allowUnreachableCode

Code after a return

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

Since: TypeScript 1.8
strict: Not in strict
In your tsconfig: "allowUnreachableCode": false

The same snippet both times. Only the option changed.

With it off

"allowUnreachableCode": true

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

Compiles clean

With it on

"allowUnreachableCode": false

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

Emits TS7027

Why the compiler bothers

This option has three states rather than two, which is why it is easy to get wrong. Left undefined it produces an editor suggestion; set to `false` it produces an error; set to `true` it produces nothing. Unreachable code is never intentional — it is an early return added during a fix, a `throw` moved up, a branch that stopped being a branch — and the only one of the three settings that stops it reaching main is the explicit `false`.

Takeaway

Undefined is not the same as `false` here. Write the `false`.

Where to go next

Try it in the Lab

22 options