Preserve modes

tsc removes all type information when it emits JavaScript. tsc-rs can keep some of it in the output, for tools that run after the compiler: runtime type libraries, documentation generators and refactoring tools.

There are three independent modes.

Type annotations#

--preserveTypeAnnotations keeps as expressions, satisfies expressions and angle-bracket assertions in the emitted code.

Input, cast.ts:

const raw = JSON.parse("{}") as { port: number };
const config = { port: raw.port, host: "localhost" } satisfies Record<string, string | number>;
export const port = <number>config.port;

Default output:

tsc-rs --target es2022 --module esnext cast.ts

const raw = JSON.parse("{}");
const config = { port: raw.port, host: "localhost" };
export const port = config.port;

With the flag:

tsc-rs --target es2022 --module esnext --preserveTypeAnnotations cast.ts

const raw = JSON.parse("{}") as { port: number };
const config = { port: raw.port, host: "localhost" } satisfies Record<string, string | number>;
export const port = <number>config.port;

Comments#

--preserveComments keeps comments even when removeComments is set. It lets a project keep removeComments in a shared config and still produce commented output for a documentation or analysis pass.

removeComments is a tsconfig.json option; there is no --removeComments command line flag. With this config:

{
  "compilerOptions": {
    "target": "es2022",
    "module": "esnext",
    "removeComments": true
  },
  "include": ["*.ts"]
}

tsc-rs -p tsconfig.json strips the comments:

export const a = 1;
export function f() { }

and tsc-rs -p tsconfig.json --preserveComments keeps them:

// leading note
export const a = 1; // trailing
/** doc */
export function f() { }

Setting "preserveComments": true in the config has the same effect as the flag.

Whitespace#

--preserveWhitespace keeps the original layout of the source where it can. This mode is partial: statements that the compiler has to transform are still re-printed.

tsc-rs --preserveWhitespace file.ts

In tsconfig.json#

All three are also accepted as compilerOptions: preserveTypeAnnotations, preserveComments and preserveWhitespace. See tsconfig support.

Related: fast emit#

--fast-emit is a different trade. It produces plain JavaScript, but copies statements that need no transform verbatim from the source instead of normalizing their formatting the way tsc would. Combined with --transpileOnly it runs two to four times faster than the structured path, and is intended for consumers that feed the result straight to a JavaScript engine.

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