How do i use a value from a function in another as a callback? Node js

Question

I´ve been making a prototype of webservice and I get confronted with the following problem: Whenever I try to use a 'var' as a callback i get: undefined.

What I'm tryng to do is:

var mysqlquery = function (){

   var vAlue = XXX;

}

var service = function (pp, ee, cb){

 var toReturn= ({                      //XML code

     Value: vAlue

     })

cb(toReturn);

};

Output should be XXX

The service runs fine and logs the values, but when i try to make it a callback to respond it is undefined, I guess because of the async of node.
Already tried making it a global or global.window or calling it inside another function, none of wich work. I don´t want to use extra modules for this, is there any method for it ? (Also tried this.)
Any tip is much apreciated, thanks.

Answer 1

You already know that defining a var in a function limits its scope to that function, but you can pass that data out of the function using a return, like so:

var mysqlquery = function (){
   return 'XXX';
}

console.log(mysqlquery())
> "XXX"

What's happening in the console.log line is that your function is being evaluated, and returns "XXX", and then it is passed to console.log.

This is the foundation of callbacks: if you have an asynchronous function, you can pass a callback function into it to feed in the result of the async function:

function print(res) {
  console.log(res)
}

function asyncThing(cb) {
  var ten = 5 + 5
  window.setTimeout(cb.bind(this, ten), 5000)
}

asyncThing(print)
... [wait five seconds]...
> 10

How do i use a value from a function in another as a callback? Node js

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