TypeScript's whole value proposition is moving errors from "found by a user in production" to "found by the compiler before you even run the code." It is not a separate language runtime — it is a static type layer on top of JavaScript that disappears entirely by the time your code executes. Getting real value out of it means understanding a handful of ideas that don't exist in plain JavaScript at all: structural typing, narrowing, generics, and the compiler's own configuration knobs. This is a complete reference, not a quick overview — work through it in order, or jump straight to the section you need from the sidebar.