Introduction to useState

In React when we want to manage individual state there is a function we can import called useState that gives us a getter and setter letting React know when it needs to re-render.

import React, { useState } from 'react';

const myComponent = () => {
  const [count, setCount] = 0;
  ...
}

This initializes count as a state variable that can be accessed with count and set with setCount.

Incrementing a Counter

If we wanted to write a function that incremented count, we could add a button.

...
return (
    <>
      <p>Count is: {count}</p>
      <button onClick={() => setCount(count + 1)}>+1</button>
    </>
);

Here we wrapped our text and button in a React Fragment. Since components can only return one DOM parent element, Fragments allow us to return more than one without creating more divs in our markup.

Then I set the onClick for button to an inline function that increases count by 1 using setCount. If we were to do this without a state variable, i.e. const count = 0 React wouldn’t know when to re-render our data.

Managing List State

This can be used to hold any state or object however, not just numbers and strings. If we wanted to instead append to a list we might write a function that gets passed to the onSubmit of a form.

...
[list, setList] = useState([]);
...

const onFormSubmit(event) {
  event.preventDefault();
  list.push(form.name);
  setList(list);
}

Handling Form Input

Changing all our input onChange methods as well to update the state of form when the data changes.

...
<label>
  <p className={"text-label"}>Name:</p>
  <input
    onChange={handleSetForm}
    type="text"
    name="name"
  />
</label>
...

...
[form, setForm] = useState({ name: '' });
...

const handleSetForm = ({ target: { name, value } }: any) => {
  setForm((form) => ({ ...form, [name]: value }));
};

This updates our form state whenever the data changes. Allowing us to use form to append to our list above whenever the submit button is clicked.