You're building a tiny tool. Maybe a notepad. Maybe a settings panel. Maybe a draft autosave for a form. You don't want a database. You don't want a backend. You don't even want a login. localStorage.setItem(key, value) solves the problem in one line and you go home. Until your tool grows up, the data outgrows 5MB, three browser tabs trip over each other, the user clears their cookies, and suddenly you have a support ticket that boils down to "I lost my work." This is the localStorage post I wish I'd had three years ago. When it's the right tool, when it isn't, and the specific footguns that make production-grade local persistence harder than it looks. What localStorage actually is localStorage is a synchronous key-value store, scoped to an origin (protocol + domain + port), persisted to disk by the browser. Both keys and values are strings. That's the whole interface: localStorage . setItem ( ' username ' , ' jane ' ) const value = localStorage . getItem ( ' username ' ) // 'jane' localStorage .…