erasableSyntaxOnly

Only the parts that erase

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

Since: TypeScript 5.8
strict: Not in strict
In your tsconfig: "erasableSyntaxOnly": true

The same snippet both times. Only the option changed.

With it off

"erasableSyntaxOnly": false

export enum Level {
  Low,
  High,
}

Compiles clean

With it on

"erasableSyntaxOnly": true

export enum Level {
  Low,
  High,
}

Emits TS1294

Why the compiler bothers

Most of TypeScript is annotation: delete it and you have the JavaScript. A handful of features are not, and they are the reason a file cannot simply have its types stripped. Node, Deno and browsers running TypeScript directly all take the stripping route, so this flag is what tells you, at compile time, whether your source can go through it — rather than at the moment somebody tries.

Takeaway

A union of string literals does what an enum did, and erases.

Where to go next

22 options