tsconfig

Not an option so much as a subscription: it turns on eight other checks, and keeps turning on the ones added after you wrote your tsconfig.

Without it, `null` and `undefined` are members of every type, so `string` silently means "string, or nothing at all".

A parameter the compiler cannot infer becomes `any` — this makes it say so instead.

A function that accepts less cannot stand in for one that must accept more — obvious when stated, and unchecked without this flag.

Without it, these three accept any arguments at all, because their declared types say `any[]`.

Declaring `token: string` says the property is a string. Before this flag, nothing checked that it ever became one.

Inside a plain nested function, `this` is not the object you wrote it in — and without this flag it is silently `any`.

JavaScript can throw anything — a string, undefined, a number — so a caught value is `unknown` rather than `any`.

Reading a key that was never written gives `undefined` at runtime. This is the flag that says so in the type.

`{ retries?: number }` normally also permits `{ retries: undefined }`. This flag makes the question mark mean only "may be absent".

Reserves `obj.key` for properties that were actually declared, and pushes everything from an index signature to `obj["key"]`.

Requires the `override` keyword on any member that replaces one from the base class.

A function where some branches return a value and others fall off the end is almost always a missing case.

Flags a `case` that does work and then runs into the next one, while still permitting the deliberate empty fallthrough.

An unread local is usually the leftover of a change that was only half made.

Reports parameters that are declared and never read, with a leading underscore as the documented way to say you meant it.

Set it to `false` — the default only warns in the editor, and a warning is not a build failure.

An unused label is usually not a label at all — it is an object literal or a type annotation that parsed as one.

An import used only as a type must be written `import type`, so the emitted JavaScript is what the source shows.

Rejects the constructs that cannot be compiled correctly without looking at other files — which is how every fast bundler compiles.

Bans the TypeScript constructs that emit runtime code — enums, namespaces, parameter properties, `declare` class fields.

`import "./styles.css"` was never checked to resolve at all, so a typo in one was invisible.