docs(examples): add example for spy functions (#1222)

examples/spy-functions.ts

@@ -0,0 +1,49 @@
+/**
+ * @title Spy functions
+ * @difficulty intermediate
+ * @tags cli, deploy
+ * @run <url>
+ * @resource {https://jsr.io/@std/testing/doc/mock#spying} Spy docs on JSR
+ * @resource {https://www.typescriptlang.org/docs/handbook/release-notes/typescript-5-2.html} Typescript docs for `using` keyword
+ * @group Testing
+ *
+ * A function spy allow us to assert that a function was called with the correct arguments and a specific number of times.
+ * This snippet demonstrates how to spy on a function using a mock function.
+ */
+
+import { assertSpyCall, assertSpyCalls, spy } from "jsr:@std/testing/mock";
+
+function log(message: string) {
+  console.log(message);
+}
+
+const logger = { log };
+
+Deno.test("logger uses the log function", () => {
+  // Create a spy on the `logger.log` method.
+  const logSpy = spy(logger, "log");
+
+  // Call the `logger.log` method.
+  logger.log("Hello, world!");
+
+  try {
+    // Assert that the `log` function was called just once.
+    assertSpyCalls(logSpy, 1);
+
+    // Assert that the `log` function was called with the correct arguments.
+    assertSpyCall(logSpy, 0, { args: ["Hello, world!"] });
+  } finally {
+    // Restore the logger.log function to stop spying it.
+    logSpy.restore();
+  }
+});
+
+Deno.test("Creating a spy with the using keyword", () => {
+  // method spys are disposable, so we can create them with the keyword `using`
+  using logSpy = spy(logger, "log");
+
+  logger.log("Disposable spy");
+
+  // Spys created with the `using` keyword are automatically restored at the end of the test
+  assertSpyCalls(logSpy, 1);
+});