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Lab: Fetch API

Overview

This lab walks you through using the Fetch API, a simple interface for fetching resources, and an improvement over the XMLHttpRequest API.

What you will learn

  • How to use the Fetch API to request resources
  • How to make GET, HEAD, and POST requests with fetch
  • How to read & set custom headers
  • The usage and limitations of CORS

What you should know

  • Basic JavaScript and HTML
  • Familiarity with the concept and basic syntax of ES2015 Promises

What you will need

  • Computer with terminal/shell access
  • Connection to the internet
  • A browser that supports Fetch
  • A text editor
  • Node and npm

1. Get set up

If you have not downloaded the repository and installed the LTS version of Node.js, follow the instructions in Setting up the labs.

Open your computer's command line. Navigate into the fetch-api-lab/app/ directory and start a local development server:

cd fetch-api-lab/app
npm install
node server.js

You can terminate the server at any time with Ctrl-c.

Open your browser and navigate to localhost:8081/. You should see a page with buttons for making requests (they won't work yet).

Open the fetch-api-lab/app/ folder in your preferred text editor. The app/ folder is where you will be building the lab.

This folder contains:

  • echo-servers/ contains files that are used for running test servers
  • examples/ contains sample resources that we use in experimenting with fetch
  • js/main.js is the main JavaScript for the app, and it is where you will write all your code
  • index.html is the main HTML page for our sample site/application
  • package-lock.json and package.json are configuration files for our development server and echo server dependencies
  • server.js is a node development server

2. Fetching a resource

The Fetch API has a relatively simple interface. This section explains how to write a basic HTTP request using fetch.

2.1 Fetch a JSON file

In js/main.js, the app's Fetch JSON button is attached to the fetchJSON function.

Update the fetchJSON function to request the examples/animals.json file and log the response:

function fetchJSON() {
  fetch('examples/animals.json')
    .then(logResult)
    .catch(logError);
}

Save the script and refresh the page. Click Fetch JSON. The console should log the fetch response.

Explanation

The fetch method accepts the path for the resource we want to retrieve as a parameter, in this case examples/animals.json. fetch returns a promise that resolves to a Response object. If the promise resolves, the response is passed to the logResult function. If the promise rejects, the catch takes over and the error is passed to the logError function.

Response objects represent the response to a request. They contain the response body and also useful properties and methods.

2.2 Test invalid responses

Examine the logged response in the console. Note the values of the status, url, and ok properties.

Replace the examples/animals.json resource in fetchJSON with examples/non-existent.json. The updated fetchJSON function should now look like:

function fetchJSON() {
  fetch('examples/non-existent.json')
    .then(logResult)
    .catch(logError);
}

Save the script and refresh the page. Click Fetch JSON again to try and fetch this non-existent resource.

Observe that the fetch completed successfully, and didn't trigger the catch block. Now find the status, URL, and ok properties of the new response.

The values should be different for the two files (do you understand why?). If you got any console errors, do the values match up with the context of the error?

Explanation

Why didn't a failed response activate the catch block? This is an important note for fetch and promises—bad responses (like 404s) still resolve! A fetch promise only rejects if the request was unable to complete, so you must always check the validity of the response. We will validate responses in the next section.

For more information

2.3 Check response validity

We need to update our code to check the validity of responses.

In main.js, add a function to validate responses:

function validateResponse(response) {
  if (!response.ok) {
    throw Error(response.statusText);
  }
  return response;
}

Then replace fetchJSON with the following code:

function fetchJSON() {
  fetch('examples/non-existent.json')
    .then(validateResponse)
    .then(logResult)
    .catch(logError);
}

Save the script and refresh the page. Click Fetch JSON. Check the console. Now the response for examples/non-existent.json should trigger the catch block.

Replace examples/non-existent.json in the fetchJSON function with the original examples/animals.json. The updated function should now look like:

function fetchJSON() {
  fetch('examples/animals.json')
    .then(validateResponse)
    .then(logResult)
    .catch(logError);
}

Save the script and refresh the page. Click Fetch JSON. You should see that the response is being logged successfully as before.

Explanation

Now that we have added the validateResponse check, bad responses (like 404s) throw an error and the catch takes over. This allows us to handle failed responses and prevents unexpected responses from propagating down the fetch chain.

2.4 Read the response

Fetch responses are represented as ReadableStreams ( streams spec) and must be read in order to access the body of the response. Response objects have methods for doing this.

In main.js, add a readResponseAsJSON function with the following code:

function readResponseAsJSON(response) {
  return response.json();
}

Then replace the fetchJSON function with the following code:

function fetchJSON() {
  fetch('examples/animals.json') // 1
  .then(validateResponse) // 2
  .then(readResponseAsJSON) // 3
  .then(logResult) // 4
  .catch(logError);
}

Save the script and refresh the page. Click Fetch JSON. Check the console to see that the JSON from examples/animals.json is being logged (instead of the Response object).

Explanation

Let's review what is happening.

Step 1. Fetch is called on a resource, examples/animals.json. Fetch returns a promise that resolves to a Response object. When the promise resolves, the response object is passed to validateResponse.

Step 2. validateResponse checks if the response is valid (is it a 200?). If it isn't, an error is thrown, skipping the rest of the then blocks and triggering the catch block. This is particularly important. Without this check bad responses are passed down the chain and could break later code that may rely on receiving a valid response. If the response is valid, it is passed to readResponseAsJSON.

Step 3. readResponseAsJSON reads the body of the response using the Response.json() method. This method returns a promise that resolves to JSON. Once this promise resolves, the JSON data is passed to logResult. (If the promise from response.json() rejects, the catch block is triggered.)

Step 4. Finally, the JSON data from the original request to examples/animals.json is logged by logResult.

For more information

3. Fetch an image

Fetch is not limited to JSON. In this example we will fetch an image and append it to the page.

In main.js, write a showImage function with the following code:

function showImage(responseAsBlob) {
  const container = document.getElementById('img-container');
  const imgElem = document.createElement('img');
  container.appendChild(imgElem);
  const imgUrl = URL.createObjectURL(responseAsBlob);
  imgElem.src = imgUrl;
}

Then add a readResponseAsBlob function that reads responses as a Blob:

function readResponseAsBlob(response) {
  return response.blob();
}

Update the fetchImage function with the following code:

function fetchImage() {
  fetch('examples/fetching.jpg')
    .then(validateResponse)
    .then(readResponseAsBlob)
    .then(showImage)
    .catch(logError);
}

Save the script and refresh the page. Click Fetch image. You should see an adorable dog fetching a stick on the page (it's a fetch joke!).

Explanation

In this example an image is being fetched, examples/fetching.jpg. Just like in the previous exercise, the response is validated with validateResponse. The response is then read as a Blob (instead of JSON as in the previous section). An image element is created and appended to the page, and the image's src attribute is set to a data URL representing the Blob.

For more information

4. Optional: Fetch text

This section is an optional challenge.

Update the fetchText function to

  1. fetch /examples/words.txt
  2. validate the response with validateResponse
  3. read the response as text (hint: see Response.text())
  4. and display the text on the page

You can use this showText function as a helper for displaying the final text:

function showText(responseAsText) {
  const message = document.getElementById('message');
  message.textContent = responseAsText;
}

Save the script and refresh the page. Click Fetch text. If you've implemented fetchText correctly, you should see added text on the page.

For more information

5. Using HEAD requests

By default, fetch uses the GET method, which retrieves a specific resource. But fetch can also use other HTTP methods.

5.1 Make a HEAD request

Replace the headRequest function with the following code:

function headRequest() {
  fetch('examples/words.txt', {
    method: 'HEAD'
  })
  .then(validateResponse)
  .then(readResponseAsText)
  .then(logResult)
  .catch(logError);
}

Save the script and refresh the page. Click HEAD request. Observe that the logged text content is empty.

Explanation

The fetch method can receive a second optional parameter, init. This parameter enables the configuration of the fetch request, such as the request method, cache mode, credentials, and more.

In this example we set the fetch request method to HEAD using the init parameter. HEAD requests are just like GET requests, except the body of the response is empty. This kind of request can be used when all you want is metadata about a file but don't need to transport all of the file's data.

5.2 Optional: Find the size of a resource

Let's look at the Headers of the fetch response for examples/words.txt to determine the size of the file.

Update the headRequest function to log the content-length property of the response headers (hint: see the headers documentation and get method).

After you have updated the code, save the file and refresh the page. Click HEAD request. The console should log the size (in bytes) of examples/words.txt.

Explanation

In this example, the HEAD method is used to request the size (in bytes) of a resource (represented in the content-length header) without actually loading the resource itself. In practice this could be used to determine if the full resource should be requested (or even how to request it).

Optional: Find out the size of examples/words.txt using another method and confirm that it matches the value from the response header (you can look up how to do this for your specific operating system—bonus points for using the command line!).

For more information

6. Using POST requests

Fetch can also send data with POST requests.

6.1 Set up an echo server

For this example you need to run an echo server. From the fetch-api-lab/app/ directory run the following command (if you command line is blocked by the localhost:8081 server, open a new command line window or tab):

node echo-servers/cors-server.js

This command starts up a simple server at localhost:5000/ that echoes back the requests sent to it.

You can terminate this server at any time with ctrl+c.

6.2 Make a POST request

Replace the postRequest function with the following code (make sure you have defined the showText function from section 4 if you didn't complete the section):

function postRequest() {
  fetch('http://localhost:5000/', {
    method: 'POST',
    body: 'name=david&message=hello'
  })
    .then(validateResponse)
    .then(readResponseAsText)
    .then(showText)
    .catch(logError);
}

Save the script and refresh the page. Click POST request. Observe the sent request echoed on the page. It should contain the name and message (note that we are not yet getting data from the form).

Explanation

To make a POST request with fetch, we use the init parameter to specify the method (similar to how we set the HEAD method in the previous section). This is also where we set the body of the request, in this case a simple string. The body is the data we want to send.

When data is sent as a POST request to localhost:5000/, the request is echoed back as the response. The response is then validated with validateResponse, read as text, and displayed on the page.

In practice, this server would represent a 3rd party API.

6.3 Optional: Use the FormData interface

You can use the FormData interface to easily grab data from forms.

In the postRequest function, instantiate a new FormData object from the msg-form form element:

const formData = new FormData(document.getElementById('msg-form'));

Then replace the value of the body parameter with the formData variable.

Save the script and refresh the page. Fill out the form (the Name and Message fields) on the page, and then click POST request. Observe the form content displayed on the page.

Explanation

The FormData constructor can take in an HTML form, and create a FormData object. This object is populated with the form's keys and values.

For more information

7. Optional: CORS and custom headers

7.1 Start a non-cors echo server

Stop the previous echo server (by pressing ctrl+c from the command line) and start a new echo server from the fetch-lab-api/app/ directory by running the following command:

node echo-servers/no-cors-server.js

This command sets up another simple echo server, this time at localhost:5001/. This server, however, is not configured to accept cross origin requests.

7.2 Fetch from the new server

Now that the new server is running at localhost:5001/, we can send a fetch request to it.

Update the postRequest function to fetch from localhost:5001/ instead of localhost:5000/. After you have updated the code, save the file, refresh the page, and then click POST Request.

You should get an error in the console indicating that the cross-origin request is blocked because the CORS Access-Control-Allow-Origin header is missing.

Update the fetch in the postRequest function with the following code, which uses no-cors mode (as the error log suggests), and removes the calls to validateResponse and readResponseAsText (see explanation below):

function postRequest() {
  const formData = new FormData(document.getElementById('msg-form'));
  fetch('http://localhost:5001/', {
    method: 'POST',
    body: formData,
    mode: 'no-cors'
  })
    .then(logResult)
    .catch(logError);
}

Save the script and refresh the page. Then fill out the message form and click POST Request.

Observe the response object logged in the console.

Explanation

Fetch (and XMLHttpRequest) follow the same-origin policy. This means that browsers restrict cross-origin HTTP requests from within scripts. A cross-origin request occurs when one domain (for example http://foo.com/) requests a resource from a separate domain (for example http://bar.com/).

Since our app's server has a different port number than the two echo servers, requests to either of the echo servers are considered cross-origin. The first echo server, however, running on localhost:5000/, is configured to support CORS (you can open echo-servers/cors-server.js and examine the configuration). The new echo server, running on localhost:5001/, is not (which is why we get an error).

Using mode: no-cors allows fetching an opaque response. This allows use to get a response, but prevents accessing the response with JavaScript (which is why we can't use validateResponse, readResponseAsText, or showResponse). The response can still be consumed by other API's or cached by a service worker.

7.3 Modify request headers

Fetch also supports modifying request headers. Stop the localhost:5001 (no CORS) echo server and restart the localhost:5000 (CORS) echo server from section 6:

node echo-servers/cors-server.js

Restore the previous version of the postRequest function that fetches from localhost:5000/:

function postRequest() {
  const formData = new FormData(document.getElementById('msg-form'));
  fetch('http://localhost:5000/', {
    method: 'POST',
    body: formData
  })
    .then(validateResponse)
    .then(readResponseAsText)
    .then(showText)
    .catch(logError);
}

Now use the Header interface to create a Headers object inside the postRequest function called messageHeaders with the Content-Type header equal to application/json.

Then set the headers property of the init object to be the messageHeaders variable.

Update the body property to be a stringified JSON object, such as:

JSON.stringify({ lab: 'fetch', status: 'fun' })

After you have updated the code, save the file and refresh the page. Then click POST Request.

Observe that the echoed request now has a Content-Type of application/json (as opposed to multipart/form-data as it had previously).

Now add a custom Content-Length header to the messageHeaders object and give the request an arbitrary size.

After you have updated the code, save the file, refresh the page, and click POST Request. Observe that this header is not modified in the echoed request.

Explanation

The Header interface enables the creation and modification of Headers objects. Some headers, like Content-Type can be modified by fetch. Others, like Content-Length, are guarded and can't be modified (for security reasons).

7.4 Set custom request headers

Fetch supports setting custom headers.

Remove the Content-Length header from the messageHeaders object in the postRequest function. Add the custom header X-Custom with an arbitrary value (for example 'X-CUSTOM': 'hello world').

Save the script, refresh the page, and then click POST Request.

You should see that the echoed request has the X-Custom property that you added.

Now add a Y-Custom header to the Headers object. Save the script, refresh the page, and click POST Request.

You should get an error similar to this in the console:

Fetch API cannot load http://localhost:5000/. Request header field y-custom is not allowed by Access-Control-Allow-Headers in preflight response.

Explanation

Like cross-origin requests, custom headers must be supported by the server from which the resource is requested. In this example, our echo server is configured to accept the X-Custom header but not the Y-Custom header (you can open echo-servers/cors-server.js and look for Access-Control-Allow-Headers to see for yourself). Anytime a custom header is set, the browser performs a preflight check. This means that the browser first sends an OPTIONS request to the server, to determine what HTTP methods and headers are allowed by the server. If the server is configured to accept the method and headers of the original request, then it is sent, otherwise an error is thrown.

For more information

Solution code

To get a copy of the working code, navigate to the solution folder.

Congratulations!

You now know how to use the Fetch API!

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