Callback not provided pattern in NodeJS

Question

I usually write NodeJS code for hours, and there is a common situation I'm not sure how to face. Given the following async function call (where we forget the needed callback):

function foo (callback) {
    // Perform a task and call callback when finished
    callback();
}
foo(); // We forget the callback

Which is the better way to handle this? I find two options:

Make foo() function more robust adding this as the first line: callback = callback || function () {}.
Let it crash when foo() tries to call callback() and it does not exist.

Maybe a third option (which I prefer) would be the best one: throwing a custom error if the callback is not provided, as the first lines of foo(). For instance:

function foo (callback) {
    if (!callback) {
        throw new Error('No callback provided');
    }
    // More stuff here
    callback();
}

My question is: Is there any well-known patten to solve this situation? Which approach do you think is the better one? Do you solve this in a different way?

Are you tried to use Promises for async functions? It brings some new approaches for designing without callbacks. — raidendev
– raidendev, Commented Jul 11, 2014 at 9:21

fmsf · Accepted Answer · 2014-07-11 09:22:42Z

4

It all depends if your callback is mandatory or not.

If it is mandatory then you should fail fast and do the throw new Error('No callback provided');.
If it is optional then the standard practice is that you just call it if it exists. Instead of creating a new function, just add a check in the place where it is gonna get called:
```
if ( callback ) {
   callback();
}
```

or in a single line:

callback && callback();

answered Jul 11, 2014 at 9:22

fmsf

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2 Comments

Ben Fortune Over a year ago

Would typeof callback === "function" work better here?

fmsf Over a year ago

@BenFortune if you have a variable called "callback", semantically places it as type function, I rather call it and have the code fail if it is not a function. That way it is easier to know that something wrong is being stored in the callback.

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