Array Filtering for Stage 2

(and also grouping)

To Recap

filter keeps items which return true

This is the opposite behavior that some users expect.

• Completely valid to think a coffee filter acts on the liquid
• For me, a coffee filter acts on the grounds
• The end result is still coffee

Proposal is to add an “filter out” method

Would remove items which return true

• The goal isn’t to “just make negation easier”
• It’s easy to add a ! to any predicate, or wrap in a negate HOF
• Instead, it’s to help with an incorrect intuition

Real Code!

Filtering out items

Use filterReject to reject the item

• The fact filterReject exists helps intuition with filter
• filterReject is the one that rejects, filter must be the one that selects

Array partitioning/grouping

Partitioning an array

Partitioning is special form of filtering

• Collects both the selections and rejections

Issues with Partition

• What order is the return value?
• [trues, falses], or [falses, trues]?
• Well, false == 0, so falses first?
• Functional languages put trues first.

Grouping an Array

groupBy is a more generic form of partition

• Items are grouped together into arrays
• The returned value is used as the key to the array in output object

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Grouping is extremely flexible

Needed to implement a stable sort in older node. We’re grouping nodes into integer keys, and sorting the keys. Concat, and voilà.

Also known as bucketing.

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babel/babel#13021

What return value?

• Ecosystem uses normal object
• Could use prototype-less object to avoid inherited bugs
• Could be a Map, though this would be a large divergence

Difference from python itertools.groupby

Python's itertools doesn't combine all groups, only sequential groups

• Lodash/Underscore precedence is all groups
• Do we need a new name?

Do we treat array holes as undefined?

• ES6+ methods were supposed to pretend holes don't exist
1. find treats holes as undefined
2. flatMap skips holes

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Discussion

Stage 2?

(Both filterRejct and groupBy)