建構值
我們使用 Promise 的方式：new Promise((resolve) => resolve());
一開始建立的時候會拿到一個 callback，這個 callback 擁有兩個 function
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| class Promise {
constructor(callback) {
this.status = "pending";
this.value = "";
this.onFulfilledCallbacks = [];
this.onRejectedCallbacks = [];
const resolve = (value) => {
if (this.status === "pending") {
this.status = "fulfilled";
this.value = value;
this.onFulfilledCallbacks.forEach((fn) => fn(value));
}
};
const reject = (value) => {
if (this.status === "pending") {
this.status = "rejected";
this.value = value;
this.onRejectedCallbacks.forEach((fn) => fn(value));
}
};
try {
callback(resolve, reject);
} catch (err) {
reject(err);
}
}
then(onFulfilled, onRejected) {
return new Promise((resolve, reject) => {
if (this.status === "pending") {
this.onFulfilledCallbacks.push(() => {
try {
const fulfilledFromLastPromise = onFulfilled(this.value);
if (fulfilledFromLastPromise instanceof Promise) {
fulfilledFromLastPromise.then(resolve, reject);
} else {
resolve(fulfilledFromLastPromise);
}
} catch (err) {
reject(err);
}
});
this.onRejectedCallbacks.push(() => {
try {
const rejectedFromLastPromise = onRejected(this.value);
if (rejectedFromLastPromise instanceof Promise) {
rejectedFromLastPromise.then(resolve, reject);
} else {
reject(rejectedFromLastPromise);
}
} catch (err) {
reject(err);
}
});
}
if (this.status === "fulfilled") {
try {
const fulfilledFromLastPromise = onFulfilled(this.value);
if (fulfilledFromLastPromise instanceof Promise) {
fulfilledFromLastPromise.then(resolve, reject);
} else {
resolve(fulfilledFromLastPromise);
}
} catch (err) {
reject(err);
}
}
if (this.status === "rejected") {
try {
const rejectedFromLastPromise = onRejected(this.value);
if (rejectedFromLastPromise instanceof Promise) {
rejectedFromLastPromise.then(resolve, reject);
} else {
reject(rejectedFromLastPromise);
}
} catch (err) {
reject(err);
}
}
});
}
}
|
