如何实现一个异步的reduce

async function asyncAdd(a, b) {
    return new Promise((resolve) => {
        setTimeout(() => {
            resolve(a + b);
        }, 500);
    });
}

function asyncReduce(fn) {
    return async function (arr) {
        return arr.reduce((old, newOne) => {
            // 简化
            return Promise.resolve(old).then((data) => {
                return fn.call(this, data, newOne);
            });
            // return new Promise((resolve) => {
            //     Promise.resolve(old).then(data => {
            //         resolve(fn(data, newOne));
            //     });
            // });
        });
    };
}

asyncReduce(asyncAdd)([1, 2, 3]).then(data => {
    console.log(data);
});

function reduce(arr, cb, initial = null) {
    function iterator(res, leftArr) {
        if (leftArr.length == 1) {
            return cb(res, leftArr[0]);
        }

        return cb(res, leftArr[0]).then(
            result => iterator(result, leftArr.slice(1))
        );
    }

    if (initial) {
        return iterator(initial, arr);
    }
    else {
        return iterator(arr[0], arr.slice(1));
    }
}

// 测试用例
function cb(res, i) {
    return new Promise(function (resolve, reject) {
        setTimeout(() => {
            resolve(res + i);
        }, 1000);
    });
}

reduce([1, 2, 3, 4, 5], cb);
reduce([1, 2, 3, 4, 5], cb, 10);