Answer 1

Promise._all = function (promises) {
    return new Promise((resolve, reject) => {
        const result = [], resolved = 0
        const length = promises.length
        const addResult = (item, index) => {
            result[index] = item
            resolved++
            if (resolved === length) {
                resolve(result)
            }
        }
        promises.forEach((promise, index) => {
            if (typeof promise.then === 'function') {
                // thenable
                promise.then((item) => {
                    addResult(item, index)
                }).catch(reject)
            } else {
                // 非promise类型直接透传
                addResult(promise, index)
            }
        })
    })
}

Answer 2

@gzwgq222 直接用lodash吧

Answer 3

4,2,1

Answer 4

厉害了 大神