proposal-flatMap VS proposal-array-grouping

If you're doing a series of mutations specific to a custom type in that fashion, then there isn't much benefit. But imagine you want to chain operations on an existing type, whether it's a built-in like String, or a type introduced by a different package like sqlite.Database. To use method chaining you'd have to monkey-patch them, which can lead to all sorts of problems: colliding with other packages that want to monkey-patch the same types, confusing the reader when unfamiliar and seemingly undocumented methods appear (why does the SQLite documentation not mention this sqlite.Database#drop method? Which of my installed packages added that? ), your package breaking when the thing you're monkey-patching changes its API, altering object iterations, etc. The troubles with Prototype.js and MooTools in the '00s showed some of the headaches monkey-patching can lead to when it comes to JS especially, where users expect ever-changing browsers to work with 25-year-old code. (I still support changing flatten to smoosh, though.) You might subclass the existing type or encapsulate it in a new type, but then you're potentially breaking the user's typechecking, preventing the use of any other packages that want to do the same thing, escaping tree shaking, and becoming incompatible with unexpected types (what if the user already subclassed sqlite.Database themselves, and want to apply your transformation to that?).

Global iterator type is coming: https://github.com/tc39/proposal-iterator-helpers

But a method named `groupBy` on iterators traditionally means a different thing: https://github.com/tc39/proposal-array-grouping/issues/51#is...

Global iterable type it's too late for, since there's many extant iterables in the language and on the web which don't have it in their prototype chain and can't reasonably be changed.

Array grouping to combine like data into groups using Object.groupBy() and Map.groupBy().

Array Grouping: A proposal to make grouping of array items easier.

They are coming out with array.group() soon, which is supposed to also fulfil the use-case of partitioning :) - https://github.com/tc39/proposal-array-grouping.

https://github.com/tc39/proposal-array-grouping for those curious

I'm also looking forward to map.emplace() and arr.group() and iterator helpers.

At the end of the day, it comes down to whether browser vendors are willing to ship the feature even though it breaks said websites, and they do have visitor data to back those decisions. If the rule was "never break any website ever", you'd be able to block proposals you don't personally like by crafting a website that "gets broken" by that proposal.

At least in the array grouping proposal, they did evaluate amount and popularity of possibly breaking sites, before eventually renaming the method from .groupBy to .group

https://github.com/tc39/proposal-array-grouping/issues/37

The proposal is described here.

Remarkably, this is now a TC39 proposal.

Proposal, Polyfill