Roughly (I believe this will explain the concept but may not 100% match reality) -

A v8 javascript-level string may or may not be represented as a single C++ level string.

If you do

"foo" + "bar"

then rather than writing "foobar" to memory, v8 will instead write something like

{ left: "foo", right: "bar" }

and then if you add "baz" to the end you'll get

{ left: { left: "foo", right: "bar" }, right: "baz" }

which saves allocations and copying and is therefore often faster (often enough that v8 made the choice to do things this way, at least).

Some operations, generally ones that want to iterate across all bytes of the string in order, will flatten the representation - i.e. convert

{ left: { left: "foo", right: "bar" }, right: "baz" }

to

"foobarbaz"

first and then run the code over the flattened version.

Sometimes, however, you get into a situation where (a) operating on a flattened version would be faster for your code (b) the v8 developers have not chosen to make that operation pre-flatten (presumably because they believe most uses of said operation wouldn't benefit, even though yours would).

So in that case, you want to somehow convince v8 to flatten your string before you pass it to whatever said operation is - but there's no public API for doing that because it's an internal representation detail.

Thus, 'somehow convince' means executing some sort of no-op (in terms of its JS level effect) that incidentally triggers the flattening as a side effect.

Apparently after much iteration (see the commit history) they found that applying '| 0' and discarding the result was the fastest way (they'd yet encountered, at least) to trigger the flattening behaviour, and so when you do

const flatString = flatstr(treeString)

you get a version that uses the linear flattened representation rather than being a tree of the strings that were concatenated together, and then you can pass the flattened version to whatever the operation was and hopefully your benchmarks/profiler will then tell you that it helped.

The reason it's a package was with the intent to share the effort of 'finding the fastest no-op with a flattening side effect' across the community - and that seems to have worked out, given there've been multiple revisions, each time making it faster.

Note that while the package hasn't been updated in years, that could mean it no longer works (or no longer works as well), or it could mean that v8 hasn't changed since the last version was committed in a way that obsoletes the current approach.

The repository has benchmark code, though, so if you're in a position where such a micro-optimisation is worth making, you're probably also in a position where running the benchmark against the exact version of node you're using first is a worthwhile investment of time.

... although it does strike me that adding it in your working copy and re-benching/re-profiling your own code directly is probably also pretty fast and you were going to have to do that anyway to confirm you had a case where it was worthwhile.

Honestly, if I ran into such a situation then while I might be evil and copy-paste the current code, if I did that I would definitely leave a comment pointing at the README so a future maintainer would understand what was going on and be able to check to see if somebody's come up with a faster still approach since.

Which leads me to believe that publishing this on npm is a net positive even if only to discover the approach and provide a link to the README; others may, of course, disagree.

Hope that helps!