feat(coverage): replace acorn with @babel/parser for JS and TS support (#152)

Author: sohil-kshirsagar

package-lock.json

@@ -108,9 +108,8 @@
     "typescript": "^5.0.0"
   },
   "dependencies": {
+    "@babel/parser": "^7.29.2",
     "@use-tusk/drift-core-node": "^0.1.8",
-    "acorn": "^8.14.0",
-    "acorn-typescript": "^1.4.13",
     "ast-v8-to-istanbul": "^1.0.0",
     "import-in-the-middle": "^1.14.4",
     "js-yaml": "^4.1.0",

package.json

@@ -80,3 +80,139 @@ test("takeAndProcessSnapshot: skips coverage files without user scripts", async
     fs.rmSync(tmpDir, { recursive: true, force: true });
   }
 });
+
+// --- Babel plugin selection and parsing tests ---
+
+test("Babel plugin selection: TypeScript files get typescript and decorators-legacy plugins", (t) => {
+  const scriptPath = "/project/src/service.ts";
+  const isTypeScript = /\.(ts|tsx|mts|cts)$/.test(scriptPath);
+  const isTSX = /\.tsx$/.test(scriptPath);
+  const babelPlugins: string[] = isTypeScript
+    ? ["typescript", "decorators-legacy", ...(isTSX ? ["jsx"] : [])]
+    : ["decorators-legacy"];
+
+  t.deepEqual(babelPlugins, ["typescript", "decorators-legacy"]);
+});
+
+test("Babel plugin selection: TSX files get typescript, decorators-legacy, and jsx plugins", (t) => {
+  const scriptPath = "/project/src/Component.tsx";
+  const isTypeScript = /\.(ts|tsx|mts|cts)$/.test(scriptPath);
+  const isTSX = /\.tsx$/.test(scriptPath);
+  const babelPlugins: string[] = isTypeScript
+    ? ["typescript", "decorators-legacy", ...(isTSX ? ["jsx"] : [])]
+    : ["decorators-legacy"];
+
+  t.deepEqual(babelPlugins, ["typescript", "decorators-legacy", "jsx"]);
+});
+
+test("Babel plugin selection: .mts files get typescript and decorators-legacy plugins", (t) => {
+  const scriptPath = "/project/src/module.mts";
+  const isTypeScript = /\.(ts|tsx|mts|cts)$/.test(scriptPath);
+  const isTSX = /\.tsx$/.test(scriptPath);
+  const babelPlugins: string[] = isTypeScript
+    ? ["typescript", "decorators-legacy", ...(isTSX ? ["jsx"] : [])]
+    : ["decorators-legacy"];
+
+  t.deepEqual(babelPlugins, ["typescript", "decorators-legacy"]);
+});
+
+test("Babel plugin selection: .cts files get typescript and decorators-legacy plugins", (t) => {
+  const scriptPath = "/project/src/commonjs.cts";
+  const isTypeScript = /\.(ts|tsx|mts|cts)$/.test(scriptPath);
+  const isTSX = /\.tsx$/.test(scriptPath);
+  const babelPlugins: string[] = isTypeScript
+    ? ["typescript", "decorators-legacy", ...(isTSX ? ["jsx"] : [])]
+    : ["decorators-legacy"];
+
+  t.deepEqual(babelPlugins, ["typescript", "decorators-legacy"]);
+});
+
+test("Babel plugin selection: JavaScript files get only decorators-legacy plugin", (t) => {
+  const scriptPath = "/project/src/service.js";
+  const isTypeScript = /\.(ts|tsx|mts|cts)$/.test(scriptPath);
+  const isTSX = /\.tsx$/.test(scriptPath);
+  const babelPlugins: string[] = isTypeScript
+    ? ["typescript", "decorators-legacy", ...(isTSX ? ["jsx"] : [])]
+    : ["decorators-legacy"];
+
+  t.deepEqual(babelPlugins, ["decorators-legacy"]);
+});
+
+test("Babel plugin selection: .mjs files get only decorators-legacy plugin", (t) => {
+  const scriptPath = "/project/src/module.mjs";
+  const isTypeScript = /\.(ts|tsx|mts|cts)$/.test(scriptPath);
+  const isTSX = /\.tsx$/.test(scriptPath);
+  const babelPlugins: string[] = isTypeScript
+    ? ["typescript", "decorators-legacy", ...(isTSX ? ["jsx"] : [])]
+    : ["decorators-legacy"];
+
+  t.deepEqual(babelPlugins, ["decorators-legacy"]);
+});
+
+test("Babel plugin selection: .cjs files get only decorators-legacy plugin", (t) => {
+  const scriptPath = "/project/src/commonjs.cjs";
+  const isTypeScript = /\.(ts|tsx|mts|cts)$/.test(scriptPath);
+  const isTSX = /\.tsx$/.test(scriptPath);
+  const babelPlugins: string[] = isTypeScript
+    ? ["typescript", "decorators-legacy", ...(isTSX ? ["jsx"] : [])]
+    : ["decorators-legacy"];
+
+  t.deepEqual(babelPlugins, ["decorators-legacy"]);
+});
+
+// --- Babel parser integration tests ---
+
+test("Babel parser: can parse TypeScript with parameter decorators (decorators-legacy)", (t) => {
+  // eslint-disable-next-line @typescript-eslint/no-var-requires
+  const babelParser = require("@babel/parser");
+  const tsCode = `
+class Service {
+  constructor(@inject('repo') private repo: any) {}
+}
+`;
+
+  // Should parse without error using decorators-legacy
+  const ast = babelParser.parse(tsCode, {
+    sourceType: "module",
+    plugins: ["typescript", "decorators-legacy"],
+  });
+
+  t.truthy(ast);
+  t.is(ast.type, "File");
+});
+
+test("Babel parser: can parse TSX with JSX syntax", (t) => {
+  // eslint-disable-next-line @typescript-eslint/no-var-requires
+  const babelParser = require("@babel/parser");
+  const tsxCode = `
+const Component = () => <div>Hello</div>;
+export { Component };
+`;
+
+  // Should parse without error using typescript + jsx + decorators-legacy
+  const ast = babelParser.parse(tsxCode, {
+    sourceType: "module",
+    plugins: ["typescript", "decorators-legacy", "jsx"],
+  });
+
+  t.truthy(ast);
+  t.is(ast.type, "File");
+});
+
+test("Babel parser: can parse JavaScript with class decorators", (t) => {
+  // eslint-disable-next-line @typescript-eslint/no-var-requires
+  const babelParser = require("@babel/parser");
+  const jsCode = `
+@decorator
+class Example {}
+`;
+
+  // Should parse without error using decorators-legacy
+  const ast = babelParser.parse(jsCode, {
+    sourceType: "module",
+    plugins: ["decorators-legacy"],
+  });
+
+  t.truthy(ast);
+  t.is(ast.type, "File");
+});

src/core/coverageProcessor.test.ts

@@ -179,7 +179,7 @@ function resolveSourceCode(scriptPath: string, projectRoot: string): {
       }
     }
 
-    // Fallback: read the .ts file directly — acorn-typescript can parse it.
+    // Fallback: read the .ts file directly — @babel/parser can parse it.
     // This handles ts-node, ts-node-dev, and --experimental-strip-types.
     const code = fs.readFileSync(scriptPath, "utf-8");
     return { code, resolvedPath: scriptPath, sourceMap: null };
@@ -208,8 +208,6 @@ export async function processV8CoverageFile(
   // Using require() avoids loading them on every SDK startup (adds ~50ms + memory).
   // eslint-disable-next-line @typescript-eslint/no-var-requires
   const { convert } = require("ast-v8-to-istanbul");
-  // eslint-disable-next-line @typescript-eslint/no-var-requires
-  const acorn = require("acorn");
   const data: V8CoverageData = preParsedData ?? JSON.parse(fs.readFileSync(v8FilePath, "utf-8"));
   const coverage: CoverageResult = {};
 
@@ -226,34 +224,35 @@ export async function processV8CoverageFile(
 
       // Try parsing as script first (CJS), fall back to module (ESM).
       // Track which succeeded — CJS modules have a V8 wrapper that shifts byte offsets.
-      // For .ts/.tsx files run via --experimental-strip-types, use acorn-typescript
-      // plugin since acorn can't parse TypeScript syntax natively.
+      // Use @babel/parser for all files — handles JS and TypeScript uniformly,
+      // including decorators, generics, import type, etc.
       let isCJS = false;
       const isTypeScript = /\.(ts|tsx|mts|cts)$/.test(scriptPath);
-      const parserOptions: Record<string, unknown> = {
-        ecmaVersion: "latest",
-        locations: true,
-      };
-
-      // Resolve the parser: use acorn-typescript for .ts files, plain acorn for .js
-      let parser = acorn;
-      if (isTypeScript) {
-        try {
-          const { tsPlugin } = require("acorn-typescript");
-          parser = acorn.Parser.extend(tsPlugin()) as typeof acorn;
-        } catch {
-          // acorn-typescript not available — plain acorn will be used
-          // (may fail for TS files, but the outer try/catch handles that)
-        }
-      }
+      const babelParser = require("@babel/parser");
+      const isTSX = /\.tsx$/.test(scriptPath);
+      // Use decorators-legacy (not decorators) to support TypeScript's experimental
+      // parameter decorators used by inversify, NestJS, TypeORM, etc.
+      // This covers ~99% of TS codebases. If a project uses TC39 Stage 3 decorators
+      // (experimentalDecorators: false in tsconfig.json), we'd need to switch to the
+      // "decorators" plugin instead. Could be inferred from tsconfig.json if needed.
+      const babelPlugins: string[] = isTypeScript
+        ? ["typescript", "decorators-legacy", ...(isTSX ? ["jsx"] : [])]
+        : ["decorators-legacy"];
 
-      // Try script (CJS) first, fall back to module (ESM)
       let ast;
       try {
-        ast = parser.parse(code, { ...parserOptions, sourceType: "script" });
+        ast = babelParser.parse(code, {
+          sourceType: "script",
+          plugins: babelPlugins,
+          ranges: true,
+        });
         isCJS = true;
       } catch {
-        ast = parser.parse(code, { ...parserOptions, sourceType: "module" });
+        ast = babelParser.parse(code, {
+          sourceType: "module",
+          plugins: babelPlugins,
+          ranges: true,
+        });
       }
 
       // Strip sourceMappingURL from code passed to convert() — we already loaded