手写精简版Promise

[chunhuigao]

class MyPromise {
	constructor(fn) {
		this.status = 'pedding';
		this.value = undefined; //成功理由
		this.reason = undefined; //失败理由
		this.onFulfilledList = [];
		this.onRejectedList = [];

		const resolve = (result) => {
			setTimeout(() => {
				if (this.status === 'pedding') {
					this.status = 'resolved';
					this.value = result;
					this.onFulfilledList.forEach((fn) => fn(this.value));
				}
			}, 0);
		};
		const reject = (error) => {
			setTimeout(() => {
				if (this.status === 'pedding') {
					this.status = 'rejected';
					this.reason = error;
					this.onRejectedList.forEach((fn) => fn(this.reason));
				}
			}, 0);
		};
		try {
			fn(resolve, reject);
		} catch (error) {
			reject(error);
		}
	}

	then(onFulfilled, onRejected) {
		if (this.status === 'pedding') {
			return new MyPromise((resolve, reject) => {
				this.onFulfilledList.push(() => {
					try {
						const x = onFulfilled(this.value);
						if (x instanceof MyPromise) {
							x.then(resolve, reject);
						} else {
							resolve(x);
						}
					} catch (error) {
						reject(error);
					}
				});
				this.onRejectedList.push(() => {
					try {
						const x = onFulfilled(this.value);
						if (x instanceof MyPromise) {
							x.then(resolve, reject);
						} else {
							resolve(x);
						}
					} catch (error) {
						reject(error);
					}
				});
			});
		} else if (this.status === 'resolved') {
			onFulfilled(this.value);
		} else if (this.status === 'rejected') {
			onRejected(this.reason);
		}
	}
}