Knex sometime return an empty string - mysql

Question

I'm using knex with MYSQL. I have a function that I called to show the data to the user, Also I'm using a view table which has 5 right join on it and I think it will take some time to return values from the table plus I added the WHERE condition on my knex and it looks like this :
var showClass = (teacherId , ClassId){
return new Promise((resolve , reject)=>{
knex.select().from('v_cardex_details').where({teacherId }).andWhere({id : ClassId}).then(classes =>{
resolve(classes)
}).catch(err=>{
console.error(`Show Teacher class Error: ${err}`)
reject (err)
})
})
}
and I call this general function to response some request something like this
exports.EditClass = (req,res)=>{
knex('Table').update({//Some update stuff here}).then(()=>{
showClass(req.user.id, req.params.id).then(data=>{
return res.status(200).json({data , message:''})
})
}).catch()
}
With the same input, this function after updating returns value and some times it returns an empty string, especially when it's on the hosting server most of the time it returns nothing but { message : '' }

Answer 1

Try to create simplified code by removing all the unnecessary wrappers and you might find where your problem is. AFAIK there is no way that that your {data , message:''} would create an object containing just {message: ''} without any additional attributes.
> var data = []
undefined
> {data, foo:1}
{ data: [], foo: 1 }
> data = undefined
undefined
> {data, foo:1}
{ data: undefined, foo: 1 }
> {data1, foo:1}
ReferenceError: data1 is not defined
The problem you are experiencing does not exist in from the code you have shared (though there are syntax errors and other problems).
EDIT:
res.json() uses JSON.stringify() to convert js object to JSON strings. So if value of data in your code is undefined instead of and array, that could explain the behavior you are experiencing:
λ node
> JSON.stringify({ test: undefined })
'{}'
As you can see JSON.stringify() omits the attributes with value undefined from the output JSON string.