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/scripts/spy_functions.ts

Spy functions

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