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JavaScript

Posted by henriquegiuliani
henriquegiuliani
12,296 Points

Why my callback isn't running?

I'm trying to understand callbacks in JavaScript but i don't know why isn't running...

Here's the code:

function sum(n1, n2, callback){
  return n1 + n2;
}

sum(4,5,function(){
  console.log("Result: " + this);
});

console.log("processing ...");

It doesn't display the result when i run the script... it should appeear "processing..." first and then the result.

2 Answers

Bryan Peters
Bryan Peters
11,996 Points

When you use a callback pattern, you don't use the return. Your callback function should receive the value as a parameter. Try this - it uses the callback if provided, otherwise it returns the value normally.

function sum(n1, n2, callback) {
  var result = n1 + n2;
  if (callback) {
    callback(result);
  } else {
   return result;
  }
}

sum(4,5,function(val){
  console.log("Result: " + val);
});

console.log("processing ...");

Note: There are other use cases for callbacks that don't return a value, in that case you would just call callback() in your function, and the callback function would not have a parameter. Example:

function startProcess(callback) {
  console.log("process started");    
  //do stuff
   callback()
}

function processDone(){
  console.log("process done");
}

startProcess(processDone);
henriquegiuliani
henriquegiuliani
12,296 Points

thanks for the answer... but what does it mean "if(callback)" ???

Bryan Peters
Bryan Peters
11,996 Points

if (callback) checks to see if a callback function was received. If it was, it sends the value via the callback, otherwise it sends the value back via return.

so you could do:

var mySum = sum(3,6);
console.log(mySum);

//or

sum(3, 6, function(val){  console.log(val);  })
henriquegiuliani
henriquegiuliani
12,296 Points
henriquegiuliani
henriquegiuliani
12,296 Points

Hm, thanks a lot!! Much clearer now hehe