How to use toHaveLengthAsync method in Jest

This jest config worked for me :

{
"jest": {
    "moduleNameMapper": {
      ".*css$": "<rootDir>/src/stub.css",
    }
}

(Remember to create stub.css)

I think that if you don't want to use relative paths, you have to define additional moduleDirectories, like

// package.json
{
  "jest": {
    "moduleFileExtensions": ["js", "jsx"],
    "moduleDirectories": ["node_modules", "bower_components", "src"],
  }
} 

More about moduleDirectories in jest docs and configuring jest to find files.

The Jest docs state it is an alias of test. So they are exactly the same from a functional point of view. They exist both to enable to make a readable English sentence from your test.

You can use jest-runner-groups to define and run tests in groups. Once it's installed and added to the Jest configuration, you can tag your tests using docblock notation, like here:

/**
 * Foo tests
 *
 * @group group1/subgroup1
 * @group unit/foo
 */

describe( 'Foo class', () => {
    ...
} );

/**
 * Bar tests
 *
 * @group group1/subgroup2
 * @group unit/bar
 */

describe( 'Bar class', () => {
    ...
} );

Update your jest configuration to specify a new runner:

// jest.config.js
module.exports = {
    ...
    runner: "groups"
};

Then to run a specific group, you need to use --group= argument:

// Using the Jest executable
jest --group=mygroup

// Or npm
npm test -- --group=mygroup

You can also use multiple --group arguments to run multiple groups:

// Will execute tests in the unit/bar and unit/foo groups
npm test -- --group=unit/bar --group=unit/foo

// Will execute tests in the unit group (including unit/bar and unit/foo groups)
npm test -- --group=unit

Another possibility is that ts-jest is slow. There was an issue about that, and it was not completely resolved.

There are various workarounds discussed. They consist of setting isolatedModules=true and also --maxWorkers=1. That is, in jest.config.js

'use strict';

module.exports = {
    preset: 'ts-jest',
    testEnvironment: 'node',
    globals: {
        'ts-jest': {
            isolatedModules: true
        }
    },
}

and run

yarn test --maxWorkers=1

Could be worth trying. Alternatively, it is possible to forgo ts-jest and use babel transpilation.

you can do it like this:

I suppose your button is in some component and iam using that component's name as ComponentName

import React from 'react';
import { shallow } from 'enzyme';
import ComponentName from './ComponentName'

describe('Test Button component', () => {
  it('Test click event', () => {

    const component = shallow((<ComponentName />));
    component.find('button').simulate('click');
    //write an expectation here if suppose you are setting state in your deleteData function you can do like this
    component.update();//if you are setting state
    expect(component.state().stateVariableName).toEqual(value you are expecting after setState in deleteData);  
  });
});

Edit: for plain test of function call we can use spyOn:

  it('calls click event', () => {
    const FakeFun = jest.spyOn(ComponentName.prototype, 'deleteData');
    const component = shallow((<ComponentName />));
    component.find('button').simulate('click');
    component.update();
    expect(FakeFun).toHaveBeenCalled();
  });

You can use jest.spyOn(object, methodName, accessType?) method to mock dataService.retrieveData method. And, your serviceFunction function only has one statement, so you can test lambdaService function with it together.

E.g.

index.js:

const dataService = require('./dataService');

exports.lambdaService = async event => {
  let response = await serviceFunction(event.id);
  if (response.code == 200) {
    return response;
  } else {
    return {
      statusCode: response.code,
      body: JSON.stringify({
        message: response.message
      })
    };
  }
};

const serviceFunction = async id => {
  return await dataService.retrieveData(id);
};

dataService.js:

module.exports = {
  retrieveData: async id => {
    return { code: 200, data: 'real data' };
  }
};

index.spec.js:

const { lambdaService } = require('.');
const dataService = require('./dataService');

describe('lambdaService', () => {
  beforeEach(() => {
    jest.restoreAllMocks();
  });
  test('should return data', async () => {
    const mResponse = { code: 200, data: 'mocked data' };
    const mEvent = { id: 1 };
    const retrieveDataSpy = jest.spyOn(dataService, 'retrieveData').mockResolvedValueOnce(mResponse);
    const actualValue = await lambdaService(mEvent);
    expect(actualValue).toEqual(mResponse);
    expect(retrieveDataSpy).toBeCalledWith(mEvent.id);
  });

  test('should return error message', async () => {
    const mResponse = { code: 500, message: 'Internal server error' };
    const mEvent = { id: 1 };
    const retrieveDataSpy = jest.spyOn(dataService, 'retrieveData').mockResolvedValueOnce(mResponse);
    const actualValue = await lambdaService(mEvent);
    expect(actualValue).toEqual({ statusCode: 500, body: JSON.stringify({ message: mResponse.message }) });
    expect(retrieveDataSpy).toBeCalledWith(mEvent.id);
  });

  test('should throw an error', async () => {
    const mEvent = { id: 1 };
    const retrieveDataSpy = jest.spyOn(dataService, 'retrieveData').mockRejectedValueOnce(new Error('network error'));
    await expect(lambdaService(mEvent)).rejects.toThrowError(new Error('network error'));
    expect(retrieveDataSpy).toBeCalledWith(mEvent.id);
  });
});

Unit test result with coverage report:

 PASS  src/stackoverflow/58623194/index.spec.js
  lambdaService
    ✓ should return data (6ms)
    ✓ should return error message (4ms)
    ✓ should throw an error (2ms)

----------------|----------|----------|----------|----------|-------------------|
File            |  % Stmts | % Branch |  % Funcs |  % Lines | Uncovered Line #s |
----------------|----------|----------|----------|----------|-------------------|
All files       |       90 |      100 |    66.67 |       90 |                   |
 dataService.js |       50 |      100 |        0 |       50 |                 3 |
 index.js       |      100 |      100 |      100 |      100 |                   |
----------------|----------|----------|----------|----------|-------------------|
Test Suites: 1 passed, 1 total
Tests:       3 passed, 3 total
Snapshots:   0 total
Time:        4.619s

Source code: https://github.com/mrdulin/jest-codelab/tree/master/src/stackoverflow/58623194

In Jest you have to pass a function into expect(function).toThrow(<blank or type of error>).

Example:

test("Test description", () => {
  const t = () => {
    throw new TypeError();
  };
  expect(t).toThrow(TypeError);
});

Or if you also want to check for error message:

test("Test description", () => {
  const t = () => {
    throw new TypeError("UNKNOWN ERROR");
  };
  expect(t).toThrow(TypeError);
  expect(t).toThrow("UNKNOWN ERROR");
});

If you need to test an existing function whether it throws with a set of arguments, you have to wrap it inside an anonymous function in expect().

Example:

test("Test description", () => {
  expect(() => {http.get(yourUrl, yourCallbackFn)}).toThrow(TypeError);
});

For those who are committed to using nodemon, nodemon -x "npm test" has worked for me.

A little explanation

nodemon --help says:

-x, --exec app ........... execute script with "app", ie. -x "python -v".

In our case npm test is set to run tests by configuring our package.json

For example:

"scripts": {
  "test": "mocha"
},