noUnusedLocals

A variable nothing reads

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

Since: TypeScript 2.0
strict: Not in strict
In your tsconfig: "noUnusedLocals": true

The same snippet both times. Only the option changed.

With it off

"noUnusedLocals": false

export function run() {
  const unused = 1
  return 2
}

Compiles clean

With it on

"noUnusedLocals": true

export function run() {
  const unused = 1
  return 2
}

Emits TS6133

Why the compiler bothers

The value is not in the tidiness — a bundler drops these anyway. It is that an unread variable is evidence: something was extracted and the call site was never updated, or a condition was rewritten and one branch stopped being reachable. The diagnostic is a hint rather than an error in editors, so it greys out rather than shouts, and it belongs in the build precisely because a grey line is easy to walk past.

Takeaway

It is a hint in the editor and an error in the build. Only one of those gets acted on.

Where to go next

Try it in the Lab

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