class Promise {
constructor(fn) {
this.status = 'pending';
this.onFulfilledCallbacks = [];
this.onRejectedCallbacks = [];
// 更改成功后的状态
const resolve = value => {
if (this.status === 'pending') {
this.status = 'fulfilled';
this.onFulfilledCallbacks.forEach(fn => fn(value));
}
}
// 更改失败后的状态
const reject = err => {
if (this.status === 'pending') {
this.status = 'rejected';
this.onRejectedCallbacks.forEach(fn => fn(err));
}
}
try {
fn(resolve, reject)
} catch(err) {
reject(err)
}
}
then(onFulfilled, onRejected) {
onFulfilled = typeof onFulfilled === 'function' ? onFulfilled : v => v;
onRejected = typeof onRejected === 'function' ? onRejected : err => {throw err};
// 为了链式调用，这里直接返回一个 Promise
return new Promise((resolve, reject) => {
const fulfilledMicrotask = value => {
queueMicrotask(() => {
try {
const res = onFulfilled(value);
res instanceof Promise ? res.then(resolve, reject) : resolve(res);
} catch(err) {
reject(err);
}
})
}
const rejectedMicrotask = value => {
queueMicrotask(() => {
try {
const res = onRejected(value);
res instanceof Promise ? res.then(resolve, reject) : reject(res);
} catch(err) {
reject(err);
}
})
}
this.onFulfilledCallbacks.push(fulfilledMicrotask);
this.onRejectedCallbacks.push(rejectedMicrotask);
});
}
static resolve(value) {
if (value instanceof Promise) {
return value;
}
return new Promise(resolve => {
resolve(value);
});
}
static reject(err) {
return new Promise((resolve, reject) => {
reject(err);
});
}
}
