explicit return values, why and how does this work

[koin612]

hey,

I used deepdash's filterdeep method and wrote a predicate which evaluates a boolean expression like

(val, key, parent) => key === 'key' && (val === 'val1' || val === 'val2')

this doesnt work the result would be null, but if I explicit return true everything is fine (like in the official examples).

(val, key, parent) => { if (...) return true }

So, from the docs (https://deepdash.io/#iteratee) I figured out that this is because of "returns - return false explicitly to prevent iteration over current value's children"?

Is my assumption correct and what is this called in JS? I'd like to know more about this but I fail to find anything related to this. Can someone explain or point out some words so I can check?

Edit: I now think it's because it works with undefined for seen nodes so just returning true/false would not work?

[YuriGor]

Hi, looks like you mixing docs for eachDeep and filterDeep.

eachDeep will not care about returned value unless it's exactly boolean false.
In this case eachDeep will skip current children if any, so it's kind of 'break' in for loop, but local for current depth level.
This is similar to convention of Lodash each method.

filterDeep similar to Lodash or native array filter method expects true or false from predicate to decide keep current value or not.
Because filterDeep case is much more complex, I extended this convention and also accept undefined as possible return result. If "truthy" reply will be "yes" and "falsey" response will be "no", undefined means "Don't know yet"

In case of undefined current value will be preserved in result only in case some children values will also be preserved. If finally in result such value will be childless - it will be cleared out.
filterDeep deep doesn't care too much about true or false result data type, 0 or null will also work as false. It only cares about undefined as separate from false reply, so undefined is not falsey from filterDeep point of view.

If you are not sure yet you can read about Truthy and Falsy values in JS.

Note: if in some case you function will not return something - it will mean function will still return undefined implicitly.
I guess this is your point of confusion. So you can of coarse return just result of evaluation of some boolean value, you don't need to return exactly true or false, in case of eachDeep just make sure your false result is really boolen, not undefined or 0 wich in boolean expressions are considered as false, but if you check it with === operator - it will be different.
And in case of filterDeep you can return any truthy/falsy values, but be carefull with undefined because it's a separate reply.

To have better idea how deepdash works with complex nested data - I recomment first to learn how Lodash / JS array forEach and filter methods work. They work similar, but Lodash also supports objects, not only arrays, and in Lodash there is return false ==> break convention exists, while native JS forEach doesn't support it.

[koin612]

Hey YuriGor,

thank for your detailed explanation, really appreciate it!
I think I got messed up by the definition of a predicate. For me it's like P: X -> {true,false} and so I got stuck with that thinking and messed up also understanding the documentation.

But I think I got it now and it makes perfectly sense :)

[YuriGor]

Sure, documentation is not strongest side of this lib, so if you have some motivation / resources - feel free to improve docs, I am always open for PR.
docs sources live here: https://github.com/YuriGor/deepdash/tree/master/docs