Exclusive Tests - Mocha

:::caution Exclusive tests are incompatible with parallel mode. :::

The exclusivity feature allows you to run only the specified suite or test-case by appending .only() to the function. Here's an example of executing only a particular suite:

describe("Array", function () {
  describe.only("#indexOf()", function () {
    // ...
  });
});

Note: All nested suites will still be executed.

Here's an example of executing an individual test case:

describe("Array", function () {
  describe("#indexOf()", function () {
    it.only("should return -1 unless present", function () {
      // ...
    });

    it("should return the index when present", function () {
      // ...
    });
  });
});

Previous to v3.0.0, .only() used string matching to decide which tests to execute; this is no longer the case. In v3.0.0 or newer, .only() can be used multiple times to define a subset of tests to run:

describe("Array", function () {
  describe("#indexOf()", function () {
    it.only("should return -1 unless present", function () {
      // this test will be run
    });

    it.only("should return the index when present", function () {
      // this test will also be run
    });

    it("should return -1 if called with a non-Array context", function () {
      // this test will not be run
    });
  });
});

You may also choose multiple suites:

describe("Array", function () {
  describe.only("#indexOf()", function () {
    it("should return -1 unless present", function () {
      // this test will be run
    });

    it("should return the index when present", function () {
      // this test will also be run
    });
  });

  describe.only("#concat()", function () {
    it("should return a new Array", function () {
      // this test will also be run
    });
  });

  describe("#slice()", function () {
    it("should return a new Array", function () {
      // this test will not be run
    });
  });
});

But tests will have precedence:

describe("Array", function () {
  describe.only("#indexOf()", function () {
    it.only("should return -1 unless present", function () {
      // this test will be run
    });

    it("should return the index when present", function () {
      // this test will not be run
    });
  });
});

:::note Hooks, if present, will still be executed. :::

:::tip Be mindful not to commit usages of .only() to version control, unless you really mean it! To do so one can run mocha with the option --forbid-only in the continuous integration test command (or in a git precommit hook). :::