Examine the following code, which is used to output files in the specified directory

var fs=require('fs');

function sleep(numberMillis) {     //sleep方法，暂停numberMillis毫秒
    var now = new Date(); 
    var exitTime = now.getTime() + numberMillis; 
    while (true) { 
        now = new Date(); 
        if (now.getTime() > exitTime) 
            return; 
    } 
}

fs.readdir(__dirname,function(err,files){    //文件读取方法
    console.log('');
    if(!files.length){
        console.log('找不到文件 \n');
    }else{
        console.log('文件如下:');
        function file(i){
            var filename=files[i];
            fs.stat(__dirname+'/'+filename,function(err,stat){
                if(stat.isDirectory()){
                    console.log(' '+i+'文件夹：'+filename+'/...');
                }else{
                    console.log(' '+i+'文件：'+filename+'');
                }
            });
            i++;
            //sleep(1000);    //。。。。。。。。######暂停1s    
            if(i==files.length){
                console.log('输出完毕');
            }else{
                file(i);    //否则递归调用
            }
        }
        file(0);        //开始执行
    }
});

I can’t understand the result output by the above code. According to the logic of this program design, all files in the directory should be listed first, and then the counter i==files.length should be output before "Output completed" is the final word.

As shown in the picture above, not only is the terminal word "output completed" output first, but the printing order is completely messed up, and the printing order of multiple runs is also unstable.

I was wondering if there was a time difference in asynchronous calls, so I removed the sleep(1000) comment part of the above code, but the result was still inconsistent with what I expected: The "file is as follows" prompt was output first, and then stopped after 1s. , the subsequent statements are issued almost at the same time, without pause, and the terminal word "output completed" is still the first to be printed .

To sum up, what I want to know is

1. Why is the printing result unstable and why is the prompt "output completed" output first?

2. Why after adding the pause statement sleep(1000), it only pauses for 1s at the beginning, and then it no longer pauses but outputs simultaneously?

3. How to solve this problem, that is, how to make the prompt "Output Complete" finally output with as few modifications as possible?

Thank you for your advice!