实现一个promise
`const PENDING = "PENDING";
const FULFILLED = "FULFILLED";
const REJECTED = "REJECTED";
class MyPromise {
constructor(executor) {
this.status = PENDING; // 初始状态
this.value = undefined; // 成功的值
this.reason = undefined; // 失败的原因
this.onFulfilledCallbacks = []; // 成功回调队列
this.onRejectedCallbacks = []; // 失败回调队列
const resolve = (value) => {
if (this.status === PENDING) {
this.status = FULFILLED;
this.value = value;
this.onFulfilledCallbacks.forEach((fn) => fn());
}
};
const reject = (reason) => {
if (this.status === PENDING) {
this.status = REJECTED;
this.reason = reason;
this.onRejectedCallbacks.forEach((fn) => fn());
}
};
try {
executor(resolve, reject);
} catch (error) {
reject(error);
}
}
then(onFulfilled, onRejected) {
onFulfilled = typeof onFulfilled === "function" ? onFulfilled : (v) => v;
onRejected =
typeof onRejected === "function"
? onRejected
: (err) => {
throw err;
};
const promise2 = new MyPromise((resolve, reject) => {
if (this.status === FULFILLED) {
setTimeout(() => {
try {
const x = onFulfilled(this.value);
resolve(x);
} catch (error) {
reject(error);
}
});
}
if (this.status === REJECTED) {
setTimeout(() => {
try {
const x = onRejected(this.reason);
resolve(x);
} catch (error) {
reject(error);
}
});
}
if (this.status === PENDING) {
this.onFulfilledCallbacks.push(() => {
setTimeout(() => {
try {
const x = onFulfilled(this.value);
resolve(x);
} catch (error) {
reject(error);
}
});
});
this.onRejectedCallbacks.push(() => {
setTimeout(() => {
try {
const x = onRejected(this.reason);
resolve(x);
} catch (error) {
reject(error);
}
});
});
}
});
return promise2;
}
}
// 测试代码
new MyPromise((resolve, reject) => {
setTimeout(() => resolve("成功"), 1000);
})
.then((res) => {
console.log(res); // 输出: 成功
return "下一步";
})
.then((res) => console.log(res)); // 输出: 下一步`
