Testing

To define a test you need to call with a name and function to be tested. There are two styles you can use.

Assertions

There are some useful assertion utilities at to make testing easier:

import {
  assertArrayIncludes,
  assertEquals,
} from "https://deno.land/std@$STD_VERSION/testing/asserts.ts";
Deno.test("hello world", () => {
  const x = 1 + 2;
  assertEquals(x, 3);
});

You can also test asynchronous code by passing a test function that returns a promise. For this you can use the async keyword when defining a function:

import { delay } from "https://deno.land/std@$STD_VERSION/async/delay.ts";
Deno.test("async hello world", async () => {
  const x = 1 + 2;
  // await some async task
  await delay(100);
  if (x !== 3) {
    throw Error("x should be equal to 3");
});

Certain actions in Deno create resources in the resource table (learn more here). These resources should be closed after you are done using them.

For each test definition, the test runner checks that all resources created in this test have been closed. This is to prevent resource ‘leaks’. This is enabled by default for all tests, but can be disabled by setting the sanitizeResources boolean to false in the test definition.

The same is true for async operation like interacting with the filesystem. The test runner checks that each operation you start in the test is completed before the end of the test. This is enabled by default for all tests, but can be disabled by setting the sanitizeOps boolean to false in the test definition.

Deno.test({
  name: "leaky test",
  fn() {
    Deno.open("hello.txt");
  },
  sanitizeResources: false,
  sanitizeOps: false,

To run the test, call deno test with the file that contains your test function. You can also omit the file name, in which case all tests in the current directory (recursively) that match the glob {*_,*.,}test.{js,mjs,ts,jsx,tsx} will be run. If you pass a directory, all files in the directory that match this glob will be run.

To see all runtime options with deno test, you can reference the command line help:

deno help test

Filtering

There are a number of options to filter the tests you are running.

Tests can be run individually or in groups using the command line --filter option.

The filter flags accept a string or a pattern as value.

Assuming the following tests:

Deno.test({ name: "my-test", fn: myTest });
Deno.test({ name: "test-1", fn: test1 });

This command will run all of these tests because they all contain the word “test”.

deno test --filter "test" tests/

On the flip side, the following command uses a pattern and will run the second and third tests.

Within the tests themselves, you have two options for filtering.

Filtering out (Ignoring these tests)

Sometimes you want to ignore tests based on some sort of condition (for example you only want a test to run on Windows). For this you can use the ignore boolean in the test definition. If it is set to true the test will be skipped.

Deno.test({
  name: "do macOS feature",
  ignore: Deno.build.os !== "darwin",
  fn() {
    doMacOSFeature();
  },
});

Filtering in (Only run these tests)

Sometimes you may be in the middle of a problem within a large test class and you would like to focus on just that test and ignore the rest for now. For this you can use the only option to tell the test framework to only run tests with this set to true. Multiple tests can set this option. While the test run will report on the success or failure of each test, the overall test run will always fail if any test is flagged with only, as this is a temporary measure only which disables nearly all of your tests.

Deno.test({
  name: "Focus on this test only",
  only: true,
  fn() {
    testComplicatedStuff();
  },
});

If you have a long running test suite and wish for it to stop on the first failure, you can specify the --failfast flag when running the suite.

deno test --failfast

Test coverage

Deno will automatically determine test coverage for your code if you specify the --coverage flag when starting . Coverage is determined on a line by line basis for modules that share the parent directory with at-least one test module that is being executed.

This coverage information is acquired directly from the JavaScript engine (V8). Because of this, the coverage reports are very accurate.

When all tests are done running a summary of coverage per file is printed to stdout. In the future there will be support for lcov output too.