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Learn how to handle errors and submit data with the fetch method

The browser's fetch method is deliberately low-level. This means there are certain things you'll almost always need to do to make requests in a real application.

HTTP errors

fetch is only concerned with making HTTP requests. From this perspective as long as it receives a response it was successful, even if that response says something like 500 server error. Most of the time in your application code you want to treat non-200 status codes as errors.

Challenge

1. Open workshop.html in your editor

2. Add a fetch call to "https://echo.oliverjam.workers.dev/status/404" (this always returns a 404)

3. Add a .then() and .catch(). Which of these runs? What does the response look like?

We need to handle HTTP responses we don't want. We can do this by checking the response.ok property. This will be true for successful status codes (like 200) and false for unsuccessful ones (like 404 or 502).

Challenge

1. Edit your .then() to check the response's ok property

2. If the response is not okay throw a new error with the status property of the response

3. Now does your .catch() run?

Submitting data

fetch allows us to make any kind of HTTP request we like. So far we have made GET requests, but those won't allow us to submit data to a server. To do that we'll need to configure some options by passing a second argument to fetch. E.g.

fetch("example.com", {
  method: "POST",
});

This options object can include lots of properties. Here are some useful ones:

• method: to use methods other than GET

• headers: to send extra info about the request. e.g. if we're submitting JSON we should set the "content-type" header to "application/json"

• body: to send information to the server. If we're sending JSON we also need to JSON.stringify the data.

Challenge

1. Edit your fetch to send a POST request to "https://echo.oliverjam.workers.dev/json"

2. Send a JSON body containing an object with whatever properties you like

3. Don't forget the "content-type"!

User input

So far we've only hard-coded our requests. In reality they're usually triggered by a user submitting a form or clicking a button. There are several different ways we can access form data in our JavaScript.

Forms

Forms are the semantically correct element for receiving user input. We should use them even when we're using JS to handle the request (rather than relying on the native browser submission).

We can add a handler for the submit event like this:

const myForm = document.querySelector("form");

myForm.addEventListener("submit", event => {
  event.preventDefault();
  // handle the submission yourself here
}))

event.preventDefault() will stop the browser trying to send the request for you. We want to handle the request with fetch instead.

In order to send our request we have to get hold of the values the user entered. There are a few ways we could do this.

Challenge: querySelector

We can use querySelector to directly access each input element, then get its value. For example document.querySelector("#username").value.

1. Create a form with two inputs and a submit button

2. Add a "submit" event handler to the form (don't forget preventDefault)

3. Use querySelector to get each input's value

4. Use fetch to POST the data as JSON to the same URL as before

5. Log the response you get from the server

Challenge: new FormData()

There is a built-in API that mirrors a form's native behaviour. We can use new FormData(myForm) to create a FormData interface. This is what the form would send if we didn't call preventDefault(), and contains all the input values.

If we want to submit this as JSON we need to turn it into a normal object. You can do this with Object.fromEntries(data). Note: fromEntries() is relatively new and isn't supported in older browsers.

Challenge

1. Edit your previous solution

2. Use new FormData() to get all the input values

3. Turn the FormData into an object to submit

Workshop

We're going to make a Pokémon search page using the PokéAPI.

1. Create a form with a search input and submit button

2. When the form is submitted request the Pokémon the user typed from "https://pokeapi.co/api/v2/pokemon/NAME"

3. If the request succeeds show the Pokémon's name and sprite

4. If the request fails show a relevant error to the user

Stretch goals

If you have extra time try using some of the other data in the response body to show e.g. the pokémon's types or stats. Write some CSS to make it pretty!

Solution preview