尝试了用promise，但输出编号发现还是异步执行   

 for(let i =0;i<res.data.length;i++)
   {
    var id = res.data[i].uid;
    new Promise((resolve, reject) => {
        console.log(id)
        db.collection('Student')
        .where({
            _id:id
        }).get()
        .then(res=>{
            if(res.data.length==0)
            {
                db.collection('Teacher')
                .where({
                    _id:id
                }).get()
                .then(res=>{
                    that.data.headlist.push(res.data[0].pic)
                    that.setData({
                        headlist:that.data.headlist,
                    })
                    setTimeout(function () {
                        console.log(i)
                        resolve()
                    }, 500)
                    
                }).catch(res=>{
                    console.log(res)
                })
            }
            else
            {
                that.data.headlist.push(res.data[0].pic)
                that.setData({
                    headlist:that.data.headlist,
                })
                setTimeout(function () {
                    console.log(i)
                    resolve()
                }, 500)
            }
        })
    }) 
 }