feat: add transform loader

README.md

@@ -31,7 +31,11 @@ It is possible to use Amaro as an external loader to execute TypeScript files.
 This allows the installed Amaro to override the Amaro version used by Node.js.
 
 ```bash
-node --experimental-strip-types --import="amaro/register" script.ts
+node --experimental-strip-types --import="amaro/strip" script.ts
+```
+
+```bash
+node --experimental-transform-types --import="amaro/transform" script.ts
 ```
 
 ### How to update SWC

esbuild.config.js

@@ -1,24 +1,46 @@
 const { copy } = require("esbuild-plugin-copy");
-const esbuild = require("esbuild");
+const { build } = require("esbuild");
 
-esbuild.build({
-	entryPoints: ["src/index.ts"],
-	bundle: true,
-	platform: "node",
-	target: "node20",
-	outdir: "dist",
-	plugins: [
-		copy({
-			assets: {
-				from: ["./src/register/register.mjs"],
-				to: ["."],
-			},
-		}),
-		copy({
-			assets: {
-				from: ["./lib/LICENSE", "./lib/package.json"],
-				to: ["."],
-			},
-		}),
+const copyPlugin = copy({
+	assets: [
+		{
+			from: ["./src/register/register-strip.mjs"],
+			to: ["."],
+		},
+		{
+			from: ["./src/register/register-transform.mjs"],
+			to: ["."],
+		},
+		{
+			from: ["./lib/LICENSE", "./lib/package.json"],
+			to: ["."],
+		},
 	],
 });
+
+(async () => {
+	await build({
+		entryPoints: ["src/index.ts"],
+		bundle: true,
+		platform: "node",
+		target: "node20",
+		outfile: "dist/index.js",
+		plugins: [copyPlugin],
+	});
+
+	await build({
+		entryPoints: ["src/strip-loader.ts"],
+		bundle: false,
+		outfile: "dist/strip-loader.js",
+		platform: "node",
+		target: "node20",
+	});
+
+	await build({
+		entryPoints: ["src/transform-loader.ts"],
+		bundle: false,
+		outfile: "dist/transform-loader.js",
+		platform: "node",
+		target: "node20",
+	});
+})();

package.json

@@ -36,7 +36,11 @@
 	},
 	"exports": {
 		".": "./dist/index.js",
-		"./register": "./dist/register.mjs"
+		"./strip": "./dist/register-strip.mjs",
+		"./transform": "./dist/register-transform.mjs"
 	},
-	"files": ["dist", "LICENSE.md"]
-}
+	"files": [
+		"dist",
+		"LICENSE.md"
+	]
+}

src/index.ts

@@ -1,2 +1 @@
 export { transformSync } from "./transform.ts";
-export { load } from "./loader.ts";

src/register/register-strip.mjs

@@ -0,0 +1,3 @@
+import { register } from "node:module";
+
+register("./strip-loader.js", import.meta.url);

src/register/register-transform.mjs

@@ -0,0 +1,3 @@
+import { register } from "node:module";
+
+register("./transform-loader.js", import.meta.url);

src/loader.ts → src/strip-loader.ts

@@ -1,6 +1,6 @@
 import type { LoadFnOutput, LoadHookContext } from "node:module";
 import type { Options } from "../lib/wasm";
-import { transformSync } from "./index.ts";
+import { transformSync } from "./index.js";
 
 type NextLoad = (
 	url: string,
@@ -20,14 +20,21 @@ export async function load(
 			...context,
 			format: "module",
 		});
-		if (source == null)
+
+		if (source == null) {
 			throw new Error("Source code cannot be null or undefined");
+		}
+
 		const { code } = transformSync(source.toString(), {
 			mode: "strip-only",
 		} as Options);
+
 		return {
 			format: format.replace("-typescript", ""),
-			source: code,
+			// Source map is not necessary in strip-only mode. However, to map the source
+			// file in debuggers to the original TypeScript source, add a sourceURL magic
+			// comment to hint that it is a generated source.
+			source: `${code}\n\n//# sourceURL=${url}`,
 		};
 	}
 	return nextLoad(url, context);

src/transform-loader.ts

@@ -0,0 +1,47 @@
+import type { LoadFnOutput, LoadHookContext } from "node:module";
+import type { Options } from "../lib/wasm";
+import { transformSync } from "./index.js";
+
+type NextLoad = (
+	url: string,
+	context?: LoadHookContext,
+) => LoadFnOutput | Promise<LoadFnOutput>;
+
+export async function load(
+	url: string,
+	context: LoadHookContext,
+	nextLoad: NextLoad,
+) {
+	const { format } = context;
+	if (format.endsWith("-typescript")) {
+		// Use format 'module' so it returns the source as-is, without stripping the types.
+		// Format 'commonjs' would not return the source for historical reasons.
+		const { source } = await nextLoad(url, {
+			...context,
+			format: "module",
+		});
+
+		if (source == null) {
+			throw new Error("Source code cannot be null or undefined");
+		}
+
+		const { code, map } = transformSync(source.toString(), {
+			mode: "transform",
+			sourceMap: true,
+			filename: url,
+		} as Options);
+
+		let output = code;
+
+		if (map) {
+			const base64SourceMap = Buffer.from(map).toString("base64");
+			output = `${code}\n\n//# sourceMappingURL=data:application/json;base64,${base64SourceMap}`;
+		}
+
+		return {
+			format: format.replace("-typescript", ""),
+			source: output,
+		};
+	}
+	return nextLoad(url, context);
+}

src/transform.ts

@@ -3,6 +3,11 @@ import swc from "../lib/wasm.js";
 
 const DEFAULT_OPTIONS = {
 	mode: "strip-only",
+	// default transform will only work when mode is "transform"
+	transform: {
+		verbatimModuleSyntax: true,
+		nativeClassProperties: true,
+	},
 } as Options;
 
 export function transformSync(

test/loader.test.js

@@ -6,7 +6,7 @@ test("should work as a loader", async () => {
 	const result = await spawnPromisified(process.execPath, [
 		"--experimental-strip-types",
 		"--no-warnings",
-		"--import=./dist/register.mjs",
+		"--import=./dist/register-strip.mjs",
 		fixturesPath("hello.ts"),
 	]);
 
@@ -19,7 +19,7 @@ test("should work with enums", async () => {
 	const result = await spawnPromisified(process.execPath, [
 		"--experimental-strip-types",
 		"--no-warnings",
-		"--import=./dist/register.mjs",
+		"--import=./dist/register-strip.mjs",
 		fixturesPath("enum.ts"),
 	]);