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Spy functions

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.

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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);
});

Run this example locally using the Deno CLI:

deno run https://docs.deno.com/examples/spy-functions.ts

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