Jest

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Globals

In your test files, Jest puts each of these methods and objects into the global environment. You don't have to require or import anything to use them.

Methods #

Reference #

afterAll(fn) #

Runs a function after all the tests in this file have completed. If the function returns a promise, Jest waits for that promise to resolve before continuing.

This is often useful if you want to clean up some global setup state that is shared across tests.

For example:

const globalDatabase = makeGlobalDatabase();

function cleanUpDatabase(db) {
  db.cleanUp();
}

afterAll(() => {
  cleanUpDatabase(globalDatabase);
});

test('can find things', () => {
  return globalDatabase.find('thing', {}, results => {
    expect(results.length).toBeGreaterThan(0);
  });
});

test('can insert a thing', () => {
  return globalDatabase.insert('thing', makeThing(), response => {
    expect(response.success).toBeTruthy();
  });
});

Here the afterAll ensures that cleanUpDatabase is called after all tests run.

If afterAll is inside a describe block, it runs at the end of the describe block.

If you want to run some cleanup after every test instead of after all tests, use afterEach instead.

afterEach(fn) #

Runs a function after each one of the tests in this file completes. If the function returns a promise, Jest waits for that promise to resolve before continuing.

This is often useful if you want to clean up some temporary state that is created by each test.

For example:

const globalDatabase = makeGlobalDatabase();

function cleanUpDatabase(db) {
  db.cleanUp();
}

afterEach(() => {
  cleanUpDatabase(globalDatabase);
});

test('can find things', () => {
  return globalDatabase.find('thing', {}, results => {
    expect(results.length).toBeGreaterThan(0);
  });
});

test('can insert a thing', () => {
  return globalDatabase.insert('thing', makeThing(), response => {
    expect(response.success).toBeTruthy();
  });
});

Here the afterEach ensures that cleanUpDatabase is called after each test runs.

If afterEach is inside a describe block, it only runs after the tests that are inside this describe block.

If you want to run some cleanup just once, after all of the tests run, use afterAll instead.

beforeAll(fn) #

Runs a function before any of the tests in this file run. If the function returns a promise, Jest waits for that promise to resolve before running tests.

This is often useful if you want to set up some global state that will be used by many tests.

For example:

const globalDatabase = makeGlobalDatabase();

beforeAll(() => {
  // Clears the database and adds some testing data.
  // Jest will wait for this promise to resolve before running tests.
  return globalDatabase.clear().then(() => {
    return globalDatabase.insert({testData: 'foo'});
  });
});

// Since we only set up the database once in this example, it's important
// that our tests don't modify it.
test('can find things', () => {
  return globalDatabase.find('thing', {}, results => {
    expect(results.length).toBeGreaterThan(0);
  });
});

Here the beforeAll ensures that the database is set up before tests run. If setup was synchronous, you could just do this without beforeAll. The key is that Jest will wait for a promise to resolve, so you can have asynchronous setup as well.

If beforeAll is inside a describe block, it runs at the beginning of the describe block.

If you want to run something before every test instead of before any test runs, use beforeEach instead.

beforeEach(fn) #

Runs a function before each of the tests in this file runs. If the function returns a promise, Jest waits for that promise to resolve before running the test.

This is often useful if you want to reset some global state that will be used by many tests.

For example:

const globalDatabase = makeGlobalDatabase();

beforeEach(() => {
  // Clears the database and adds some testing data.
  // Jest will wait for this promise to resolve before running tests.
  return globalDatabase.clear().then(() => {
    return globalDatabase.insert({testData: 'foo'});
  });
});

test('can find things', () => {
  return globalDatabase.find('thing', {}, results => {
    expect(results.length).toBeGreaterThan(0);
  });
});

test('can insert a thing', () => {
  return globalDatabase.insert('thing', makeThing(), response => {
    expect(response.success).toBeTruthy();
  });
});

Here the beforeEach ensures that the database is reset for each test.

If beforeEach is inside a describe block, it runs for each test in the describe block.

If you only need to run some setup code once, before any tests run, use beforeAll instead.

describe(name, fn) #

describe(name, fn) creates a block that groups together several related tests in one "test suite". For example, if you have a myBeverage object that is supposed to be delicious but not sour, you could test it with:

const myBeverage = {
  delicious: true,
  sour: false,
};

describe('my beverage', () => {
  test('is delicious', () => {
    expect(myBeverage.delicious).toBeTruthy();
  });

  test('is not sour', () => {
    expect(myBeverage.sour).toBeFalsy();
  });
});

This isn't required - you can just write the test blocks directly at the top level. But this can be handy if you prefer your tests to be organized into groups.

You can also nest describe blocks if you have a hierarchy of tests:

const binaryStringToNumber = binString => {
  if (!/^[01]+$/.test(binString)) {
    throw new CustomError('Not a binary number.');
  }

  return parseInt(binString, 2);
};

describe('binaryStringToNumber', () => {
  describe('given an invalid binary string', () => {
    test('composed of non-numbers throws CustomError', () => {
      expect(() => binaryStringToNumber('abc')).toThrowError(CustomError);
    });

    test('with extra whitespace throws CustomError', () => {
      expect(() => binaryStringToNumber('  100')).toThrowError(CustomError);
    });
  });

  describe('given a valid binary string', () => {
    test('returns the correct number', () => {
      expect(binaryStringToNumber('100')).toBe(4);
    });
  });
});

describe.only(name, fn) #

Also under the alias: fdescribe(name, fn)

You can use describe.only if you want to run only one describe block:

describe.only('my beverage', () => {
  test('is delicious', () => {
    expect(myBeverage.delicious).toBeTruthy();
  });

  test('is not sour', () => {
    expect(myBeverage.sour).toBeFalsy();
  });
});

describe('my other beverage', () => {
  // ... will be skipped
});

describe.skip(name, fn) #

Also under the alias: xdescribe(name, fn)

You can use describe.skip if you do not want to run a particular describe block:

describe('my beverage', () => {
  test('is delicious', () => {
    expect(myBeverage.delicious).toBeTruthy();
  });

  test('is not sour', () => {
    expect(myBeverage.sour).toBeFalsy();
  });
});

describe.skip('my other beverage', () => {
  // ... will be skipped
});

Using describe.skip is often just an easier alternative to temporarily commenting out a chunk of tests.

require.requireActual(moduleName) #

Returns the actual module instead of a mock, bypassing all checks on whether the module should receive a mock implementation or not.

require.requireMock(moduleName) #

Returns a mock module instead of the actual module, bypassing all checks on whether the module should be required normally or not.

test(name, fn) #

Also under the alias: it(name, fn)

All you need in a test file is the test method which runs a test. For example, let's say there's a function inchesOfRain() that should be zero. Your whole test could be:

test('did not rain', () => {
  expect(inchesOfRain()).toBe(0);
});

The first argument is the test name; the second argument is a function that contains the expectations to test.

If a promise is returned from test, Jest will wait for the promise to resolve before letting the test complete.

For example, let's say fetchBeverageList() returns a promise that is supposed to resolve to a list that has lemon in it. You can test this with:

test('has lemon in it', () => {
  return fetchBeverageList().then(list => {
    expect(list).toContain('lemon');
  });
});

Even though the call to test will return right away, the test doesn't complete until the promise resolves as well.

test.only(name, fn) #

Also under the aliases: it.only(name, fn) or fit(name, fn)

When you are debugging a large codebase, you will often only want to run a subset of tests. You can use .only to specify which tests are the only ones you want to run.

For example, let's say you had these tests:

test.only('it is raining', () => {
  expect(inchesOfRain()).toBeGreaterThan(0);
});

test('it is not snowing', () => {
  expect(inchesOfSnow()).toBe(0);
});

Only the "it is raining" test will run, since it is run with test.only.

Usually you wouldn't check code using test.only into source control - you would use it just for debugging, and remove it once you have fixed the broken tests.

test.skip(name, fn) #

Also under the aliases: it.skip(name, fn) or xit(name, fn) or xtest(name, fn)

When you are maintaining a large codebase, you may sometimes find a test that is temporarily broken for some reason. If you want to skip running this test, but you don't want to just delete this code, you can use test.skip to specify some tests to skip.

For example, let's say you had these tests:

test('it is raining', () => {
  expect(inchesOfRain()).toBeGreaterThan(0);
});

test.skip('it is not snowing', () => {
  expect(inchesOfSnow()).toBe(0);
});

Only the "it is raining" test will run, since the other test is run with test.skip.

You could simply comment the test out, but it's often a bit nicer to use test.skip because it will maintain indentation and syntax highlighting.

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