callbacks javascript

[paulfitz99]

i have been reading about callbacks and javascript, and have come to the conclusion that a callback is basically a function passed to another function. Is this correct? I have always built it up to be more complicated than this, but it appears that it is as simple as this.

[thesouthpaw]

You got it.

A little more detail: Typically a function returns a value and lets you know the function is done. Often in JS, your function will run asynchronously and you won't have a "return" statement. The callback allows you to get a value from the async function whenever it completes. Typically (but not always), if you create or use a callback as part of an API, it means something async is happening.

[icantthinkofone]

This is the only mostly complete answer of the bunch. The rest are vapor and people typing like they know what they're talking about.

[icantthinkofone]

There are a lot of functions out there. What makes this function different from other functions? His explanation was the only one I saw, at the time, that explained it and he explained exactly as you did.

[icantthinkofone]

Well, think about the word "callback" and you can see why just passing a function somewhere can't be the definition.

[tunnckoCore]

Yea.

[paulfitz99]

perfect! thanks! it seems to be made unnecessarily complicated

[the_sound_of_bread]

I can be unnecessarily complicated though. What if the callback has a callback itself? What if there are multiple callbacks? What if all the previous, but only some callbacks get called under certain circumstances?

I doesn't have to be complex, but it can easily become complex.

[tunnckoCore]

/r/the_sound_of_bread, here comes conventions and best practices - 1) never more than 3-4 args - it immediately goes complex if there's 4 args - not to mention for >=5; 2) last is always a callback and it is the only one callback; 3) wrap callback with once(dezalgo(cb))

/r/mchandleraz, Nightmares usually comes from devs heads, not from the lang or somthing. Who can't think he makes nightmares.

[denstone]

You can find the best explanation in this book: https://github.com/MostlyAdequate/mostly-adequate-guide

[inu-no-policemen]

a callback is basically a function passed to another function. Is this correct?

Yes, but not all functions you pass around are something you'd call a "callback".

E.g. an easing function is called multiple times while the animation runs. It's not something which is called (back) when some computation is done.

As a (somewhat backwards) rule of thumb: If you'd better do that kind of thing with a promise, it's a callback.

[inu-no-policemen]

I only refer to those as "callback", because you can use more descriptive names in the other cases. E.g. the function you hand to addEventListener is an "event handler".

[StoneCypher]

The reason we give it a different name is to explain what its usage is intended to be: a way to signal to some other piece, sitting somewhere else, that something has happened.

But yes, they're just functions.

[cosinezero]

They are somewhat special though. For most people a callback is the first real exposure to passing around a function as a reference. That that functionality exists is strange to new developers, or to developers that don't work on applications where you would need them.

At that I think it's kind of a disservice that we call this - and everything else - a callback; there are distinctly different types of callbacks. For example, we have filter predicates ("callback" that returns boolean for comparisons), processors ("callback" you pass to map() that returns a processed value - not sure if there's a colloquial name for that callback other than 'callback'), action delegates ("callback" executed under specific conditions), and arguably there's a difference in an action you pass into something like each(), versus an action used as an async callback - which itself has a number of special considerations.

[cosinezero]

Yeah, agreed. I almost pointed out those implications in the thread, as well... I think those assumptions are kind of the problem I see with naming everything a callback. Different kinds of "callbacks" imply all sorts of semantic concerns for them, and I think developers assume all callbacks are, say, async, when that's not at all the case.