npm / immutable

immutable

What changed in immutable 5, from its official release notes, upgrade guides and advisories. Sources checked 2026-10-18.

1 breaking4 security4 fixes and other

See which of these apply to your code, ranked, in your agent:

npx @unstale/cli init

immutable 59 changes

On a lazy Seq of unknown size, has(index) checks whether the index exists instead of searching for a value equal to the index.
5.1.7release notes

DoS patches for CVE-2026-59879 and CVE-2026-59880 were ported onto branch 3.x.
3.8.4, 4.3.9, 5.1.8release notes security advisory security advisory
Guards oversized bounds in setListBounds, addressing a security vulnerability affecting List.
3.8.4, 4.3.9, 5.1.8release notes security advisory release notes
Indexes large hash-collision buckets for faster lookups in Map.
3.8.4, 4.3.9, 5.1.8release notes security advisory release notes
Immutable is vulnerable to Prototype Pollution
3.8.3, 4.3.8, 5.1.5security advisory release notes release notes release notes

List now preserves undefined values when grown past 32 elements.
5.1.9release notes
Repeat.lastIndexOf now returns size - 1 instead of size for the last index.
5.1.7release notes
The type for reduce is improved when no initial value is given.
5.1.7release notes
The reverseFactory iterator now reads reversedSequence.size instead of this, fixing reversed-sequence iteration behavior.
5.1.6release notes