The bug report that's always the same shape
“It works most of the time, but sometimes it crashes” is one of the most common bug reports in a growing JavaScript codebase, and it's almost always the same root cause wearing a different disguise: a function assumed an object had a field that, under some condition nobody tested, it didn't. An API response changed shape slightly. A prop was optional somewhere and required somewhere else. None of these are exotic bugs — they're exactly the class of error a type system exists to catch before the code ships.
Why teams delay adopting it
The usual objection is migration cost, and it's a fair one for a codebase with real production traffic. Nobody wants to pause feature work for a rewrite. The good news is that a full rewrite is not actually how a realistic TypeScript migration goes — incremental adoption lets a codebase convert file by file, with an explicit, visible escape hatch for the parts not converted yet, rather than an all-or-nothing cutover.
What incremental adoption actually looks like
Turn on TypeScript checking against existing JS files first, catching type errors without converting anything yet
Convert new files and heavily-modified files as you touch them, rather than scheduling a dedicated migration sprint
Type the boundaries first — API responses, form inputs, database models — since that's where the highest-value, highest-frequency bugs live
Turn on strict mode last, once the bulk of the codebase is typed, rather than fighting it from day one
The part beyond bug prevention
The less-discussed benefit is onboarding. A typed codebase tells a new engineer what a function expects and returns without needing to trace through five other files or ask around. When we inherit an existing JavaScript codebase from a client, an incremental TypeScript migration is often one of the first things we propose — not because it's exciting work, but because it measurably reduces the number of “how does this even work” questions in the following months.