Promise basics
Thu Dec 23 2021 11:42:55 GMT+0000 (Coordinated Universal Time)
Saved by
@bapateprachee
#javascript
>> promises basic concept
// promise example
const getSomething = () => {
return new Promise((resolve, reject) => {
// do something (fetch data)
// resolve('some data');
reject('some error');
});
};
// getSomething().then(data => {
// console.log('promise resolved:', data);
// }).catch(err => {
// console.log('promise rejected:', err);
// });
>> promise chaining
const getTodos = (resource) => {
return new Promise((resolve, reject) => {
const request = new XMLHttpRequest();
request.addEventListener('readystatechange', () => {
if(request.readyState === 4 && request.status === 200){
const data = JSON.parse(request.responseText);
resolve(data);
} else if (request.readyState === 4){
reject('could not fetch the data');
}
});
request.open('GET', resource);
request.send();
});
};
getTodos('json/luigi.json').then(data => {
console.log('promise 1 resolved:', data);
return getTodos('json/mario.json');
}).then(data => {
console.log('promise 2 resolved:', data);
return getTodos('json/shaun.json');
}).then(data => {
console.log('promise 3 resolved:', data);
}).catch(err => {
console.log('promise rejected:', err);
});
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