Jest actually uses Jasmine, so you can use fail just like before.

Sample call:

fail('it should not reach here');

Here's the definition from the TypeScript declaration file for Jest:

declare function fail(error?: any): never;

If you know a particular call should fail you can use expect.

expect(() => functionExpectedToThrow(param1)).toThrow();
// or to test a specific error use
expect(() => functionExpectedToThrow(param1)).toThrowError();

See Jest docs for details on passing in a string, regex, or an Error object to test the expected error in the toThrowError method.

For an async call use .rejects

// returning the call
return expect(asyncFunctionExpectedToThrow(param1))
  .rejects();
// or to specify the error message
// .rejects.toEqual('error message');

With async/await you need to mark the test function with async

it('should fail when calling functionX', async () => {
  await expect(asyncFunctionExpectedToThrow(param1))
    .rejects();
  // or to specify the error message
  // .rejects.toEqual('error message');
}

See documentation on .rejects and in the tutorial.

Also please note that the Jasmine fail function may be removed in a future version of Jest, see Yohan Dahmani's comment. You may start using the expect method above or do a find and replace fail with throw new Error('it should not reach here'); as mentioned in other answers. If you prefer the conciseness and readability of fail you could always create your own function if the Jasmine one gets removed from Jest.

function fail(message) {
  throw new Error(message);
}

You can do it by throwing an error. For example:

test('Obi-Wan Kenobi', () => {
  throw new Error('I have failed you, Anakin')
})

Copy/pasta failing test:

it('This test will fail', done => {
  done.fail(new Error('This is the error'))
})