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JavaScript Where You’ll Find Promises in the Wild?

Posted by Cristian-Adrian Frasineanu
Cristian-Adrian Frasineanu
5,101 Points

Why there aren't error-first callbacks?

For me it seems a little bit strange that we pass in the resolver and then the rejector for .then() method. I've worked mostly with APIs which required the error handler first, isn't that a standard? I might be wrong.

Thanks, Cristian

2 Answers

Steven Parker
Steven Parker
243,215 Points
Steven Parker
Steven Parker
243,215 Points

:point_right: Even more of a standard (and a bit of a necessity) is that required arguments come before optional arguments. And the failure method in a .then() is optional.

Cristian-Adrian Frasineanu
Cristian-Adrian Frasineanu
5,101 Points

Thanks, is it me or only Node.js with the V8 engine use error-first callbacks? I understand that it is a necessity, especially in a loosely typed language like JavaScript to have the optional arguments at the end, but my question now is: if ES5 and ES2015 offer native promises why the callbacks in Node.js are different? Is there an irony or they just wanted to disallow this flexibility when using callbacks?

I haven't searched, but I guess it is impossible to replace the error parameter in Node.js and use a chained .catch() method.

Steven Parker
Steven Parker
243,215 Points
Steven Parker
Steven Parker
243,215 Points

I wasn't aware of the difference in Node, but I haven't been using it for very long. My JS experience is almost entirely client-side up to now.

I haven't seen it yet myself, but if the previous method call doesn't return the promise, you wouldn't be able to chain.

Cristian-Adrian Frasineanu
Cristian-Adrian Frasineanu
5,101 Points
Cristian-Adrian Frasineanu
Cristian-Adrian Frasineanu
5,101 Points

Yes, correct. I've searched it and it seems that only Node adopted this implementation after all. I believe this is just for prioritizing the error handling inside the callback and not by catching it.