回调返回值是否有意义？

Question

Commonly, an async function in js returns a promise wrapping either the resolved value or error message. If promise style do not apply, a callback can be passed as a parameter to that async function, in which the result of the async function is accessible.

Just wonder if it is meaningful for such a callback to have ar eturn statement? If so, who can receive the returned value and is there any useful use case?

Answer 1

Here's a piece of code to illustrate the concept you are trying to understand.

var callback = function(err, result){
    if(err){
      console.log(err);
      return err;
    }
    callAnotherFunction(result);
};

function getRandom(cb){
   //do something
   var result = Math.random();
   if(result > 0.5){
     return cb(result);
   }
   return cb(null, result);
}
getRandom(callback);

The function getRandom() will return an error if the random number is less than 0.5, a value otherwise. The callback then handles the response differently depending on the values passed to the callback function. As you can see, the return statement inside the callback makes a big difference. By the way, the above example demonstrates a standard error-first design pattern in NodeJS.

Answer 2

Theoretically, any async function should return something like a handler, which you can use to track progress, or cancel the on-going async actions, example:

var timer = setTimeout(..)
clearTimeout(timer)

Because the return value is the only chance that you can get this handler in a sync manner, but it seems people don't quite care about this and invent all kinds of stuff to solve async cancellation problem, please read more about this on my note on action-js .

In a perfect world, we should get a handler for every async function:

handler = fs.readFile('...', function(err, data){...});
xhr = ajax('...', function(err, data){...});
...