error = false;
if(error){
reject("Something wrong in </>, Try setting me TRUE and check in console");
}
else{
resolve();
}
},2000);
})
}
New_Post({name:"War Machine",message:"I'm here to protect"})
.then(Create_Post)
.catch(err => console.log(err));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)