Build a simple component

Build a simple component in isolation

We’ll build our UI following a Component-Driven Development (CDD) methodology. It’s a process that builds UIs from the “bottom up” starting with components and ending with screens. CDD helps you scale the amount of complexity you’re faced with as you build out the UI.


Task component in three states

Task is the core component in our app. Each task displays slightly differently depending on exactly what state it’s in. We display a checked (or unchecked) checkbox, some information about the task, and a “pin” button, allowing us to move tasks up and down the list. Putting this together, we’ll need these props:

  • title – a string describing the task
  • state - which list is the task currently in and is it checked off?

As we start to build Task, we first write our test states that correspond to the different types of tasks sketched above. Then we use Storybook to build the component in isolation using mocked data. We’ll manually test the component’s appearance given each state as we go.

Get setup

First, let’s create the task component and its accompanying story file: src/components/Task.js and src/components/Task.stories.js.

We’ll begin with a basic implementation of the Task, simply taking in the attributes we know we’ll need and the two actions you can take on a task (to move it between lists):

// src/components/Task.js

import React from 'react';

export default function Task({ task: { id, title, state }, onArchiveTask, onPinTask }) {
  return (
    <div className="list-item">
      <input type="text" value={title} readOnly={true} />

Above, we render straightforward markup for Task based on the existing HTML structure of the Todos app.

Below we build out Task’s three test states in the story file:

// src/components/Task.stories.js

import React from 'react';
import { action } from '@storybook/addon-actions';

import Task from './Task';

export default {
  component: Task,
  title: 'Task',
  // Our exports that end in "Data" are not stories.
  excludeStories: /.*Data$/,

export const taskData = {
  id: '1',
  title: 'Test Task',
  state: 'TASK_INBOX',
  updatedAt: new Date(2018, 0, 1, 9, 0),

export const actionsData = {
  onPinTask: action('onPinTask'),
  onArchiveTask: action('onArchiveTask'),

export const Default = () => <Task task={{ ...taskData }} {...actionsData} />;

export const Pinned = () => <Task task={{ ...taskData, state: 'TASK_PINNED' }} {...actionsData} />;

export const Archived = () => (
  <Task task={{ ...taskData, state: 'TASK_ARCHIVED' }} {...actionsData} />

There are two basic levels of organization in Storybook: the component and its child stories. Think of each story as a permutation of a component. You can have as many stories per component as you need.

  • Component

    • Story
    • Story
    • Story

To tell Storybook about the component we are documenting, we create a default export that contains:

  • component -- the component itself,
  • title -- how to refer to the component in the sidebar of the Storybook app,
  • excludeStories -- exports in the story file that should not be rendered as stories by Storybook.

To define our stories, we export a function for each of our test states to generate a story. The story is a function that returns a rendered element (i.e. a component with a set of props) in a given state---exactly like a Stateless Functional Component.

action() allows us to create a callback that appears in the actions panel of the Storybook UI when clicked. So when we build a pin button, we’ll be able to determine in the test UI if a button click is successful.

As we need to pass the same set of actions to all permutations of our component, it is convenient to bundle them up into a single actionsData variable and use React's {...actionsData} props expansion to pass them all at once. <Task {...actionsData}> is equivalent to <Task onPinTask={actionsData.onPinTask} onArchiveTask={actionsData.onArchiveTask}>.

Another nice thing about bundling the actions into actionsData is that you can export that variable and use the actions in stories for components that reuse this component, as we'll see later.

When creating a story we use a base task (taskData) to build out the shape of the task the component expects. This is typically modelled from what the true data looks like. Again, export-ing this shape will enable us to reuse it in later stories, as we'll see.

Actions help you verify interactions when building UI components in isolation. Oftentimes you won't have access to the functions and state you have in context of the app. Use action() to stub them in.


We'll need to make a couple of changes to the Storybook configuration so it notices not only our recently created stories, but also allows us to use the CSS file that was changed in the previous chapter.

Start by changing your Storybook configuration file (.storybook/main.js) to the following:

// .storybook/main.js

module.exports = {
  stories: ['../src/components/**/*.stories.js'],
  addons: [

After completing the change above, inside the .storybook folder, add a new file called preview.js with the following:

// .storybook/preview.js

import '../src/index.css';

Once we’ve done this, restarting the Storybook server should yield test cases for the three Task states:

Build out the states

Now we have Storybook setup, styles imported, and test cases built out, we can quickly start the work of implementing the HTML of the component to match the design.

The component is still basic at the moment. First write the code that achieves the design without going into too much detail:

// src/components/Task.js

import React from 'react';

export default function Task({ task: { id, title, state }, onArchiveTask, onPinTask }) {
  return (
    <div className={`list-item ${state}`}>
      <label className="checkbox">
          defaultChecked={state === 'TASK_ARCHIVED'}
        <span className="checkbox-custom" onClick={() => onArchiveTask(id)} />
      <div className="title">
        <input type="text" value={title} readOnly={true} placeholder="Input title" />

      <div className="actions" onClick={event => event.stopPropagation()}>
        {state !== 'TASK_ARCHIVED' && (
          // eslint-disable-next-line jsx-a11y/anchor-is-valid
          <a onClick={() => onPinTask(id)}>
            <span className={`icon-star`} />

The additional markup from above combined with the CSS we imported earlier yields the following UI:

Specify data requirements

It’s best practice to use propTypes in React to specify the shape of data that a component expects. Not only is it self documenting, it also helps catch problems early.

// src/components/Task.js

import React from 'react';
import PropTypes from 'prop-types';

export default function Task({ task: { id, title, state }, onArchiveTask, onPinTask }) {
  // ...

Task.propTypes = {
  task: PropTypes.shape({
    id: PropTypes.string.isRequired,
    title: PropTypes.string.isRequired,
    state: PropTypes.string.isRequired,
  onArchiveTask: PropTypes.func,
  onPinTask: PropTypes.func,

Now a warning in development will appear if the Task component is misused.

An alternative way to achieve the same purpose is to use a JavaScript type system like TypeScript to create a type for the component properties.

Component built!

We’ve now successfully built out a component without needing a server or running the entire frontend application. The next step is to build out the remaining Taskbox components one by one in a similar fashion.

As you can see, getting started building components in isolation is easy and fast. We can expect to produce a higher-quality UI with fewer bugs and more polish because it’s possible to dig in and test every possible state.

Automated Testing

Storybook gave us a great way to manually test our application UI during construction. The ‘stories’ will help ensure we don’t break our Task's appearance as we continue to develop the app. However, it is a completely manual process at this stage, and someone has to go to the effort of clicking through each test state and ensuring it renders well and without errors or warnings. Can’t we do that automatically?

Snapshot testing

Snapshot testing refers to the practice of recording the “known good” output of a component for a given input and then flagging the component whenever the output changes in future. This complements Storybook, because it’s a quick way to view the new version of a component and check out the changes.

Make sure your components render data that doesn't change, so that your snapshot tests won't fail each time. Watch out for things like dates or randomly generated values.

With the Storyshots addon a snapshot test is created for each of the stories. Use it by adding the following development dependencies:

yarn add -D @storybook/addon-storyshots react-test-renderer

Then create an src/storybook.test.js file with the following in it:

// src/storybook.test.js

import initStoryshots from '@storybook/addon-storyshots';

That's it, we can run yarn test and see the following output:

Task test runner

We now have a snapshot test for each of our Task stories. If we change the implementation of Task, we’ll be prompted to verify the changes.

Next Chapter
Composite component
Assemble a composite component out of simpler components