同时允许最大任务量

function limitRequest(tasks,n){
    return new Promise((resolve,reject) => {
        let index = 0//当前第几个执行
        let start = 0//当前执行几个
        let finish = 0//总共执行完几个
        let arr = []//执行完结果
        let run = () => {
            if(finish == tasks.length){
                resolve(arr)
                return
            }
            while(start <= n && index < tasks.length){
                start++
                let cur = index
                tasks[cur]().then((res) => {
                    arr[cur] = res
                }).catch((err) => {
                    arr[cur] = err
                }).finally(()=>{
                    index++
                    finish++
                    start--
                    run()
                })
            }
        }
        run()
    })
}