You are currently looking at the v6.0 - v8.2 docs (Reason v3.6 syntax edition). You can find the latest API docs here.
(These docs cover all versions between v3 to v8 and are equivalent to the old BuckleScript docs before the rebrand)
Storage
t
REtype t
getItem
REexternal getItem: (string, t) => option<string> = "getItem"
setItem
REexternal setItem: (string, string, t) => unit = "setItem"
removeItem
REexternal removeItem: (string, t) => unit = "removeItem"
clear
REexternal clear: t => unit = "clear"
key
REexternal key: (int, t) => option<string> = "key"
length
REexternal length: t => int = "length"
localStorage
RE[@bs.val] external localStorage: t = "localStorage"
sessionStorage
RE[@bs.val] external sessionStorage: t = "sessionStorage"