How to display error messages in a React application

API calls may return errors, learn how to deal with them

In React we often have to work with external APIs. We typically grab data from a remote server and we display it in our application.

We saw that one very popular library for making http requests to remote servers is Axios. Axios lets us use methods like get(), post(), and others that call the corresponding http methods that deal with getting, posting, updating and deleting data from an API.

One good place where we want to put Axios calls is inside the componentDidMount() function of our class components.
componentDidMount() gets called by React automatically when the component mounts in our application. If we place the call to Axios in there, it will be called at the appropriate moment and the data retrieved will be available to the component state, ready to be displayed.

Possible API errors

Not all calls to external APIs are successful, though. In fact, it's very possible that a remote server is down or some other blockage prevents the data we are looking for to be accessed.

In these cases, Axios will return an error. It's common practice to notify the user that an error has occurred by triggering some kind of notification like displaying an error message in our web page.

How do we display error messages?

Let's say we want to display an error message at the top of our view when something bad happens. In order to display the message we need to have the message sitting ready in our component state.
Let's add an errorMessage property to our state object with the value of an empty string as the initial state.

state = {
  items: [],
  errorMessage: ''
}

We place our Axios call inside componentDidMount() and when the call is successful, we set the state to the value returned in the API response.

componentDidMount() {
  axios.get('http://localhost:3333/items')
    .then(response => this.setState({items: response.data}))
    .catch(err => { console.log(err) })
}

But when there is an error, the data won't be available inside then(), and the catch() method will be called instead. The error object returned by the API will be passed in there.
At this point, what we need to do is grab the error and update the errorMessage property in our state using setState().

In the code below, I show this operation. In the catch branch I call setState() with an object that updates errorMessage with whatever error is returned by the API.

componentDidMount() {
  axios.get('http://localhost:3333/items')
    .then(response => this.setState({items: response.data}))
    .catch(err => { 
      this.setState({errorMessage: err.message});
    })
}

Now that we have the error in our state all we have to do is display it at the
top of our web page. How do we do that?

Display the error

There are many ways to do it but we like to create a conditional statement to
display the error. The conditional statement basically needs to say:

"if we have an errorMessage on the state, display an h3 element with the errorMessage value. However, if errorMessage is empty, don't display anything."

To translate this if condition into code we could use a plain old if statement, but we can also use a fancy way of doing it.

We use the shortcut operator &&.

The && operator is placed in the middle of a statement.

• It first evaluates the left side of the statement.
• If the left side is true, then the right side of the statement is executed.
• If the left side is not true, && will not do anything with the right side.

In the code below we use the && operator to display the error message only if the errorMessage property on the state is not empty:

{ this.state.errorMessage &&
  <h3 className="error"> { this.state.errorMessage } </h3> }

This is saying: if this.state.errorMessage is true, display the error message.

Remember, we need to enclose this statement in brackets because we are writing Javascript code inside JSX.

In summary

• API calls to external resources can get stuck and return errors instead of the expected data.
• In this case we catch the error and we display it in our application, so the user knows something went wrong.
• We display the error using a conditional statement that shows the error only if it exists.

As you can see, it's very easy to write code that display error messages inside our React application.

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