基于 Promise A+ 规范使用Typescript实现Promise

以下是一个简单的基于Promise/A+规范的Typescript实现的示例：

interface IPromise<T> {
    then<U>(onFulfilled?: (value: T) => U | IPromise<U>, onRejected?: (error: any) => U | IPromise<U>): IPromise<U>;
    catch<U>(onRejected?: (error: any) => U | IPromise<U>): IPromise<U>;
}
 
class MyPromise<T> implements IPromise<T> {
    private state: 'pending' | 'fulfilled' | 'rejected' = 'pending';
    private value: T | undefined;
    private reason: any;
    private onFulfilledCallbacks: Array<(value: T) => void> = [];
    private onRejectedCallbacks: Array<(reason: any) => void> = [];
 
    constructor(executor: (resolve: (value?: T | IPromise<T>) => void, reject: (reason?: any) => void) => void) {
        executor(this.resolve.bind(this), this.reject.bind(this));
    }
 
    private resolve(value?: T | IPromise<T>) {
        if (this.state === 'pending') {
            if (value instanceof MyPromise) {
                value.then(this.resolve.bind(this), this.reject.bind(this));
            } else {
                this.state = 'fulfilled';
                this.value = value as T;
                this.onFulfilledCallbacks.forEach(callback => callback(this.value));
            }
        }
    }
 
    private reject(reason?: any) {
        if (this.state === 'pending') {
            this.state = 'rejected';
            this.reason = reason;
            this.onRejectedCallbacks.forEach(callback => callback(this.reason));
        }
    }
 
    then<U>(onFulfilled?: (value: T) => U | IPromise<U>, onRejected?: (error: any) => U | IPromise<U>): IPromise<U> {
        let promise2 = new MyPromise<U>((resolve, reject) => {
            if (this.state === 'fulfilled') {
                let x = onFulfilled ? onFulfilled(this.value as T) : this.value as U;
                resolve(x);
            } else if (this.state === 'rejected') {
                let x = onRejected ? onRejected(this.reason) : this.reason;
                reject(x);
            } else {
                this.onFulfilledCallbacks.push(() => {
                    let x = onFulfilled ? onFulfilled(this.value as T) : this.value as U;
                    resolve(x);
                });
                this.onRejectedCallbacks.push(() => {
                    let x = onRejected ? onRejected(this.reason) : this.reason;
                    reject(x);
                });
            }
        });
        r