detype

Remove the types, keep the formatting

npm i -g detype

Suppose you have a library that you want to provide usage examples for. detype can help you generate vanilla JavaScript samples from TypeScript samples automatically and remove the burden of maintaining two separate versions of what is essentially the same code.

It is a command line tool and a library that removes type annotations and other TypeScript specific syntax constructs and outputs vanilla JavaScript without altering the source formatting too much. It supports .ts, .tsx, as well as .vue files.

In other words, it turns this:

import type { ParsedPath } from "path";

let x: string;

// This comment should be kept

// This comment should be deleted
// Ditto for this
interface Foo {
	// This should go too
	bar: number;
}

// This comment should also be kept
export function bar(foo: Foo): Date {
	return new Date();
}

into this:

let x;

// This comment should be kept

// This comment should also be kept
export function bar(foo) {
	return new Date();
}

The output is very close to hand-written JavaScript, especially if you were already using Prettier for formatting.

Doesn't tsc already do that?

There are lots of tools for transpiling TypeScript into plain JavaScript (tsc, babel, swc, esbuild, sucrase etc.) but none of them is perfectly suitable for this specific use case. Most of them don't preserve the formatting at all. sucrase comes close, but it doesn't remove comments attached to TypeScript-only constructs.

detype uses Babel, a small Babel plugin to remove comments attached to TypeScript-only constructs, and Prettier under the hood. For Vue files, it also uses the tools from the VueDX project.

Magic comments

Sometimes you want the generated JavaScript to be slightly different than the TypeScript original. You can use the magic comments feature to achieve this:

Input:

// @detype: replace
// These two lines will be removed
console.log("Hello from TypeScript");
// @detype: with
// // Notice the double comments!
// console.log("Hello from JavaScript");
// @detype: end

Output:

// Notice the double comments!
console.log("Hello from JavaScript");

If you just want to remove the magic comments, you can use the -m CLI flag or the removeMagicComments function to generate uncluttered TypeScript like this:

// These two lines will be removed
console.log("Hello from TypeScript");

System requirements

detype requires Node version 14.19.3 or later.

CLI Usage

detype [-m | --remove-magic-comments] <INPUT> [OUTPUT]

  INPUT   Input file or directory

  OUTPUT  Output file or directory
    (optional if it can be inferred and it won't overwrite the source file)

  -m, --remove-magic-comments
    Remove magic comments only, don't perform ts > js transform

detype [-v | --version]

  Print version and exit

detype [-h | --help]

  Print this help and exit

Node API

// Transform TypeScript code into vanilla JavaScript without affecting the formatting
function transform(
	// Source code
	code: string,
	// File name for the source
	fileName: string,
	// Options to pass to prettier
	prettierOptions?: PrettierOptions | null,
): Promise<string>;

// Transform the input file and write the output to another file
function transformFile(
	inputFileName: string,
	outputFileName: string,
): Promise<void>;

// Remove magic comments without performing the TS to JS transform
export function removeMagicComments(
	// Source code
	code: string,
	// File name for the source
	fileName: string,
	// Options to pass to prettier
	prettierOptions?: PrettierOptions | null,
): string;

// Remove magic comments from the input file and write the output to another file
export function removeMagicCommentsFromFile(
	inputFileName: string,
	outputFileName: string,
): Promise<void>;

Change log

0.5

• BREAKING CHANGE: Drop support for Node 12
• chore: Set up CI workflows

0.4

• feature: CLI support for removing magic comments
• chore: Improve documentation

0.3

• feature: Magic comments
• feature: Expose type declarations
• fix: Better empty line handling

0.2

• feature: for Vue single file components

0.1

• Initial release

Credits

Fatih Aygün, under MIT License