Guides.rubyonrails.org

Chapters

1. An Introduction to Ajax
2. Unobtrusive JavaScript
3. Built-in Helpers
   • Remote Elements
   • Customize Remote Elements
   • Confirmations
   • Automatic disabling
   • Rails-ujs event handlers
   • Stoppable events
4. Server-Side Concerns
   • A Simple Example
5. Turbolinks
   • How Turbolinks Works
   • Page Change Events
6. Cross-Site Request Forgery (CSRF) token in Ajax
7. Other Resources

1 An Introduction to Ajax

In order to understand Ajax, you must first understand what a web browser does normally.

When you type http://localhost:3000 into your browser's address bar and hit 'Go', the browser (your 'client') makes a request to the server. It parses the response, then fetches all associated assets, like JavaScript files, stylesheets and images. It then assembles the page. If you click a link, it does the same process: fetch the page, fetch the assets, put it all together, show you the results. This is called the 'request response cycle'.

JavaScript can also make requests to the server, and parse the response. It also has the ability to update information on the page. Combining these two powers, a JavaScript writer can make a web page that can update just parts of itself, without needing to get the full page data from the server. This is a powerful technique that we call Ajax.

As an example, here's some JavaScript code that makes an Ajax request:

fetch("/test")
  .then((data) => data.text())
  .then((html) => {
    const results = document.querySelector("#results");
    results.insertAdjacentHTML("beforeend", data);
  });

This code fetches data from "/test", and then appends the result to the element with an id of results.

Rails provides quite a bit of built-in support for building web pages with this technique. You rarely have to write this code yourself. The rest of this guide will show you how Rails can help you write websites in this way, but it's all built on top of this fairly simple technique.

2 Unobtrusive JavaScript

Rails uses a technique called "Unobtrusive JavaScript" to handle attaching JavaScript to the DOM. This is generally considered to be a best-practice within the frontend community, but you may occasionally read tutorials that demonstrate other ways.

Here's the simplest way to write JavaScript. You may see it referred to as 'inline JavaScript':

<a href="#" onclick="this.style.backgroundColor='#990000';event.preventDefault();">Paint it red</a>

When clicked, the link background will become red. Here's the problem: what happens when we have lots of JavaScript we want to execute on a click?

<a href="#" onclick="this.style.backgroundColor='#009900';this.style.color='#FFFFFF';event.preventDefault();">Paint it green</a>

Awkward, right? We could pull the function definition out of the click handler, and turn it a function:

window.paintIt = function(event, backgroundColor, textColor) {
  event.preventDefault();
  event.target.style.backgroundColor = backgroundColor;
  if (textColor) {
    event.target.style.color = textColor;
  }
}

And then on our page:

<a href="#" onclick="paintIt(event, '#990000')">Paint it red</a>

That's a little bit better, but what about multiple links that have the same effect?

<a href="#" onclick="paintIt(event, '#990000')">Paint it red</a>
<a href="#" onclick="paintIt(event, '#009900', '#FFFFFF')">Paint it green</a>
<a href="#" onclick="paintIt(event, '#000099', '#FFFFFF')">Paint it blue</a>

Not very DRY, eh? We can fix this by using events instead. We'll add a data-* attribute to our link, and then bind a handler to the click event of every link that has that attribute:

function paintIt(element, backgroundColor, textColor) {
  element.style.backgroundColor = backgroundColor;
  if (textColor) {
    element.style.color = textColor;
  }
}

window.addEventListener("load", () => {
  const links = document.querySelectorAll(
    "a[data-background-color]"
  );
  links.forEach((element) => {
    element.addEventListener("click", (event) => {
      event.preventDefault();

      const {backgroundColor, textColor} = element.dataset;
      paintIt(element, backgroundColor, textColor);
    });
  });
});

<a href="#" data-background-color="#990000">Paint it red</a>
<a href="#" data-background-color="#009900" data-text-color="#FFFFFF">Paint it green</a>
<a href="#" data-background-color="#000099" data-text-color="#FFFFFF">Paint it blue</a>

We call this 'unobtrusive' JavaScript because we're no longer mixing our JavaScript into our HTML. We've properly separated our concerns, making future change easy. We can easily add behavior to any link by adding the data attribute. We can run all of our JavaScript through a minimizer and concatenator. We can serve our entire JavaScript bundle on every page, which means that it'll get downloaded on the first page load and then be cached on every page after that. Lots of little benefits really add up.

3 Built-in Helpers

3.1 Remote Elements

Rails provides a bunch of view helper methods written in Ruby to assist you in generating HTML. Sometimes, you want to add a little Ajax to those elements, and Rails has got your back in those cases.

Because of Unobtrusive JavaScript, the Rails "Ajax helpers" are actually in two parts: the JavaScript half and the Ruby half.

Unless you have disabled the Asset Pipeline, rails-ujs provides the JavaScript half, and the regular Ruby view helpers add appropriate tags to your DOM.

You can read below about the different events that are fired dealing with remote elements inside your application.

3.1.1 form_with

form_with is a helper that assists with writing forms. To use Ajax for your form you can pass the :local option to form_with.

<%= form_with(model: @article, id: "new-article", local: false) do |form| %>
  ...
<% end %>

This will generate the following HTML:

<form id="new-article" action="/articles" accept-charset="UTF-8" method="post" data-remote="true">
  ...
</form>

Note the data-remote="true". Now, the form will be submitted by Ajax rather than by the browser's normal submit mechanism.

You probably don't want to just sit there with a filled out <form>, though. You probably want to do something upon a successful submission. To do that, bind to the ajax:success event. On failure, use ajax:error. Check it out:

window.addEventListener("load", () => {
  const element = document.querySelector("#new-article");
  element.addEventListener("ajax:success", (event) => {
    const [_data, _status, xhr] = event.detail;
    element.insertAdjacentHTML("beforeend", xhr.responseText);
  });
  element.addEventListener("ajax:error", () => {
    element.insertAdjacentHTML("beforeend", "<p>ERROR</p>");
  });
});

Obviously, you'll want to be a bit more sophisticated than that, but it's a start.

3.1.2 link_to

link_to is a helper that assists with generating links. It has a :remote option you can use like this:

<%= link_to "an article", @article, remote: true %>

which generates

<a href="/articles/1" data-remote="true">an article</a>

You can bind to the same Ajax events as form_with. Here's an example. Let's assume that we have a list of articles that can be deleted with just one click. We would generate some HTML like this:

<%= link_to "Delete article", @article, remote: true, method: :delete %>

and write some JavaScript like this:

window.addEventListener("load", () => {
  const links = document.querySelectorAll("a[data-remote]");
  links.forEach((element) => {
    element.addEventListener("ajax:success", () => {
      alert("The article was deleted.");
    });
  });
});

3.1.3 button_to

button_to is a helper that helps you create buttons. It has a :remote option that you can call like this:

<%= button_to "An article", @article, remote: true %>

this generates

<form action="/articles/1" class="button_to" data-remote="true" method="post">
  <input type="submit" value="An article" />
</form>

Since it's just a <form>, all of the information on form_with also applies.

3.2 Customize Remote Elements

It is possible to customize the behavior of elements with a data-remote attribute without writing a line of JavaScript. You can specify extra data- attributes to accomplish this.

3.2.1 data-method

Activating hyperlinks always results in an HTTP GET request. However, if your application is RESTful, some links are in fact actions that change data on the server, and must be performed with non-GET requests. This attribute allows marking up such links with an explicit method such as "post", "put" or "delete".

The way it works is that, when the link is activated, it constructs a hidden form in the document with the "action" attribute corresponding to "href" value of the link, and the method corresponding to data-method value, and submits that form.

3.2.2 data-url and data-params

Certain elements of your page aren't actually referring to any URL, but you may want them to trigger Ajax calls. Specifying the data-url attribute along with the data-remote one will trigger an Ajax call to the given URL. You can also specify extra parameters through the data-params attribute.

This can be useful to trigger an action on check-boxes for instance:

<input type="checkbox" data-remote="true"
    data-url="/update" data-params="id=10" data-method="put">

3.2.3 data-type

It is also possible to define the Ajax dataType explicitly while performing requests for data-remote elements, by way of the data-type attribute.

3.3 Confirmations

You can ask for an extra confirmation of the user by adding a data-confirm attribute on links and forms. The user will be presented with a JavaScript confirm() dialog containing the attribute's text. If the user chooses to cancel, the action doesn't take place.

Adding this attribute on links will trigger the dialog on click, and adding it on forms will trigger it on submit. For example:

<%= link_to "Dangerous zone", dangerous_zone_path,
  data: { confirm: 'Are you sure?' } %>

This generates:

<a href="..." data-confirm="Are you sure?">Dangerous zone</a>

The attribute is also allowed on form submit buttons. This allows you to customize the warning message depending on the button which was activated. In this case, you should not have data-confirm on the form itself.

The default confirmation uses a JavaScript confirm dialog, but you can customize this by listening to the confirm event, which is fired just before the confirmation window appears to the user. To cancel this default confirmation, have the confirm handler return false.

3.4 Automatic disabling

It is also possible to automatically disable an input while the form is submitting by using the data-disable-with attribute. This is to prevent accidental double-clicks from the user, which could result in duplicate HTTP requests that the backend may not detect as such. The value of the attribute is the text that will become the new value of the button in its disabled state.

This also works for links with data-method attribute.

For example:

<%= form_with(model: Article.new) do |form| %>
  <%= form.submit data: { disable_with: "Saving..." } %>
<% end %>

This generates a form with:

<input data-disable-with="Saving..." type="submit">

3.5 Rails-ujs event handlers

Rails 5.1 introduced rails-ujs and dropped jQuery as a dependency. As a result the Unobtrusive JavaScript (UJS) driver has been rewritten to operate without jQuery. These introductions cause small changes to custom events fired during the request:

Example usage:

document.body.addEventListener("ajax:success", (event) => {
  const [data, status, xhr] = event.detail;
});

3.6 Stoppable events

You can stop execution of the Ajax request by running event.preventDefault() from the handlers methods ajax:before or ajax:beforeSend. The ajax:before event can manipulate form data before serialization and the ajax:beforeSend event is useful for adding custom request headers.

If you stop the ajax:aborted:file event, the default behavior of allowing the browser to submit the form via normal means (i.e. non-Ajax submission) will be canceled and the form will not be submitted at all. This is useful for implementing your own Ajax file upload workaround.

Note, you should use return false to prevent an event for jquery-ujs and event.preventDefault() for rails-ujs.

4 Server-Side Concerns

Ajax isn't just client-side, you also need to do some work on the server side to support it. Often, people like their Ajax requests to return JSON rather than HTML. Let's discuss what it takes to make that happen.

4.1 A Simple Example

Imagine you have a series of users that you would like to display and provide a form on that same page to create a new user. The index action of your controller looks like this:

class UsersController < ApplicationController
  def index
    @users = User.all
    @user = User.new
  end
  # ...

The index view (app/views/users/index.html.erb) contains:

<b>Users</b>

<ul id="users">
<%= render @users %>
</ul>

<br>

<%= form_with model: @user do |form| %>
  <%= form.label :name %><br>
  <%= form.text_field :name %>
  <%= form.submit %>
<% end %>

The app/views/users/_user.html.erb partial contains the following:

<li><%= user.name %></li>

The top portion of the index page displays the users. The bottom portion provides a form to create a new user.

The bottom form will call the create action on the UsersController. Because the form's remote option is set to true, the request will be posted to the UsersController as an Ajax request, looking for JavaScript. In order to serve that request, the create action of your controller would look like this:

  # app/controllers/users_controller.rb
  # ......
  def create
    @user = User.new(params[:user])

    respond_to do |format|
      if @user.save
        format.html { redirect_to @user, notice: 'User was successfully created.' }
        format.js
        format.json { render json: @user, status: :created, location: @user }
      else
        format.html { render action: "new" }
        format.json { render json: @user.errors, status: :unprocessable_entity }
      end
    end
  end

Notice the format.js in the respond_to block: that allows the controller to respond to your Ajax request. You then have a corresponding app/views/users/create.js.erb view file that generates the actual JavaScript code that will be sent and executed on the client side.

var users = document.querySelector("#users");
users.insertAdjacentHTML("beforeend", "<%= j render(@user) %>");

5 Turbolinks

Rails ships with the Turbolinks library, which uses Ajax to speed up page rendering in most applications.

5.1 How Turbolinks Works

Turbolinks attaches a click handler to all <a> tags on the page. If your browser supports PushState, Turbolinks will make an Ajax request for the page, parse the response, and replace the entire <body> of the page with the <body> of the response. It will then use PushState to change the URL to the correct one, preserving refresh semantics and giving you pretty URLs.

If you want to disable Turbolinks for certain links, add a data-turbolinks="false" attribute to the tag:

<a href="..." data-turbolinks="false">No turbolinks here</a>.

5.2 Page Change Events

You'll often want to do some sort of processing upon page load. Using the DOM, you'd write something like this:

window.addEventListener("load", () => {
  alert("page has loaded!");
});

However, because Turbolinks overrides the normal page loading process, the event that this relies upon will not be fired. If you have code that looks like this, you must change your code to do this instead:

document.addEventListener("turbolinks:load", () => {
  alert("page has loaded!");
});

For more details, including other events you can bind to, check out the Turbolinks README.

6 Cross-Site Request Forgery (CSRF) token in Ajax

When using another library to make Ajax calls, it is necessary to add the security token as a default header for Ajax calls in your library. To get the token:

const token = document.getElementsByName(
  "csrf-token"
)[0].content;

You can then submit this token as a X-CSRF-Token header for your Ajax request. You do not need to add a CSRF token for GET requests, only non-GET ones.

You can read more about about Cross-Site Request Forgery in the Security guide.

7 Other Resources

Here are some helpful links to help you learn even more:

• rails-ujs wiki
• Railscasts: Unobtrusive JavaScript
• Railscasts: Turbolinks

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