手写一个promise
时间: 2023-05-11 07:05:24 浏览: 73
Promise 是 JavaScript 中的一种异步编程解决方案,可以避免回调地狱,以下是手写一个 Promise 的代码示例:
```
class MyPromise {
constructor(executor) {
this.status = 'pending';
this.value = undefined;
this.reason = undefined;
this.onResolvedCallbacks = [];
this.onRejectedCallbacks = [];
const resolve = (value) => {
if (this.status === 'pending') {
this.status = 'fulfilled';
this.value = value;
this.onResolvedCallbacks.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 : value => value;
onRejected = typeof onRejected === 'function' ? onRejected : reason => { throw reason };
const promise2 = new MyPromise((resolve, reject) => {
if (this.status === 'fulfilled') {
setTimeout(() => {
try {
const x = onFulfilled(this.value);
this.resolvePromise(promise2, x, resolve, reject);
} catch (error) {
reject(error);
}
}, 0);
} else if (this.status === 'rejected') {
setTimeout(() => {
try {
const x = onRejected(this.reason);
this.resolvePromise(promise2, x, resolve, reject);
} catch (error) {
reject(error);
}
}, 0);
} else {
this.onResolvedCallbacks.push(() => {
setTimeout(() => {
try {
const x = onFulfilled(this.value);
this.resolvePromise(promise2, x, resolve, reject);
} catch (error) {
reject(error);
}
}, 0);
});
this.onRejectedCallbacks.push(() => {
setTimeout(() => {
try {
const x = onRejected(this.reason);
this.resolvePromise(promise2, x, resolve, reject);
} catch (error) {
reject(error);
}
}, 0);
});
}
});
return promise2;
}
resolvePromise(promise2, x, resolve, reject) {
if (promise2 === x) {
return reject(new TypeError('Chaining cycle detected for promise'));
}
let called = false;
if (x !== null && (typeof x === 'object' || typeof x === 'function')) {
try {
const then = x.then;
if (typeof then === 'function') {
then.call(x, y => {
if (called) return;
called = true;
this.resolvePromise(promise2, y, resolve, reject);
}, r => {
if (called) return;
called = true;
reject(r);
});
} else {
resolve(x);
}
} catch (error) {
if (called) return;
called = true;
reject(error);
}
} else {
resolve(x);
}
}
catch(onRejected) {
return this.then(null, onRejected);
}
finally(onFinally) {
return this.then(