How to support 2 types of callback in Javascript

Question

In the intercom-client github I see this code:

client.users.list(function (d) {
  // d is the response from the server
});

// Or

client.users.list(function (err, d) {
  // err is an error response object, or null
  // d is a successful response object, or null
});

My question is: How intercom know if I pass funcrtion(err,d) or just function(data).

I check the source code and found that they use bluebird library. How bluebird does it?

I want that my function will do the same.

In other words:

function myFunction(data,callback){
  if (callbackListenToErrArgument(callback)) 
       callback(null,data)
   else
       callback(data)

}

How to implement callbackListenToErrArgument function?

Answer 1

One can inspect the .length property on the function getting passed. The .length property is the number of arguments defined for the function.

function hasErrorArgument(callback) {
  if (callback.length < 2) { return false; } // Has 1 or 0.
  else return true; // has 2 or more
} // Yes, I know it can be reduced to one line. Sue me.

Please note: This is bad practice. You should provide a uniform signature to the callbacks you accept. (err, data) => {} is a good signature to follow.

Or even better, have your list() function return a Promise object: (Promises are natively supported only in some browsers. Check compatibility before use, or use a polyfill or a library).

client.users.list()
  .then(listOfUsers => {
    // Use data here
  })
  .catch(err => {
    // Handle errors here
  });