Add a parallel option to groups.

source/group.ts

@@ -4,23 +4,32 @@ import {type Result, Test, TestContext} from "./test.js";
 export async function setup(
   name: string,
   fn: (group: Group) => Promise<void>,
+  options?: GroupOptions,
 ): Promise<Group> {
-  const group = new Group(name);
+  const group = new Group(name, options);
   await fn(group);
   await group.run();
   return group;
 }
 
+/** Options for test groups. */
+export type GroupOptions = {
+  parallel?: boolean;
+};
+
 /** A collection of tests. */
-export class Group {
+export class Group implements GroupOptions {
   public context: TestContext = new TestContext();
+  public parallel: boolean;
   public results: Result[] = [];
   public tests: Test[] = [];
 
   private _afterAll: (() => Promise<void>) | undefined;
   private _beforeAll: (() => Promise<void>) | undefined;
 
-  constructor(public name: string) {}
+  constructor(public name: string, options?: GroupOptions) {
+    this.parallel = options?.parallel ?? false;
+  }
 
   /** Set a function to run after all tests have finished. */
   afterAll(fn: Group["_afterAll"]): void {
@@ -48,9 +57,18 @@ export class Group {
       await this._beforeAll();
     }
 
-    const results = await Promise.all(
-      this.tests.map(async (test) => test.run(this.context)),
-    );
+    let results: Result[];
+    if (this.parallel) {
+      results = await Promise.all(
+        this.tests.map(async (test) => test.run(this.context)),
+      );
+    } else {
+      results = [];
+      for (const test of this.tests) {
+        // eslint-disable-next-line no-await-in-loop
+        results.push(await test.run(this.context));
+      }
+    }
 
     if (this._afterAll !== undefined) {
       await this._afterAll();

source/test.ts

@@ -1,24 +1,31 @@
 export class AssertionError extends Error {
   public readonly actual: string;
   public readonly expected: string;
+  public readonly title: string | undefined;
 
-  constructor(message: string, actual: any, expected: any) {
+  constructor(message: string, actual: any, expected: any, title?: string) {
     super(message);
 
     this.actual = JSON.stringify(actual);
     this.expected = JSON.stringify(expected);
+    this.title = title;
   }
 }
 
 /** Test execution context with assertions. */
 export class TestContext {
   /** Assert strict equality with `===`. */
-  equals<T>(actual: T, expected: T): void {
+  equals<T>(actual: T, expected: T, title?: string): void {
     if (actual === expected) {
       return;
     }
 
-    throw new AssertionError("Failed equals assertion", actual, expected);
+    throw new AssertionError(
+      "Failed equals assertion",
+      actual,
+      expected,
+      title,
+    );
   }
 }
 
@@ -120,6 +127,7 @@ export class Result implements ResultData {
 
     if (this.error !== undefined) {
       message += `\n  %c${this.error.message}`;
+      message += this.error.title === undefined ? "" : `: ${this.error.title}`;
       message += `\n  |   Actual: ${this.error.actual}`;
       message += `\n  | Expected: ${this.error.expected}`;
       styles.push("color: pink;");

tests/index.ts

@@ -34,24 +34,30 @@ void setup("add", async (group) => {
   });
 });
 
-void setup("subtract", async (group) => {
-  group.beforeAll(async () => {
-    console.log("subtract beforeAll hook");
-  });
+void setup(
+  "subtract",
+  async (group) => {
+    group.beforeAll(async () => {
+      console.log("subtract beforeAll hook");
+    });
 
-  group.afterAll(async () => {
-    console.log("subtract afterAll hook");
-  });
+    group.afterAll(async () => {
+      console.log("subtract afterAll hook");
+    });
 
-  group.test("subtract(1, 1) = 0", async (test) => {
-    test.equals(await subtract(1, 1), 0);
-  });
+    group.test("subtract(1, 1) = 0", async (test) => {
+      test.equals(await subtract(1, 1), 0);
+    });
 
-  group.skip("subtract(1, 1) = 1", async (test) => {
-    test.equals(await subtract(1, 1), 1);
-  });
+    group.skip("subtract(1, 1) = 1", async (test) => {
+      test.equals(await subtract(1, 1), 1);
+    });
 
-  group.test("subtract(1, 1) = 2", async (test) => {
-    test.equals(await subtract(1, 1), 2);
-  });
-});
+    group.test("subtract(1, 1) = 2", async (test) => {
+      test.equals(await subtract(1, 1), 2, "extra title");
+    });
+  },
+  {
+    parallel: false,
+  },
+);