Data validation using RxJS - functional-programming

Question

I have the following function that validates that rangeFrom is not superior to rangeTo and that the rangeFrom does not already exist in the list of ranges.
How can rewrite this using RxJS?
const isTagAlreadyExist = (tags, currentTag) => _(tags)
.filter(x => x.id !== currentTag.id)
.some(x => _.inRange(currentTag.rangeTo, x.rangeFrom, x.rangeTo))
.value();
const validateRangeFrom = (tags, currentTag) => {
const errors = {};
if (isNumeric(currentTag.rangeFrom)) {
if (!_.inRange(currentTag.rangeFrom, 0, currentTag.rangeTo)) {
errors.rangeFrom = 'FROM_TAG_CANNOT_BE_GREATER_THAN_TO_TAG';
} else if (isTagAlreadyExist(tags, currentTag)) {
errors.rangeFrom ='TAG_ALREADY_EXISTS';
}
}
return {
errors
};
};

Answer 1

The question is: what parts do you want to rewrite to rxjs? Those are two pure functions that run synchronously from what I can see, I do not really see much a usecase for rxjs here - of course you could always utilize your functions within an rxjs stream:
const validateRangeFrom$ = (tags, currentTag) => {
return Observable.of(currentTag)
.map(tag => validateRangeFrom(tags, tag));
}
validateRangeFrom$(myTags, currentTag)
.subscribe(errors => console.log(errors));
But as you can see, this does not make much sense if you simply wrap it inside a stream, the essence of useful reactive programming is, that everything is reactive, not just some small parts, so for your example, you should start with having tags$ and currentTag$ as observables - let's assume that you have that, then you could do something like:
const tags$: Observable<ITag[]>... // is set somewhere, and emits a new array whenever it is changed
const currentTag$: Observable<ITag>... // is set somewhere and emits the tag whenever a new currentTag is set
const validateRangeFrom$ = Observable
.combineLatest(tags$, currentTag$, (tags, tag) => ({tags, tag}))
.map(({tags, tag}) => validateRangeFrom(tags, tag));
validateRangeFrom$.subscribe(errors => console.log(errors));
This will automatically trigger the validation for you whenever a new tags-array is emitted or a new currentTag is selected/set - but again: your validation-method is kept the same - as even in reactive programming you have to do validation and logic-operations at some point, the reactive part usually just concerns the flow of the data (see: tags$ and currentTag$)