Testing

This module provides a few basic utilities to make testing easier and consistent in Deno.

Usage

testing/asserts.ts module provides range of assertion helpers. If the assertion is false an AssertionError will be thrown which will result in pretty-printed diff of failing assertion.

• equal() - Deep comparison function, where actual and expected are compared deeply, and if they vary, equal returns false.
• assert() - Expects a boolean value, throws if the value is false.
• assertFalse() - Expects a boolean value, throws if the value is true.
• assertEquals() - Uses the equal comparison and throws if the actual and expected are not equal.
• assertNotEquals() - Uses the equal comparison and throws if the actual and expected are equal.
• assertStrictEquals() - Compares actual and expected strictly, therefore for non-primitives the values must reference the same instance.
• assertAlmostEquals() - Make an assertion that actual is almost equal to expected, according to a given epsilon (defaults to 1e-7)
• assertInstanceOf() - Make an assertion that actual is an instance of expectedType.
• assertNotInstanceOf() - Make an assertion that actual is not an instance of expectedType.
• assertStringIncludes() - Make an assertion that actual includes expected.
• assertMatch() - Make an assertion that actual match RegExp expected.
• assertNotMatch() - Make an assertion that actual not match RegExp expected.
• assertArrayIncludes() - Make an assertion that actual array includes the expected values.
• assertObjectMatch() - Make an assertion that actual object match expected subset object
• assertSnapshot() - Make an assertion that actual matches a snapshot
• assertThrows() - Expects the passed fn to throw. If fn does not throw, this function does. Also compares any errors thrown to an optional expected Error class and checks that the error .message includes an optional string. If there is caught error, it gets returned.
• assertRejects() - Expects the passed fn to be async and return a PromiseLike object that rejects. If the fn does not reject, this function will reject (⚠️ you should normally await this assertion). Also optionally accepts an Error class which the expected error must be an instance of, and a string which must be a substring of the error’s .message. If there is caught error, it gets returned.
• unimplemented() - Use this to stub out methods that will throw when invoked.
• unreachable() - Used to assert unreachable code.

Basic usage:

import { assertEquals } from "https://deno.land/std@0.166.0/testing/asserts.ts";

Deno.test({
  name: "testing example",
  fn(): void {
    assertEquals("world", "world");
    assertEquals({ hello: "world" }, { hello: "world" });
  },
});

Short syntax (named function instead of object):

import { assertEquals } from "https://deno.land/std@0.166.0/testing/asserts.ts";

Deno.test("example", function (): void {
  assertEquals("world", "world");
  assertEquals({ hello: "world" }, { hello: "world" });
});

Using assertStrictEquals():

import { assertStrictEquals } from "https://deno.land/std@0.166.0/testing/asserts.ts";

Deno.test("isStrictlyEqual", function (): void {
  const a = {};
  const b = a;
  assertStrictEquals(a, b);
});

// This test fails
Deno.test("isNotStrictlyEqual", function (): void {
  const a = {};
  const b = {};
  assertStrictEquals(a, b);
});

Using assertSnapshot():

For more usage information, see Snapshot Testing.

import { assertSnapshot } from "https://deno.land/std@0.166.0/testing/snapshot.ts";

Deno.test("isSnapshotMatch", async function (t): Promise<void> {
  const a = {
    hello: "world!",
    example: 123,
  };
  await assertSnapshot(t, a);
});

Using assertThrows():

import { assertThrows } from "https://deno.land/std@0.166.0/testing/asserts.ts";

Deno.test("doesThrow", function (): void {
  assertThrows((): void => {
    throw new TypeError("hello world!");
  });
  assertThrows((): void => {
    throw new TypeError("hello world!");
  }, TypeError);
  assertThrows(
    (): void => {
      throw new TypeError("hello world!");
    },
    TypeError,
    "hello",
  );
});

// This test will not pass.
Deno.test("fails", function (): void {
  assertThrows((): void => {
    console.log("Hello world");
  });
});

Using assertRejects():

import { assertRejects } from "https://deno.land/std@0.166.0/testing/asserts.ts";

Deno.test("doesThrow", async function () {
  await assertRejects(
    async () => {
      throw new TypeError("hello world!");
    },
  );
  await assertRejects(async () => {
    throw new TypeError("hello world!");
  }, TypeError);
  await assertRejects(
    async () => {
      throw new TypeError("hello world!");
    },
    TypeError,
    "hello",
  );
  await assertRejects(
    async () => {
      return Promise.reject(new Error());
    },
  );
});

// This test will not pass.
Deno.test("fails", async function () {
  await assertRejects(
    async () => {
      console.log("Hello world");
    },
  );
});

Snapshot Testing

Basic usage:

The assertSnapshot function will create a snapshot of a value and compare it to a reference snapshot, which is stored alongside the test file in the __snapshots__ directory.

// example_test.ts
import { assertSnapshot } from "https://deno.land/std@0.166.0/testing/snapshot.ts";

Deno.test("isSnapshotMatch", async function (t): Promise<void> {
  const a = {
    hello: "world!",
    example: 123,
  };
  await assertSnapshot(t, a);
});

// __snapshots__/example_test.ts.snap
export const snapshot = {};

snapshot[`isSnapshotMatch 1`] = `
{
  example: 123,
  hello: "world!",
}
`;

Calling assertSnapshot in a test will throw an AssertionError, causing the test to fail, if the snapshot created during the test does not match the one in the snapshot file.

Updating Snapshots:

When adding new snapshot assertions to your test suite, or when intentionally making changes which cause your snapshots to fail, you can update your snapshots by running the snapshot tests in update mode. Tests can be run in update mode by passing the --update or -u flag as an argument when running the test. When this flag is passed, then any snapshots which do not match will be updated.

deno test --allow-all -- --update

Note: In Powershell, you need to quote --.

deno test --allow-all "--" --update

Additionally, new snapshots will only be created when this flag is present.

Permissions:

When running snapshot tests, the --allow-read permission must be enabled, or else any calls to assertSnapshot will fail due to insufficient permissions. Additionally, when updating snapshots, the --allow-write permission must also be enabled, as this is required in order to update snapshot files.

The assertSnapshot function will only attempt to read from and write to snapshot files. As such, the allow list for --allow-read and --allow-write can be limited to only include existing snapshot files, if so desired.

Options:

The assertSnapshot function optionally accepts an options object.

// example_test.ts
import { assertSnapshot } from "https://deno.land/std@0.166.0/testing/snapshot.ts";

Deno.test("isSnapshotMatch", async function (t): Promise<void> {
  const a = {
    hello: "world!",
    example: 123,
  };
  await assertSnapshot(t, a, {
    // options
  });
});

You can also configure default options for assertSnapshot.

// example_test.ts
import { createAssertSnapshot } from "https://deno.land/std@0.166.0/testing/snapshot.ts";

const assertSnapshot = createAssertSnapshot({
  // options
});

When configuring default options like this, the resulting assertSnapshot function will function the same as the default function exported from the snapshot module. If passed an optional options object, this will take precedence over the default options, where the value provded for an option differs.

It is possible to “extend” an assertSnapshot function which has been configured with default options.

// example_test.ts
import { createAssertSnapshot } from "https://deno.land/std@0.166.0/testing/snapshot.ts";
import { stripColor } from "https://deno.land/std@0.166.0/fmt/colors.ts";

const assertSnapshot = createAssertSnapshot({
  dir: ".snaps",
});

const assertMonochromeSnapshot = createAssertSnapshot<string>(
  { serializer: stripColor },
  assertSnapshot,
);

Deno.test("isSnapshotMatch", async function (t): Promise<void> {
  const a = "\x1b[32mThis green text has had it's colours stripped\x1b[39m";
  await assertMonochromeSnapshot(t, a);
});

// .snaps/example_test.ts.snap
export const snapshot = {};

snapshot[`isSnapshotMatch 1`] = `This green text has had it's colours stripped`;

Version Control:

Snapshot testing works best when changes to snapshot files are comitted alongside other code changes. This allows for changes to reference snapshots to be reviewed along side the code changes that caused them, and ensures that when others pull your changes, their tests will pass without needing to update snapshots locally.

Behavior-driven development

With the bdd.ts module you can write your tests in a familiar format for grouping tests and adding setup/teardown hooks used by other JavaScript testing frameworks like Jasmine, Jest, and Mocha.

The describe function creates a block that groups together several related tests. The it function registers an individual test case.

Hooks

There are 4 types of hooks available for test suites. A test suite can have multiples of each type of hook, they will be called in the order that they are registered. The afterEach and afterAll hooks will be called whether or not the test case passes. The *All hooks will be called once for the whole group while the *Each hooks will be called for each individual test case.

• beforeAll: Runs before all of the tests in the test suite.
• afterAll: Runs after all of the tests in the test suite finish.
• beforeEach: Runs before each of the individual test cases in the test suite.
• afterEach: Runs after each of the individual test cases in the test suite.

If a hook is registered at the top level, a global test suite will be registered and all tests will belong to it. Hooks registered at the top level must be registered before any individual test cases or test suites.

Focusing tests

If you would like to run only specific test cases, you can do so by calling it.only instead of it. If you would like to run only specific test suites, you can do so by calling describe.only instead of describe.

There is one limitation to this when using the flat test grouping style. When describe is called without being nested, it registers the test with Deno.test. If a child test case or suite is registered with it.only or describe.only, it will be scoped to the top test suite instead of the file. To make them the only tests that run in the file, you would need to register the top test suite with describe.only too.

Ignoring tests

If you would like to not run specific individual test cases, you can do so by calling it.ignore instead of it. If you would like to not run specific test suites, you can do so by calling describe.ignore instead of describe.

Sanitization options

Like Deno.TestDefinition, the DescribeDefinition and ItDefinition have sanitization options. They work in the same way.

• sanitizeExit: Ensure the test case does not prematurely cause the process to exit, for example via a call to Deno.exit. Defaults to true.
• sanitizeOps: Check that the number of async completed ops after the test is the same as number of dispatched ops. Defaults to true.
• sanitizeResources: Ensure the test case does not “leak” resources - ie. the resource table after the test has exactly the same contents as before the test. Defaults to true.

Permissions option

Like Deno.TestDefinition, the DescribeDefintion and ItDefinition have a permissions option. They specify the permissions that should be used to run an individual test case or test suite. Set this to "inherit" to keep the calling thread’s permissions. Set this to "none" to revoke all permissions.

This setting defaults to "inherit".

There is currently one limitation to this, you cannot use the permissions option on an individual test case or test suite that belongs to another test suite. That’s because internally those tests are registered with t.step which does not support the permissions option.

Comparing to Deno.test

The default way of writing tests is using Deno.test and t.step. The describe and it functions have similar call signatures to Deno.test, making it easy to switch between the default style and the behavior-driven development style of writing tests. Internally, describe and it are registering tests with Deno.test and t.step.

Below is an example of a test file using Deno.test and t.step. In the following sections there are examples of how the same test could be written with describe and it using nested test grouping, flat test grouping, or a mix of both styles.

// https://deno.land/std@0.166.0/testing/bdd_examples/user_test.ts
import {
  assertEquals,
  assertStrictEquals,
  assertThrows,
} from "https://deno.land/std@0.166.0/testing/asserts.ts";
import { User } from "https://deno.land/std@0.166.0/testing/bdd_examples/user.ts";

Deno.test("User.users initially empty", () => {
  assertEquals(User.users.size, 0);
});

Deno.test("User constructor", () => {
  try {
    const user = new User("Kyle");
    assertEquals(user.name, "Kyle");
    assertStrictEquals(User.users.get("Kyle"), user);
  } finally {
    User.users.clear();
  }
});

Deno.test("User age", async (t) => {
  const user = new User("Kyle");

  await t.step("getAge", () => {
    assertThrows(() => user.getAge(), Error, "Age unknown");
    user.age = 18;
    assertEquals(user.getAge(), 18);
  });

  await t.step("setAge", () => {
    user.setAge(18);
    assertEquals(user.getAge(), 18);
  });
});

Nested test grouping

Tests created within the callback of a describe function call will belong to the new test suite it creates. The hooks can be created within it or be added to the options argument for describe.

// https://deno.land/std@0.166.0/testing/bdd_examples/user_nested_test.ts
import {
  assertEquals,
  assertStrictEquals,
  assertThrows,
} from "https://deno.land/std@0.166.0/testing/asserts.ts";
import {
  afterEach,
  beforeEach,
  describe,
  it,
} from "https://deno.land/std@0.166.0/testing/bdd.ts";
import { User } from "https://deno.land/std@0.166.0/testing/bdd_examples/user.ts";

describe("User", () => {
  it("users initially empty", () => {
    assertEquals(User.users.size, 0);
  });

  it("constructor", () => {
    try {
      const user = new User("Kyle");
      assertEquals(user.name, "Kyle");
      assertStrictEquals(User.users.get("Kyle"), user);
    } finally {
      User.users.clear();
    }
  });

  describe("age", () => {
    let user: User;

    beforeEach(() => {
      user = new User("Kyle");
    });

    afterEach(() => {
      User.users.clear();
    });

    it("getAge", function () {
      assertThrows(() => user.getAge(), Error, "Age unknown");
      user.age = 18;
      assertEquals(user.getAge(), 18);
    });

    it("setAge", function () {
      user.setAge(18);
      assertEquals(user.getAge(), 18);
    });
  });
});

Flat test grouping

The describe function returns a unique symbol that can be used to reference the test suite for adding tests to it without having to create them within a callback. The gives you the ability to have test grouping without any extra indentation in front of the grouped tests.

// https://deno.land/std@0.166.0/testing/bdd_examples/user_flat_test.ts
import {
  assertEquals,
  assertStrictEquals,
  assertThrows,
} from "https://deno.land/std@0.166.0/testing/asserts.ts";
import {
  describe,
  it,
} from "https://deno.land/std@0.166.0/testing/bdd.ts";
import { User } from "https://deno.land/std@0.166.0/testing/bdd_examples/user.ts";

const userTests = describe("User");

it(userTests, "users initially empty", () => {
  assertEquals(User.users.size, 0);
});

it(userTests, "constructor", () => {
  try {
    const user = new User("Kyle");
    assertEquals(user.name, "Kyle");
    assertStrictEquals(User.users.get("Kyle"), user);
  } finally {
    User.users.clear();
  }
});

const ageTests = describe({
  name: "age",
  suite: userTests,
  beforeEach(this: { user: User }) {
    this.user = new User("Kyle");
  },
  afterEach() {
    User.users.clear();
  },
});

it(ageTests, "getAge", function () {
  const { user } = this;
  assertThrows(() => user.getAge(), Error, "Age unknown");
  user.age = 18;
  assertEquals(user.getAge(), 18);
});

it(ageTests, "setAge", function () {
  const { user } = this;
  user.setAge(18);
  assertEquals(user.getAge(), 18);
});

Mixed test grouping

Both nested test grouping and flat test grouping can be used together. This can be useful if you’d like to create deep groupings without all the extra indentation in front of each line.

// https://deno.land/std@0.166.0/testing/bdd_examples/user_mixed_test.ts
import {
  assertEquals,
  assertStrictEquals,
  assertThrows,
} from "https://deno.land/std@0.166.0/testing/asserts.ts";
import {
  describe,
  it,
} from "https://deno.land/std@0.166.0/testing/bdd.ts";
import { User } from "https://deno.land/std@0.166.0/testing/bdd_examples/user.ts";

describe("User", () => {
  it("users initially empty", () => {
    assertEquals(User.users.size, 0);
  });

  it("constructor", () => {
    try {
      const user = new User("Kyle");
      assertEquals(user.name, "Kyle");
      assertStrictEquals(User.users.get("Kyle"), user);
    } finally {
      User.users.clear();
    }
  });

  const ageTests = describe({
    name: "age",
    beforeEach(this: { user: User }) {
      this.user = new User("Kyle");
    },
    afterEach() {
      User.users.clear();
    },
  });

  it(ageTests, "getAge", function () {
    const { user } = this;
    assertThrows(() => user.getAge(), Error, "Age unknown");
    user.age = 18;
    assertEquals(user.getAge(), 18);
  });

  it(ageTests, "setAge", function () {
    const { user } = this;
    user.setAge(18);
    assertEquals(user.getAge(), 18);
  });
});

Mocking

Test spies are function stand-ins that are used to assert if a function’s internal behavior matches expectations. Test spies on methods keep the original behavior but allow you to test how the method is called and what it returns. Test stubs are an extension of test spies that also replaces the original methods behavior.

Spying

Say we have two functions, square and multiply, if we want to assert that the multiply function is called during execution of the square function we need a way to spy on the multiply function. There are a few ways to achieve this with Spies, one is to have the square function take the multiply multiply as a parameter.

// https://deno.land/std@0.166.0/testing/mock_examples/parameter_injection.ts
export function multiply(a: number, b: number): number {
  return a * b;
}

export function square(
  multiplyFn: (a: number, b: number) => number,
  value: number,
): number {
  return multiplyFn(value, value);
}

This way, we can call square(multiply, value) in the application code or wrap a spy function around the multiply function and call square(multiplySpy, value) in the testing code.

// https://deno.land/std@0.166.0/testing/mock_examples/parameter_injection_test.ts
import {
  assertSpyCall,
  assertSpyCalls,
  spy,
} from "https://deno.land/std@0.166.0/testing/mock.ts";
import { assertEquals } from "https://deno.land/std@0.166.0/testing/asserts.ts";
import {
  multiply,
  square,
} from "https://deno.land/std@0.166.0/testing/mock_examples/parameter_injection.ts";

Deno.test("square calls multiply and returns results", () => {
  const multiplySpy = spy(multiply);

  assertEquals(square(multiplySpy, 5), 25);

  // asserts that multiplySpy was called at least once and details about the first call.
  assertSpyCall(multiplySpy, 0, {
    args: [5, 5],
    returned: 25,
  });

  // asserts that multiplySpy was only called once.
  assertSpyCalls(multiplySpy, 1);
});

If you prefer not adding additional parameters for testing purposes only, you can use spy to wrap a method on an object instead. In the following example, the exported _internals object has the multiply function we want to call as a method and the square function calls _internals.multiply instead of multiply.

// https://deno.land/std@0.166.0/testing/mock_examples/internals_injection.ts
export function multiply(a: number, b: number): number {
  return a * b;
}

export function square(value: number): number {
  return _internals.multiply(value, value);
}

export const _internals = { multiply };

This way, we can call square(value) in both the application code and testing code. Then spy on the multiply method on the _internals object in the testing code to be able to spy on how the square function calls the multiply function.

// https://deno.land/std@0.166.0/testing/mock_examples/internals_injection_test.ts
import {
  assertSpyCall,
  assertSpyCalls,
  spy,
} from "https://deno.land/std@0.166.0/testing/mock.ts";
import { assertEquals } from "https://deno.land/std@0.166.0/testing/asserts.ts";
import {
  _internals,
  square,
} from "https://deno.land/std@0.166.0/testing/mock_examples/internals_injection.ts";

Deno.test("square calls multiply and returns results", () => {
  const multiplySpy = spy(_internals, "multiply");

  try {
    assertEquals(square(5), 25);
  } finally {
    // unwraps the multiply method on the _internals object
    multiplySpy.restore();
  }

  // asserts that multiplySpy was called at least once and details about the first call.
  assertSpyCall(multiplySpy, 0, {
    args: [5, 5],
    returned: 25,
  });

  // asserts that multiplySpy was only called once.
  assertSpyCalls(multiplySpy, 1);
});

One difference you may have noticed between these two examples is that in the second we call the restore method on multiplySpy function. That is needed to remove the spy wrapper from the _internals object’s multiply method. The restore method is called in a finally block to ensure that it is restored whether or not the assertion in the try block is successful. The restore method didn’t need to be called in the first example because the multiply function was not modified in any way like the _internals object was in the second example.

Stubbing

Say we have two functions, randomMultiple and randomInt, if we want to assert that randomInt is called during execution of randomMultiple we need a way to spy on the randomInt function. That could be done with either of the spying techniques previously mentioned. To be able to verify that the randomMultiple function returns the value we expect it to for what randomInt returns, the easiest way would be to replace the randomInt function’s behavior with more predictable behavior.

You could use the first spying technique to do that but that would require adding a randomInt parameter to the randomMultiple function.

You could also use the second spying technique to do that, but your assertions would not be as predictable due to the randomInt function returning random values.

Say we want to verify it returns correct values for both negative and positive random integers. We could easily do that with stubbing. The below example is similar to the second spying technique example but instead of passing the call through to the original randomInt function, we are going to replace randomInt with a function that returns pre-defined values.

// https://deno.land/std@0.166.0/testing/mock_examples/random.ts
export function randomInt(lowerBound: number, upperBound: number): number {
  return lowerBound + Math.floor(Math.random() * (upperBound - lowerBound));
}

export function randomMultiple(value: number): number {
  return value * _internals.randomInt(-10, 10);
}

export const _internals = { randomInt };

The mock module includes some helper functions to make creating common stubs easy. The returnsNext function takes an array of values we want it to return on consecutive calls.

// https://deno.land/std@0.166.0/testing/mock_examples/random_test.ts
import {
  assertSpyCall,
  assertSpyCalls,
  returnsNext,
  stub,
} from "https://deno.land/std@0.166.0/testing/mock.ts";
import { assertEquals } from "https://deno.land/std@0.166.0/testing/asserts.ts";
import {
  _internals,
  randomMultiple,
} from "https://deno.land/std@0.166.0/testing/mock_examples/random.ts";

Deno.test("randomMultiple uses randomInt to generate random multiples between -10 and 10 times the value", () => {
  const randomIntStub = stub(_internals, "randomInt", returnsNext([-3, 3]));

  try {
    assertEquals(randomMultiple(5), -15);
    assertEquals(randomMultiple(5), 15);
  } finally {
    // unwraps the randomInt method on the _internals object
    randomIntStub.restore();
  }

  // asserts that randomIntStub was called at least once and details about the first call.
  assertSpyCall(randomIntStub, 0, {
    args: [-10, 10],
    returned: -3,
  });
  // asserts that randomIntStub was called at least twice and details about the second call.
  assertSpyCall(randomIntStub, 1, {
    args: [-10, 10],
    returned: 3,
  });

  // asserts that randomIntStub was only called twice.
  assertSpyCalls(randomIntStub, 2);
});

Faking time

Say we have a function that has time based behavior that we would like to test. With real time, that could cause tests to take much longer than they should. If you fake time, you could simulate how your function would behave over time starting from any point in time. Below is an example where we want to test that the callback is called every second.

// https://deno.land/std@0.166.0/testing/mock_examples/interval.ts
export function secondInterval(cb: () => void): number {
  return setInterval(cb, 1000);
}

With FakeTime we can do that. When the FakeTime instance is created, it splits from real time. The Date, setTimeout, clearTimeout, setInterval and clearInterval globals are replaced with versions that use the fake time until real time is restored. You can control how time ticks forward with the tick method on the FakeTime instance.

// https://deno.land/std@0.166.0/testing/mock_examples/interval_test.ts
import {
  assertSpyCalls,
  spy,
} from "https://deno.land/std@0.166.0/testing/mock.ts";
import { FakeTime } from "https://deno.land/std@0.166.0/testing/time.ts";
import { secondInterval } from "https://deno.land/std@0.166.0/testing/mock_examples/interval.ts";

Deno.test("secondInterval calls callback every second and stops after being cleared", () => {
  const time = new FakeTime();

  try {
    const cb = spy();
    const intervalId = secondInterval(cb);
    assertSpyCalls(cb, 0);
    time.tick(500);
    assertSpyCalls(cb, 0);
    time.tick(500);
    assertSpyCalls(cb, 1);
    time.tick(3500);
    assertSpyCalls(cb, 4);

    clearInterval(intervalId);
    time.tick(1000);
    assertSpyCalls(cb, 4);
  } finally {
    time.restore();
  }
});