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# Step 2.4: Testing TypeScript code with Jest (Demo)
[Lessons](../) | [Exercise](./exercise/) | [Demo](./demo/)
[Jest](https://jestjs.io/) is a test framework made by Facebook and is very popular in the React and wider JS ecosystems.
In this exercise, we will work on implementing simple unit tests using Jest.
## Jest Features
- Multi-threaded and isolated test runner
- Provides a fake browser-like environment if needed (window, document, DOM, etc) using jsdom
- Snapshots: Jest can create text-based snapshots of rendered components. These snapshots can be checked in and show API or large object changes alongside code changes in pull requests.
- Code coverage is integrated (`--coverage`)
- Very clear error messages showing where a test failure occurred
## How to use Jest
- Using `create-react-app` or other project generators, Jest should already be pre-configured. Running `npm test` usually will trigger it!
- A `jest.config.js` file is used for configuration
- `jsdom` might not have enough API from real browsers, for those cases, polyfills are required. Place these inside `jest.setup.js` and hook up the setup file in `jest.config.js`
- in order to use `enzyme` library to test React Components, more config bits are needed inside `jest.setup.js`
## What does a test look like?
```ts
// describe(), it() and expect() are globally exported, so they don't need to be imported when jest runs these tests
describe('Something to be tested', () => {
it('should describe the behavior', () => {
expect(true).toBe(true);
});
});
```
## Testing React components using Enzyme
[Enzyme](https://airbnb.io/enzyme/) is made by Airbnb and provides utilities to help test React components.
In a real app using ReactDOM, the top-level component will be rendered on the page using `ReactDOM.render()`. Enzyme provides a lighter-weight `mount()` function which is usually adequate for testing purposes.
`mount()` returns a wrapper that can be inspected and provides functionality like `find()`, simulating clicks, etc.
The following code demonstrates how Enzyme can be used to help test React components.
```jsx
import React from 'react';
import { mount } from 'enzyme';
import { TestMe } from './TestMe';
describe('TestMe Component', () => {
it('should have a non-clickable component when the original InnerMe is clicked', () => {
const wrapper = mount(<TestMe name="world" />);
wrapper.find('#innerMe').simulate('click');
expect(wrapper.find('#innerMe').text()).toBe('Clicked');
});
});
describe('Foo Component Tests', () => {
it('allows us to set props', () => {
const wrapper = mount(<Foo bar="baz" />);
expect(wrapper.props().bar).toBe('baz');
wrapper.setProps({ bar: 'foo' });
expect(wrapper.props().bar).toBe('foo');
wrapper.find('button').simulate('click');
});
});
```
## Advanced topics
### Mocking
Mocking functions is a large part of what makes Jest a powerful testing library. Jest actually intercepts the module loading process in Node.js, allowing it to mock entire modules if needed.
There are many ways to mock, as you'd imagine in a language as flexible as JS. We only look at the simplest case, but there's a lot of depth here.
To mock a function:
```ts
it('some test function', () => {
const mockCallback = jest.fn(x => 42 + x);
mockCallback(1);
mockCallback(2);
expect(mockCallback).toHaveBeenCalledTimes(2);
});
```
Read more about jest mocking [here](https://jestjs.io/docs/en/mock-functions.html).
### Async Testing
For testing async scenarios, the test runner needs some way to know when the scenario is finished. Jest tests can handle async scenarios using callbacks, promises, or async/await.
```ts
// Callback
it('tests callback functions', (done) => {
setTimeout(() => {
done();
}, 1000);
});
// Returning a promise
it('tests promise functions', () => {
return someFunctionThatReturnsPromise());
});
// Async/await (recommended)
it('tests async functions', async () => {
expect(await someFunction()).toBe(5);
});
```
# Demo
## Jest basics
In this repo, we can start an inner loop development of tests by running `npm test` from the root of the `frontend-bootcamp` folder.
Take a look at code inside `demo/src`:
1. `index.ts` exports a few functions for a counter as well as a function for squaring numbers. We'll use this last function to demonstrate how mocks work.
2. `multiply.ts` is a contrived example of a function that is exported
3. `index.spec.ts` is the test file
Note how tests are re-run when either test files or source files under `src` are saved.

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<!DOCTYPE html>
<html>
<head>
<link rel="stylesheet" href="../../assets/step.css" />
</head>
<body class="ms-Fabric">
<div id="markdownReadme" data-src="./README.md"></div>
<div id="app">
For this step, we look at unit testing. Run
<pre>npm test</pre>
in the command line.
</div>
<script src="../../assets/scripts.js"></script>
</body>
</html>

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archived/jest/demo/src/TestMe.spec.tsx
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import React from 'react';
import { mount } from 'enzyme';
import { TestMe } from './TestMe';
describe('TestMe Component', () => {
it('should have a non-clickable component when the original InnerMe is clicked', () => {
const wrapper = mount(<TestMe name="world" />);
wrapper.find('#innerMe').simulate('click');
expect(wrapper.find('#innerMe').text()).toBe('Clicked');
});
});

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archived/jest/demo/src/TestMe.tsx
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import React from 'react';
export interface TestMeProps {
name: string;
}
export interface TestMeState {
clicked: boolean;
}
export const TestMe = (props: TestMeProps) => {
return (
<div id="testMe">
<InnerMe name={props.name} />
</div>
);
};
export class InnerMe extends React.Component<TestMeProps, TestMeState> {
state = {
clicked: false
};
onClick = () => {
this.setState({ clicked: true });
};
render() {
return !this.state.clicked ? (
<div onClick={this.onClick} id="innerMe">
Hello {this.props.name}, Click Me
</div>
) : (
<div id="innerMe">Clicked</div>
);
}
}

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archived/jest/demo/src/index.spec.tsx
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import React from 'react';
import { mount } from 'enzyme';
describe('index', () => {
it('placeholder', () => {
});
});

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archived/jest/demo/src/index.ts
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import { multiply } from './multiply';
let counter = 0;
export function getCount() {
return counter;
}
export function increment() {
return ++counter;
}
export function decrement() {
return --counter;
}
export function square(x: number) {
return multiply(x, x);
}

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archived/jest/demo/src/multiply.ts
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export function multiply(x: number, y: number) {
return x * y;
}

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# Step 2.4: Testing TypeScript code with Jest (Exercise)
[Lessons](../) | [Exercise](./exercise/) | [Demo](./demo/)
Start the test runner by running `npm test` in the root of the `frontend-bootcamp` folder.
## Basic testing
1. Look at `exercise/src/stack.ts` for a sample implementation of a stack
2. Follow the instructions inside `stack.spec.ts` file to complete the two tests
## Enzyme Testing
1. Open up `exercise/src/TestMe.spec.tsx`
2. Fill in the test using Enzyme concepts introduced in the demo

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<!DOCTYPE html>
<html>
<head>
<link rel="stylesheet" href="../../assets/step.css" />
</head>
<body class="ms-Fabric">
<div id="markdownReadme" class="exercise" data-src="./README.md"></div>
<div id="app">
For this step, we look at unit testing. Run
<pre>npm test</pre>
in the command line.
</div>
<script src="../../assets/scripts.js"></script>
</body>
</html>

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import React from 'react';
import { mount } from 'enzyme';
import { TestMe } from './TestMe';
describe('TestMe Component', () => {
it('should render correctly when hovered', () => {
// TODO:
// 1. mount a <TestMe> Component here
// 2. use enzyme wrapper's find() method to retrieve the #innerMe element
// 3. simulate a hover with "mouseover" event via the simulate() API
// 4. make assertions with expect on the text() of the #innerMe element
});
});

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archived/jest/exercise/src/TestMe.tsx
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import React from 'react';
export interface TestMeProps {
name: string;
}
export interface TestMeState {
enabled: boolean;
}
export const TestMe = (props: TestMeProps) => {
return (
<div id="testMe">
<InnerMe name={props.name} />
</div>
);
};
export class InnerMe extends React.Component<TestMeProps, TestMeState> {
state = {
enabled: false
};
onMouseOver = () => {
this.setState({ enabled: true });
};
render() {
return !this.state.enabled ? (
<div onMouseOver={this.onMouseOver} id="innerMe">
Hello {this.props.name}, Hover Over Me
</div>
) : (
<div id="innerMe">Enabled</div>
);
}
}

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export { Stack } from './stack';
export { TestMe } from './TestMe';

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archived/jest/exercise/src/stack.spec.ts
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// TODO: Import the stack here
describe('Stack', () => {
it('should push item to the top of the stack', () => {
// TODO: implement test here:
// 1. Instantiate a new Stack - i.e. const stack = new Stack<number>();
// 2. Use stack push calls to add some items to the stack
// 3. Write assertions via the expect() API
});
it('should pop the item from the top of stack', () => {
// TODO: implement test here:
// 1. Instantiate a new Stack - i.e. const stack = new Stack<number>();
// 2. Use stack push calls to add some items to the stack
// 3. pop a few items off the stack
// 4. write assertions via the expect() API
});
});

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export class Stack<T> {
private _items: T[] = [];
/** Add an item to the top of the stack. */
push(item: T) {
this._items.push(item);
}
/** Remove the top item from the stack and return it. */
pop(): T {
if (this._items.length > 0) {
return this._items.pop();
}
}
/** Return the top item from the stack without removing it. */
peek(): T {
if (this._items.length > 0) {
return this._items[this._items.length - 1];
}
}
/** Get the number of items in the stack/ */
get count(): number {
return this._items.length;
}
}

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1. Open `exercise/src/components/TodoHeader.tsx` 1. Open `exercise/src/components/TodoHeader.tsx`
2. Replace the couple of TODO 2. Replace the `onAdd` with a real implementation using the `this.context` object
3. Replace the `onFilter` with a real implementation using the `this.context` object
4. Be sure to set the `contextType` of the TodoHeader component

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# Step 2.6 - Redux: Dispatching actions and examining state (Exercise) # Step 2.6: Redux: React Binding (Exercise)
[Lessons](../) | [Exercise](./exercise/) | [Demo](./demo/) [Lessons](../) | [Exercise](./exercise/) | [Demo](./demo/)
## Visualize state changes with Chrome extension ## Bind Redux Store with Class Component
If you still have `npm test` running from the previous step, stop it with `ctrl+C`. Start the app by running `npm start` from the root of the `frontend-bootcamp` folder. Click the "exercise" link under day 2 step 6. 1. Open `exercise/src/components/TodoHeader.tsx`.
1. Install the [Redux DevTools extension](https://github.com/zalmoxisus/redux-devtools-extension) 2. Just like the 2.4 exercise, implement `onAdd` and `onFilter` using `this.context.dispatch()` calls to dispatch actions.
- [Chrome](https://chrome.google.com/webstore/detail/redux-devtools/lmhkpmbekcpmknklioeibfkpmmfibljd)
- [Firefox](https://addons.mozilla.org/en-US/firefox/addon/reduxdevtools/)
2. Hit F12 (`cmd+option+I` on Mac) and open the inspector panel entitled **Redux** ## Bind Redux Store with Functional Component
3. Modify `exercise/src/index.tsx` to dispatch actions (you're not limited to adding todos; you can also remove and clear) 1. Open `exercise/src/components/TodoFooter.tsx`.
4. Explore the actions' effects using the extension 2. Follow the instructions in the file to replace the `todos` const using the `useMappedState()` hook.
## Playing with dispatching actions inside tests 3. Retrieve the dispatch function with `useDispatch()` hook.
Stop the app using `ctrl+C` and start the tests by running `npm test`.
1. Open `exercise/src/reducers/reducer.spec.ts`
2. Follow the instructions to fill out the reducer tests

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import uuid from 'uuid/v4'; import uuid from 'uuid/v4';
export const actions = { export const actions = {
addTodo: (label: string) => ({ type: 'addTodo', id: uuid(), label }) addTodo: (label: string) => ({ type: 'addTodo', id: uuid(), label }),
remove: (id: string) => ({ type: 'remove', id }),
complete: (id: string) => ({ type: 'complete', id }),
clear: () => ({ type: 'clear' }),
setFilter: (filter: string) => ({ type: 'setFilter', filter }),
edit: (id: string, label: string) => ({ type: 'edit', id, label })
}; };

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import React from 'react';
import { Stack } from 'office-ui-fabric-react';
import { TodoFooter } from './TodoFooter';
import { TodoHeader } from './TodoHeader';
import { TodoList } from './TodoList';
export const TodoApp = () => {
return (
<Stack horizontalAlign="center">
<Stack style={{ width: 400 }} gap={25}>
<TodoHeader />
<TodoList />
<TodoFooter />
</Stack>
</Stack>
);
};

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import React from 'react';
import { DefaultButton, Stack, Text } from 'office-ui-fabric-react';
import { actions } from '../actions';
import { useMappedState, useDispatch } from 'redux-react-hook';
export const TodoFooter = () => {
// TODO: make use of useMappedState(state => state) and the useDispatch functions to get
// the Redux store and dispatching actions
// HINT: const { todos } = useMappedState(...);
// HINT: useDispatch() here too.
const todos = {};
const dispatch = (...args: any[]) => {};
const itemCount = Object.keys(todos).filter(id => !todos[id].completed).length;
return (
<Stack horizontal horizontalAlign="space-between">
<Text>
{itemCount} item{itemCount === 1 ? '' : 's'} left
</Text>
<DefaultButton onClick={() => dispatch(actions.clear())}>Clear Completed</DefaultButton>
</Stack>
);
};

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import React from 'react';
import { Stack, Text, Pivot, PivotItem, TextField, PrimaryButton } from 'office-ui-fabric-react';
import { FilterTypes } from '../store';
import { actions } from '../actions';
import { StoreContext } from 'redux-react-hook';
interface TodoHeaderState {
labelInput: string;
}
export class TodoHeader extends React.Component<{}, TodoHeaderState> {
constructor(props: {}) {
super(props);
this.state = { labelInput: undefined };
}
render() {
return (
<Stack gap={10}>
<Stack horizontal horizontalAlign="center">
<Text variant="xxLarge">todos</Text>
</Stack>
<Stack horizontal gap={10}>
<Stack.Item grow>
<TextField
placeholder="What needs to be done?"
value={this.state.labelInput}
onChange={this.onChange}
styles={props => ({
...(props.focused && {
field: {
backgroundColor: '#c7e0f4'
}
})
})}
/>
</Stack.Item>
<PrimaryButton onClick={this.onAdd}>Add</PrimaryButton>
</Stack>
<Pivot onLinkClick={this.onFilter}>
<PivotItem headerText="all" />
<PivotItem headerText="active" />
<PivotItem headerText="completed" />
</Pivot>
</Stack>
);
}
private onAdd = () => {
// TODO: Fill in a dispatch call to add the todo item
// HINT: this.context.dispatch(...);
this.setState({ labelInput: undefined });
};
private onChange = (evt: React.FormEvent<HTMLInputElement>, newValue: string) => {
this.setState({ labelInput: newValue });
};
private onFilter = (item: PivotItem) => {
// TODO: Fill in the dispatch call to set the filter
// HINT: this.context.dispatch(...);
};
}
// TODO: set the context type of this Class to StoreContext

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import React from 'react';
import { Stack } from 'office-ui-fabric-react';
import { TodoListItem } from './TodoListItem';
import { useMappedState } from 'redux-react-hook';
export const TodoList = () => {
const { filter, todos } = useMappedState(state => state);
const filteredTodos = Object.keys(todos).filter(id => {
return filter === 'all' || (filter === 'completed' && todos[id].completed) || (filter === 'active' && !todos[id].completed);
});
return (
<Stack gap={10}>
{filteredTodos.map(id => (
<TodoListItem key={id} id={id} />
))}
</Stack>
);
};

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import React from 'react';
import { Stack, Checkbox, IconButton, TextField, DefaultButton } from 'office-ui-fabric-react';
import { actions } from '../actions';
import { StoreContext } from 'redux-react-hook';
interface TodoListItemProps {
id: string;
}
interface TodoListItemState {
editing: boolean;
editLabel: string;
}
export class TodoListItem extends React.Component<TodoListItemProps, TodoListItemState> {
constructor(props: TodoListItemProps) {
super(props);
this.state = { editing: false, editLabel: undefined };
}
render() {
const { id } = this.props;
const { todos } = this.context.getState();
const dispatch = this.context.dispatch;
const item = todos[id];
return (
<Stack horizontal verticalAlign="center" horizontalAlign="space-between">
{!this.state.editing && (
<>
<Checkbox label={item.label} checked={item.completed} onChange={() => dispatch(actions.complete(id))} />
<div>
<IconButton iconProps={{ iconName: 'Edit' }} onClick={this.onEdit} />
<IconButton iconProps={{ iconName: 'Cancel' }} onClick={() => dispatch(actions.remove(id))} />
</div>
</>
)}
{this.state.editing && (
<Stack.Item grow>
<Stack horizontal gap={10}>
<Stack.Item grow>
<TextField value={this.state.editLabel} onChange={this.onChange} />
</Stack.Item>
<DefaultButton onClick={this.onDoneEdit}>Save</DefaultButton>
</Stack>
</Stack.Item>
)}
</Stack>
);
}
private onEdit = () => {
const { id } = this.props;
const { todos } = this.context.getState();
const { label } = todos[id];
this.setState({
editing: true,
editLabel: this.state.editLabel || label
});
};
private onDoneEdit = () => {
this.context.dispatch(actions.edit(this.props.id, this.state.editLabel));
this.setState({
editing: false,
editLabel: undefined
});
};
private onChange = (evt: React.FormEvent<HTMLInputElement>, newValue: string) => {
this.setState({ editLabel: newValue });
};
}
TodoListItem.contextType = StoreContext;

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import React from 'react';
import ReactDOM from 'react-dom';
import { reducer } from './reducers'; import { reducer } from './reducers';
import { createStore } from 'redux'; import { createStore } from 'redux';
import { TodoApp } from './components/TodoApp';
import { initializeIcons } from '@uifabric/icons';
import { composeWithDevTools } from 'redux-devtools-extension'; import { composeWithDevTools } from 'redux-devtools-extension';
import { actions } from './actions'; import { StoreContext } from 'redux-react-hook';
const store = createStore(reducer, {}, composeWithDevTools()); const store = createStore(reducer, {}, composeWithDevTools());
console.log(store.getState()); initializeIcons();
// TODO: dispatch several actions and see the effects on state inside the Redux devtools ReactDOM.render(
<StoreContext.Provider value={store}>
// store.dispatch(actions.???); <TodoApp />
// store.dispatch(actions.???); </StoreContext.Provider>,
// store.dispatch(actions.???); document.getElementById('app')
);
console.log(store.getState());

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import { Store } from '../store'; import { Store } from '../store';
import { addTodo, remove, complete, clear } from './pureFunctions'; import { combineReducers } from 'redux';
import { createReducer } from 'redux-starter-kit';
function todoReducer(state: Store['todos'] = {}, action: any): Store['todos'] { export const todosReducer = createReducer<Store['todos']>(
switch (action.type) { {},
case 'addTodo': {
return addTodo(state, action.id, action.label); addTodo(state, action) {
state[action.id] = { label: action.label, completed: false };
},
case 'remove': remove(state, action) {
return remove(state, action.id); delete state[action.id];
},
case 'clear': clear(state, action) {
return clear(state); Object.keys(state).forEach(key => {
if (state[key].completed) {
delete state[key];
}
});
},
case 'complete': complete(state, action) {
return complete(state, action.id); state[action.id].completed = !state[action.id].completed;
},
edit(state, action) {
state[action.id].label = action.label;
}
} }
);
return state; export const filterReducer = createReducer<Store['filter']>('all', {
} setFilter(state, action) {
return action.filter;
}
});
export function reducer(state: Store, action: any): Store { export const reducer = combineReducers({
return { todos: todosReducer,
todos: todoReducer(state.todos, action), filter: filterReducer
filter: 'all' });
};
}

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import { addTodo, complete } from './pureFunctions';
import { Store } from '../store';
describe('TodoApp reducers', () => {
it('can add an item', () => {
const state = <Store['todos']>{};
const newState = addTodo(state, '0', 'item1');
const keys = Object.keys(newState);
expect(newState).not.toBe(state);
expect(keys.length).toBe(1);
expect(newState[keys[0]].label).toBe('item1');
expect(newState[keys[0]].completed).toBeFalsy();
});
it('can complete an item', () => {
const state = <Store['todos']>{};
let newState = addTodo(state, '0', 'item1');
const key = Object.keys(newState)[0];
newState = complete(newState, key);
expect(newState[key].completed).toBeTruthy();
});
});

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import { Store, FilterTypes } from '../store';
export function addTodo(state: Store['todos'], id: string, label: string): Store['todos'] {
return { ...state, [id]: { label, completed: false } };
}
export function remove(state: Store['todos'], id: string) {
const newTodos = { ...state };
delete newTodos[id];
return newTodos;
}
export function complete(state: Store['todos'], id: string) {
// Clone the todo, overriding
const newTodo = { ...state[id], completed: !state[id].completed };
return { ...state, [id]: newTodo };
}
export function clear(state: Store['todos']) {
const newTodos = { ...state };
Object.keys(state).forEach(key => {
if (state[key].completed) {
delete newTodos[key];
}
});
return newTodos;
}
export function setFilter(state: Store['filter'], filter: FilterTypes) {
return filter;
}

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import { createStore } from 'redux';
import { reducer } from '.';
import { actions } from '../actions';
describe('reducers', () => {
it('should add items', () => {
// 1. Use Redux's createStore() to create a store. Pass in a reducer along with the initial state.
//
// 2. Call store.dispatch() with some action messages to indicate the kind of
// action to perform (in this case, addTodo)
//
// 3. Assert with expect() on the resultant store.getState().todos
});
// Tests left for you to write:
// - remove
// - clear
// - complete
});