SOURCE

{
    function Promise(fn) {
        this.cbs = []; //收集回调函数集
        const resolve = (value) => {
            setTimeout(() => {
                this.data = value;
                this.cbs.forEach((cb) => cb(value));//执行回调的函数
            });
        }
        fn(resolve)
    }
    Promise.prototype.then = function (onResolved) {
        console.log(onResolved)
        return new Promise((resolve) => {
            this.cbs.push(() => { 
                const res = onResolved(this.data);
               
                if (res instanceof Promise) {
                    //resolve 向下执行
                    res.then(resolve)
                } else {
                    //执行cbs函数
                    resolve(res) //执行回调函数
                }
            });
        });
    }
    const fn = (resolve) => {
        setTimeout(() => {
            resolve(1);
        }, 500);
    };
    new Promise(fn).then(res => {
        console.log(res)
    });
}