what is the callback in loopback (datasource.(automigrate))

Question

Loopback datasource API offers automigrate function with an optional callback.

I see in some examples that the callback gets one parameter (err), but no definition of that.

• What form does the callback parameter have?
• Are there other possible parameters?
• How is this with the other functions?

Grandmotherly

Answer 1

Callbacks are nothing but the function which you passing as a parameter to the other function

Look at this example

function printResult(err,result) {
  if(err) {
    console.log('something went wrong');
  }else{
   console.log(result); 
  }
}

function giveMeDouble(val, cb){
  if(val!=2){
    var err = new Error("value is not 2");
    cb(err);
  }
  cb(null,2*2);
}         
// Passing printResult function as a callback to the giveMeDoubleFunction
giveMeDouble(2,printResult); 

The Other Way of doing the same

giveMeDouble(2,function(err,result){
 if(err) {
   console.log('something went wrong');
  }else{
    console.log(result);
  }
});

Generally in Loopback form of callback is the first parameter is err and the second is the success res if everything went good but you can always have more parameters depend upon the function which you are calling. In your case callback form will be

dataSource.automigrate(model, function(err,result) {
})

Anjelicaanjou

What structure do the parameters err and result have? Where did you find that? Should a good api give that information? – Grandmotherly

error can have this possible structure refer link for this docs.strongloop.com/display/public/LB/Error+object and result depends upon the function you calling for e.g Model.count(query,function(err,res){}) // in this case res will be actual count as per your query – Anjelicaanjou