feat: support ESM custom reporters

packages/istanbul-lib-report/README.md

@@ -39,3 +39,27 @@ const report = libReport.create("json", {
 // call execute to synchronously create and write the report to disk
 report.execute(context);
 ```
+
+### Custom reporters
+
+`create()` only knows the built-in reporters. To load a custom reporter use`createAsync()`, which accepts a built-in name, a package name, an absolute path or a `file://` URL.
+The module's default export (ESM) or `module.exports` (CommonJS) must be the report class:
+
+```js
+import { createAsync, ReportBase } from "@vitest/istanbul-lib-report";
+
+// my-report.mjs
+export default class MyReport extends ReportBase {
+  onStart(root, context) {
+    this.writer = context.writer.writeFile("my-report.txt");
+  }
+  onEnd() {
+    this.writer.close();
+  }
+}
+
+const report = await createAsync("/absolute/path/to/my-report.mjs", {
+  summarizer: "nested",
+});
+report.execute(context);
+```

packages/istanbul-lib-report/src/index.ts

@@ -7,7 +7,8 @@
  * @module Exports
  */
 
-import { createRequire } from "node:module";
+import { isAbsolute } from "node:path";
+import { pathToFileURL } from "node:url";
 
 import Context from "./context";
 import type { ContextOptions } from "./context";
@@ -128,19 +129,67 @@ export interface ReportOptions {
 /** names of the built-in reports */
 export type ReportType = keyof ReportOptions;
 
+type ReportConstructor = new (cfg: object) => ReportBase;
+
+function getBuiltinReport(name: string): ReportConstructor | undefined {
+  return Object.hasOwn(reports, name)
+    ? (reports as Record<string, ReportConstructor>)[name]
+    : undefined;
+}
+
+function unwrapDefault(mod: unknown): unknown {
+  return typeof mod === "object" && mod !== null && "default" in mod ? mod.default : mod;
+}
+
+/**
+ * creates an instance of a built-in report.
+ * @param name the report name, e.g. `html`, `json`, `text`
+ * @param cfg options for the report
+ */
 export function create<T extends ReportType>(
   name: T,
   cfg?: Partial<ReportOptions[T]>,
-): InstanceType<(typeof reports)[T]>;
-export function create(name: string, cfg?: object): ReportBase;
-export function create(name: string, cfg?: object): ReportBase {
-  cfg = cfg || {};
-  let Cons = (reports as Record<string, unknown>)[name] as new (cfg: object) => ReportBase;
+): InstanceType<(typeof reports)[T]> {
+  const Cons = getBuiltinReport(name);
+  if (!Cons) {
+    throw new Error(`Unknown report "${name}". Use createAsync() to load custom reports.`);
+  }
+  return new Cons(cfg ?? {}) as InstanceType<(typeof reports)[T]>;
+}
+
+/**
+ * creates an instance of a built-in or custom report.
+ *
+ * Custom reports are loaded with `import()`, so `name` may be a package name,
+ * an absolute path or a `file://` URL of an ES module or CommonJS module whose
+ * default export (or `module.exports`) is the report class.
+ * @param name the report name, package name or path
+ * @param cfg options for the report
+ */
+export async function createAsync<T extends ReportType>(
+  name: T,
+  cfg?: Partial<ReportOptions[T]>,
+): Promise<InstanceType<(typeof reports)[T]>>;
+export async function createAsync(name: string, cfg?: object): Promise<ReportBase>;
+export async function createAsync(name: string, cfg?: object): Promise<ReportBase> {
+  let Cons = getBuiltinReport(name);
 
   if (!Cons) {
-    // TODO Verify custom reporters load here fine
-    Cons = createRequire(import.meta.url)(name);
+    const specifier = isAbsolute(name) ? pathToFileURL(name).href : name;
+    const mod: unknown = await import(specifier);
+
+    Cons = unwrapDefault(mod) as ReportConstructor;
+    // CommonJS transpiled from ESM: `module.exports = { __esModule: true, default: … }`
+    if (typeof Cons === "object" && Cons !== null && "__esModule" in Cons) {
+      Cons = unwrapDefault(Cons) as ReportConstructor;
+    }
+
+    if (typeof Cons !== "function") {
+      throw new TypeError(
+        `Custom report "${name}" must export a report class as its default export (ESM) or module.exports (CommonJS)`,
+      );
+    }
   }
 
-  return new Cons(cfg);
+  return new Cons!(cfg ?? {});
 }

packages/istanbul-lib-report/test/custom-reports.test.ts

@@ -0,0 +1,81 @@
+import { fileURLToPath } from "node:url";
+
+import { createCoverageMap } from "@vitest/istanbul-lib-coverage";
+import { describe, it, expect } from "vitest";
+
+import { create, createAsync, createContext, ReportBase } from "../src/index";
+import type { PartialVisitor, ReportNode } from "../src/index";
+
+const fixture = (name: string): string =>
+  fileURLToPath(new URL(`./fixtures/custom-reports/${name}`, import.meta.url));
+
+describe("create", () => {
+  it("creates built-in reports", () => {
+    expect(create("text")).toBeInstanceOf(ReportBase);
+    expect(create("json", { file: "out.json" })).toBeInstanceOf(ReportBase);
+  });
+
+  it("throws for unknown reports", () => {
+    expect(() => create("does-not-exist" as "text")).toThrowErrorMatchingInlineSnapshot(
+      `[Error: Unknown report "does-not-exist". Use createAsync() to load custom reports.]`,
+    );
+  });
+
+  it("does not resolve names via the Object prototype", () => {
+    expect(() => create("constructor" as "text")).toThrow(/Unknown report/);
+  });
+});
+
+describe("createAsync", () => {
+  it("creates built-in reports", async () => {
+    const report = await createAsync("text");
+    expect(report).toBeInstanceOf(ReportBase);
+  });
+
+  it("does not resolve names via the Object prototype", async () => {
+    await expect(createAsync("toString")).rejects.toThrow();
+  });
+
+  it.each([
+    ["CommonJS", "cjs.cjs", "CjsReport"],
+    ["CommonJS transpiled from ESM", "cjs-transpiled.cjs", "TranspiledReport"],
+    ["ESM", "esm.mjs", "EsmReport"],
+    ["ESM with top-level await", "esm-tla.mjs", "EsmTlaReport"],
+  ])("loads a custom %s report from an absolute path", async (_, file, className) => {
+    const cfg = { file: "out.md" };
+    const report = (await createAsync(fixture(file), cfg)) as unknown as { opts: object };
+
+    expect(report.constructor.name).toBe(className);
+    expect(report.opts).toBe(cfg);
+  });
+
+  it("loads a custom report from a file:// URL", async () => {
+    const url = new URL("./fixtures/custom-reports/esm.mjs", import.meta.url).href;
+    const report = await createAsync(url);
+    expect(report.constructor.name).toBe("EsmReport");
+  });
+
+  it.each([
+    ["CommonJS", "cjs.cjs", ["start cjs", "end cjs"]],
+    ["ESM", "esm.mjs", ["start esm", "end esm"]],
+  ])("runs a custom %s report through the report tree", async (_, file, lines) => {
+    const report = (await createAsync(fixture(file))) as unknown as PartialVisitor<ReportNode> & {
+      lines: string[];
+    };
+    const context = createContext({ coverageMap: createCoverageMap({}) });
+
+    context.getTree().visit(report, context);
+
+    expect(report.lines).toEqual(lines);
+  });
+
+  it("rejects modules without a report class as default export", async () => {
+    await expect(createAsync(fixture("named-only.mjs"))).rejects.toThrow(
+      /must export a report class as its default export/,
+    );
+  });
+
+  it("rejects unresolvable reports", async () => {
+    await expect(createAsync("@vitest/this-report-does-not-exist")).rejects.toThrow();
+  });
+});

packages/istanbul-lib-report/test/fixtures/custom-reports/cjs-transpiled.cjs

@@ -0,0 +1,8 @@
+"use strict";
+// CommonJS emitted by a transpiler from `export default class …`
+Object.defineProperty(exports, "__esModule", { value: true });
+exports.default = class TranspiledReport {
+  constructor(opts) {
+    this.opts = opts;
+  }
+};

packages/istanbul-lib-report/test/fixtures/custom-reports/cjs.cjs

@@ -0,0 +1,14 @@
+"use strict";
+// A classic CommonJS custom report, as written for istanbuljs for years.
+module.exports = class CjsReport {
+  constructor(opts) {
+    this.opts = opts;
+    this.lines = [];
+  }
+  onStart() {
+    this.lines.push("start cjs");
+  }
+  onEnd() {
+    this.lines.push("end cjs");
+  }
+};

packages/istanbul-lib-report/test/fixtures/custom-reports/esm-tla.mjs

@@ -0,0 +1,7 @@
+await Promise.resolve();
+
+export default class EsmTlaReport {
+  constructor(opts) {
+    this.opts = opts;
+  }
+}

packages/istanbul-lib-report/test/fixtures/custom-reports/esm.mjs

@@ -0,0 +1,12 @@
+export default class EsmReport {
+  constructor(opts) {
+    this.opts = opts;
+    this.lines = [];
+  }
+  onStart() {
+    this.lines.push("start esm");
+  }
+  onEnd() {
+    this.lines.push("end esm");
+  }
+}

packages/istanbul-lib-report/test/fixtures/custom-reports/named-only.mjs

@@ -0,0 +1 @@
+export class NamedReport {}