How to display an alert using client-side JavaScript after a post has happened on the server side?

You would need to add a .then() method call to your handleFormSubmit method, so:

fetch('http://127.0.0.1:3000/comment').then(() => {
    alert('comment added');
})

fetch returns a Promise, and is an async method call, so in your case, alert is triggered after the fetch call is made but not necessarily completed. The .then() method is triggered once the Promise is resolved, so in order to run code once this happens, you store it in the .then() method.

Now in your specific use case, you want to handle successful calls and errored calls. But there is one other consideration - are you only using failed calls to determine whether a call is successful, or can the server's response signify an error? By default, fetch doesn't convert the response to json (assuming your data is being returned from the server as json. . For this, you would need to do the following (assuming the server returns a success/fail response as well:

fetch('http://127.0.0.1:3000/comment').then((res) => {
    return res.json();
}).then((res) => {
    if (res === 'success') {
        alert('comment added');
    } else {
        alert('An error has occurred when trying to add your comment');
    }
}).catch((e) => {
    alert('An error has occurred when trying to add your comment');

})

A few other points that I had missed originally, but from the conversation in the comment thread:

Since your API is exposed by the server as a POST request, you need to add additional options to your fetch request, changing the method to POST, as well as adding the data that you want to send to the server. To do this, you can add the following:

let options = {
    method: 'POST',
    headers: {
        "Content-Type": "application/json"
    },
    // where data is your form fields
    body: JSON.stringify(data)
}

// Replace '...' with code from above or whatever functionality you 
// need once Promise resolves
fetch('http://127.0.0.1:3000/comment', options).then(...)

For more on fetch, check out https://developer.mozilla.org/en-US/docs/Web/API/Fetch_API/Using_Fetch

To get form data values, if there aren't too many, you can try this for each of the fields: let val = document.getElementById('someId').value

Lastly, the way your form is currently set up, once you hit submit, it will refresh by default. If you want to prevent that from happening, before the fetch call, add event.preventDefault(), where event is the EventListener that was triggered (your current method signature should be sufficient for this).