Using The Partition Method

I went down a rabbit hole and found this method that had been hiding in plain sight. Partitioning a list

What was the challenge?

I’d been looking for a way to separate a list of emails into two buckets, valid and invalid, so that I could log out the invalid emails and present that to the user. It’d already been a challenge because I’m working in Rails, but that just meant that I’m going to have to add some javascripty looking ruby code 😈

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# the heavy-handed operation I wrote looked something like this
valid = []
invalid = []

valid = emails.map {|email| validator.is_valid(email)}
invalid = emails.map {|email| !email.is_valid(email)}

But lo - and as I’d hoped, a patron saint of code feedback teammate of mine came through with some constructive feedback that I could have used a year ago changed everything.

What was ‘the way’?

Enter ruby’s built-in partition method. It returns two arrays - the first containing the elements that return true and the second returns the rest 🔥 It made my day and now my method looks like:

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valid, invalid = emails.partition {|email| validator.is_valid(email)}

So the next logical step is to ask myself, “does javascript have this?”

Well, does Javascript have it?

The answer as of June in the year of our Lord 2021, is… not out of the box. But fortunately for me, our project is using lodash and sure enough, _.partition exists! Lodash partition behaves similar to the built-in ruby version, but with a slightly different syntax:

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// syntax
_.partition(collection, predicate)

It accepts two parameters. The collection to iterate over and the predicate which is the function to be invoked per iteration. Then it returns an array of grouped elements:

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// example from above where 'emailValidator' returns a bool
let [ valid, invalid ] = _.partition(emails, emailValidator(email))

Just to note, I’m destructuring the arrays into separate buckets here for the purposes of my example.

And that’s it! I’ve found a new tool to add to the toolbox, that I look forward to using much more of in the future.