Creating a Counter App with useState

Before you begin #

If you haven’t learned React basics and React Hooks basics, please visit the official site before continuing. This course assumes a basic understanding of React and React Hooks.

What we create #

We are creating a so-called counter app. There are two numbers that we can increment or decrement. We don’t structure code and put everything into one file in this lesson.

The code #

import React, { createContext, useContext, useState } from 'react';

This imports React and some hooks from the library.

const initialState = {
  count1: 0,
  count2: 0,
};

This defines initialState which includes two numbers, count1 and count2

const useValue = () => useState(initialState);

This is a handy custom hook that we can use in a provider below.

const Context = createContext(null);

This creates a context. Its value is initially null, but will be replaced by useValue().

const useGlobalState = () => {
  const value = useContext(Context);
  if (value === null) throw new Error('Please add GlobalStateProvider');
  return value;
};

This is a custom hook to return the context value. It checks whether a provider is used as expected. If the value is null, it is not updated because useValue() never returns null.

const GlobalStateProvider = ({ children }) => (
  <Context.Provider value={useValue()}>{children}</Context.Provider>
);

This is a context provider. This is a component that should be put near the root.

const Counter = ({ name }) => {
  const [state, setState] = useGlobalState();
  const count = state[name] || 0;
  const increment = () => {
    setState({ ...state, [name]: count + 1 });
  };
  const decrement = () => {
    setState({ ...state, [name]: count - 1 });
  };
  return (
    <div>
      {count}
      <button onClick={increment}>+1</button>
      <button onClick={decrement}>-1</button>
    </div>
  );
};

This is a component to display a number. It receives a name prop to specify which number to display, count1 or count2. It has two buttons to increment and decrement the number.

const App = () => (
  <GlobalStateProvider>
    <h1>Count1</h1>
    <Counter name="count1" />
    <Counter name="count1" />
    <h1>Count2</h1>
    <Counter name="count2" />
    <Counter name="count2" />
  </GlobalStateProvider>
);

This is an app component. It has GlobalStateProvider and several Counter components.

export default App;

Finally, we export the app component.

The working example #

Check out the app below.

Exercise #

Try adding a new counter, count3. Interestingly, you don’t need to modify initialState if the initial value of count3 is 0.

Next #

In the next lesson, we will learn a different pattern with useReducer.