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: javascript create promise
// base case
const promise = new Promise(executor);
// more complex example
const coinflip = (bet) => new Promise((resolve, reject) => {
const hasWon = Math.random() > 0.5 ? true : false;
if (hasWon) {
setTimeout(() => {
resolve(bet * 2);
}, 2000);
} else {
reject(new Error("You lost...")); // same as -> throw new Error ("You lost ...");
}
});
Example 4: 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);
});
};
Example 5: what is promise in javascript
Promises make async javascript easier as they are easy to use and write than callbacks. Basically , promise is just an object , that gives us either success of async opertion or failue of async operations
Example 6: javascript promises
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));