Promise 的回呼總是在同步程式碼之後執行
const promise = Promise.resolve();
promise.then(() => console.log('async'));
console.log('sync');
//sync
//async
Promise 每次呼叫時都會傳回一個新的 Promise
const p = Promise.resolve();
const chain = p.then(() => {});
console.log(p === chain); //false
承諾支持無限連結
Promise.resolve(1)
.then(value => value + 1)
.then(value => value + 1)
.then(value => console.log(value)); // 3
您可以包裝使用回調的舊程式碼，以與現代非同步/等待一起使用
function asyncOperation(callback) {
setTimeout(() => callback(null, 'Im a callback'), 1000);
}
const promisified = () => new Promise((resolve, reject) => {
asyncOperation((err, result) => {
if (err) reject(err);
else resolve(result);
});
});
promisified().then(result => console.log(result)); // "Im a callback"
如果您傳遞一個非 Promise 值，Promise.resolve() 會將其包裝為已解決的 Promise。但如果你傳遞一個 Promise，它只會回傳相同的 Promise。
const p1 = Promise.resolve('Hello');
const p2 = Promise.resolve(p1);
console.log(p1 === p2); // true
Promise.reject('Error!')
.then(() => console.log('This will not run'))
.then(() => console.log('This will also not run'))
.catch(err => console.log('Caught:', err))
.then(() => console.log('This will run'));
finally() 方法不會接收或修改已解析的值。它用於清理資源並運行，無論 Promise 解決還是拒絕。
Promise.resolve('resolved')
.then(value => console.log(value))
.finally(() => console.log('Cleanup'))
//resolved
//cleanup
一旦 Promise 被解決（解決或拒絕），它的狀態就是不可變的。此後無法更改，即使您再次嘗試解決/拒絕它。
const p = new Promise((resolve, reject) => {
resolve('First');
resolve('Second');
});
p.then(value => console.log(value)); //"First" (only the first value is used)
Promise.reject('type C error')
.catch(err => {
if (err === 'type A error') console.log('handle type A');
throw err;
})
.catch(err => {
if (err === 'type B error') console.log('handle type B');
throw err;
})
.catch(err => {
if (err === 'type C error') console.log('handle type C');
throw err;
})
async function demo() {
const result = await 42; //not a promise
console.log(result);
}
demo(); //42
就是這樣！感謝您閱讀本文。下次見！
以上是關於 Javascript Promise 的有趣事實的詳細內容。更多資訊請關注PHP中文網其他相關文章！
