fix: synchronize mounted component props

packages/test/src/index.ts

@@ -205,3 +205,9 @@ export type {
 } from "./advanced.ts";
 export { withDatabaseRollback, createFactory, captureBrowserArtifacts } from "./platform.ts";
 export type { TransactionalDatabase, BrowserArtifactPage } from "./platform.ts";
+export { detectMutations } from "./mutation.ts";
+export type {
+  MutationCase,
+  MutationReport,
+  DetectMutationsOptions,
+} from "./mutation.ts";

packages/test/src/mutation.ts

@@ -0,0 +1,52 @@
+export interface MutationCase<T> {
+  name: string;
+  value: T;
+}
+
+export interface MutationReport<O> {
+  baseline: O;
+  killed: string[];
+  survived: string[];
+}
+
+export interface DetectMutationsOptions<T, O> {
+  baseline: T;
+  mutations: Array<MutationCase<T>>;
+  exercise(value: T): O | Promise<O>;
+  equivalent?(left: O, right: O): boolean;
+}
+
+/**
+ * Run the same behavioural probe against a baseline and explicit mutations.
+ * A mutation survives when the observable result is unchanged. The helper
+ * throws with every survivor by default, making it suitable for an ordinary
+ * Bun test and CI without a second runner or source-rewriting process.
+ */
+export async function detectMutations<T, O>(
+  options: DetectMutationsOptions<T, O>,
+): Promise<MutationReport<O>> {
+  if (!options.mutations.length) {
+    throw new Error("WRN-MUTATION-EMPTY: declare at least one meaningful mutation");
+  }
+  const equivalent = options.equivalent ?? ((left, right) => Object.is(left, right));
+  const baseline = await options.exercise(options.baseline);
+  const killed: string[] = [];
+  const survived: string[] = [];
+  for (const mutation of options.mutations) {
+    if (!mutation.name.trim()) throw new Error("WRN-MUTATION-NAME: every mutation needs a name");
+    let same = false;
+    try {
+      same = equivalent(baseline, await options.exercise(mutation.value));
+    } catch {
+      // A behavioural failure is a killed mutation.
+    }
+    (same ? survived : killed).push(mutation.name);
+  }
+  if (survived.length) {
+    throw Object.assign(
+      new Error(`WRN-MUTATION-SURVIVED: ${survived.join(", ")}`),
+      { report: { baseline, killed, survived } },
+    );
+  }
+  return { baseline, killed, survived };
+}