數據庫操作套在回調函數中就出錯

實現一個注冊功能

user.post('/register', async(ctx) => {
    let {username, password, rePassword} = ctx.request.body;
    
    if (!username) {
        responseClient(ctx, 400, 2, '用戶名不可為空');
        return;
    }
    if (!password) {
        responseClient(ctx, 400, 2, '密碼不可為空');
        return;
    }
    if (password !== rePassword) {
        responseClient(ctx, 400, 2, '兩次密碼不一致');
        return;
    }   

    try{
       await User.findOne({username}, function(err, doc){
            if(err){
                 responseClient(ctx);
             }

             if(doc){
                 responseClient(ctx, 200, 1, '用戶名已存在');
                 return;  
             }else{
                 //保存到數據庫
                let user = new User({
                     username: username,
                     password: password,
                     type: 'user'
                 });

                 user.save(function(err, doc){
                     if(err){
                         console.log(err);
                     }
                     if(doc){
                         let data = {};
                         data.username = doc.username;
                         data.userType = doc.type;
                         data.userId = doc._id;
                         responseClient(ctx, 200, 0, '注冊成功', data);
                         return;
                     }
                 });   
             }
         })

           }

    }catch(e){
        responseClient(ctx);
    }
})

當我這樣寫的時候有時候會注冊成功有時候就會提示AssertionError [ERR_ASSERTION]: headers have already been sent

后來試了下這種寫法就沒問題

user.post('/register', async(ctx) => {
    let {username, password, rePassword} = ctx.request.body;
    
    if (!username) {
        responseClient(ctx, 400, 2, '用戶名不可為空');
        return;
    }
    if (!password) {
        responseClient(ctx, 400, 2, '密碼不可為空');
        return;
    }
    if (password !== rePassword) {
        responseClient(ctx, 400, 2, '兩次密碼不一致');
        return;
    }   

    try{
        let doc = await User.findOne({username})
        if(doc){
            responseClient(ctx, 200, 1, '用戶名已存在');
            return
        }else{
            let user = new User({
                    username: username,
                    password: password,
                    type: 'user'
                });

            let newUser = await user.save();
            let data = {};
            data.username = newUser.username;
            data.userType = newUser.type;
            data.userId = newUser._id;
            responseClient(ctx, 200, 0, '注冊成功', data);
            return;
        }

    }catch(e){
        responseClient(ctx);
    }
})

responseClient

    responseClient(ctx,httpCode = 500, code = 3,message='服務端異常',data={}) {
        let responseData = {};
        responseData.code = code;
        responseData.message = message;
        responseData.data = data;
        ctx.status = httpCode;
        ctx.body = responseData;
    }

請問第一種是我哪里寫錯了嗎？正常的寫法應該怎么寫？