JavaScript Promise 簡單範例說明

範例：

// new 一個 Promise 需要一個 callback, 這個 callback 可接受兩個 callback

var promiseToCleanTheRoom = new Promise(function (resolve, reject) {

console.log('promise is working');

let isClean = true;

if (isClean) {

resolve('clean'); // 執行 .then() 的 callback

} else {

reject('not clean'); // 執行 .catch() 的 callback

}

});

promiseToCleanTheRoom.then(function (fromResolve) {

// resolve

console.log('room is ' + fromResolve);

}).catch(function (fromReject) {

// reject

console.log('room is ' + fromReject);

});

必須完成某件事才能進行下一件事

例如 "打掃房間 > 丟垃圾 > 拿到冰淇淋"

let cleanRoom = function () {

return new Promise(function (resolve, reject) {

resolve(' Cleaned the room.');

});

};

let removeGarbage = function (p) {

return new Promise(function (resolve, reject) {

resolve(p + ' Remove garbage.');

});

};

let winIceCream = function (p) {

return new Promise(function (resolve, reject) {

resolve(p + ' won ice cream!');

});

};

cleanRoom().then(function (result) { // 等待 cleanRoom 的結果

return removeGarbage(result);

}).then(function (result) { // 等待 removeGarbage 的 結果

return winIceCream(result);

}).then(function (result) { // 等待 winIceCream 的結果

console.log('finished : ' + result);

});

三個 Promise 同時進行，等待全部完成：

Promise.all([cleanRoom(), removeGarbage(), winIceCream()]).then(function () {

console.log('all done');

});

只少有一個 Promise 已經先完成：

Promise.race([cleanRoom(), removeGarbage(), winIceCream()]).then(function () {

console.log('one of them is done');

});