SOURCE

// Promise.all([a, b]).then()

Promise.myAll = function(args) {
    return new Promise((resolve, reject) => {
        const results = [];
        let count = 0;
        let index = 0;

        // 这里之所以没有使用forEach等数组循环方法，主要是因为Promise.all可以传入的数据不止是数组
        // 参考：https://zhuanlan.zhihu.com/p/362648760
        for (let item of args) {
            const current = index;
            index++;
            Promise.resolve(item).then(res => {
                results[current] = res;
                count++;

                // 注意 resolve 的判断条件，没有使用传入参数的长度进行判断，因为它不一定是数组
                if (count === index) {
                    resolve(results);
                }
            }, error => {
                reject(error);
            });
        }

        if (index === 0) {
            resolve([]);
        }
    });
};

const c = new Promise((resolve) => {
    setTimeout(() => {
        resolve('哈哈哈，我定时了');
    }, 3000);
});
const a = Promise.resolve(32);
const b = Promise.resolve(77);
const d = 'dd';

Promise.myAll([c, a, b, d]).then(res => {
    console.log(res);
});