• Return plain strings as HTTP responses
• Explain how the Internet is really just sending strings!
• Replace a DIV with a string fetched over HTTP

As developers, we do our best to hide it, but all of the Internet is just a bunch of strings being passed around from place to place. Don't tell the cats.

We format our strings so that we can render them in an agreed-upon way, but it's all just strings. All you have to do is drop into your terminal and try: curl https://twitter.com.

When the browser requests this URL, this is what it sees. Just one big string of markup (HTML) and data. Then it interprets the markup and turns it into Twitter. But on the console using curl, with nothing to turn the HTML into something, we just see the string.

So if the Internet is just strings, do we always need the markup?

Included in this repo is a blog application. To start, run rake db:migrate RAILS_ENV=development, then run rake db:seed. Once both are run, launch the Rails server and navigate to /posts.

Okay, we have a pretty basic post title and content here, with a teaser to only show the first bit of the post body. We want to give them a way to interact and ask for the whole post body, so we'll add a button to fill in the rest.

We want to do this without redirecting to the post show path, so we're going to use an AJAX request to replace the truncated content with the full body.

The first thing we'll need is a route and controller action to get the body of the post.

# routes.rb
# ...
get '/posts/:id/body', to: 'posts#body'

And in our controller:

# posts_controller.rb
# ...
  def body
    post = Post.find(params[:id])
    render plain: post.description
  end

Note the render plain: call in the controller. Normally, in a RESTful action, we allow the controller to implicitly render a template with the same name as the action. Here, however, we want to explicitly render plain text, so we call render with the :plain option.

Note: There are a lot of options for render, including plain text, files, or nothing at all. Read more about render in the Layouts and Rendering Rails guide.

Now that we have that action, we can hit it by browsing to /posts/:id/body and see that we are just rendering plain text. Restart your Rails server and check it out.

Hint: The /posts page outputs the post id as part of the <h1> tag, so you can easily get an id to use to test this out.

This route isn't useful at all to the HTML portion of the site, because it has no markup, so we have to consume it another way. In other words, we just wrote our first API endpoint!

Okay, we have an API endpoint where we can get the rest of a post body. Now it's just a matter of providing a way for the user to request it, and getting the string response onto the page.

Let's start by adding a button. In our _post.html.erb partial, we want to provide a button for each post to request the body.

It has to be tied to each post so that we can know which post's body we're requesting, which means we'll need to make use of the post.id to pass along to our route.

# _post.html.erb
<h1><%= post.id %>: <%= post.title %></h2>
<div><%= truncate post.description %></div>

<button class="js-more" data-id="<%= post.id %>">More...</button>

Note: We're using the truncate helper here to only show a portion of the potentially long description string. By default, truncate will cut off everything after 30 characters.

We need a way to attach a click() event listener to the button in JavaScript. Since there will be multiple buttons on the page that need to respond to the click, we're using class="js-more" as an identifier, rather than an id attribute meant for just one element. Prefixing the class name with js- is a common way to communicate that this class is used as a JavaScript selector.

We also need to tie the button to a post, since it has to ask for the right content, so we're using a data-* attribute of data-id to hold the post.id for us.

Now that we have that, let's wire up a click() listener to our js-more buttons and request the post body.

We'll use jQuery to wire it up in document .ready() in our index.html.erb template. Why not the _post partial? We only want to do this code once, and the partial will be loaded as many times as we have posts, so putting it in the main template ensures we just use it one time.

# posts/index.html.erb
<% @posts.each do |post| %>
  <%= render partial: "post", locals: {post: post} %>
<% end %>

<script type="text/javascript" charset="utf-8">
$(function () {
  $(".js-more").on('click', function() {
    // get the id from the data attribute
    let id = $(this).data("id");
    $.get("/posts/" + id + "/body", function(data) {
      alert(data);
    });
  });
});
</script>

We've set up a click listener for all our .js-more buttons, and used the .get() method to make an AJAX request to our /posts/:id/body route using the id we stored in the data-id attribute on the button.

So, if we reload the page and click "more" on a post, we should get an alert with the body of that post.

The last thing we need to do is swap out that alert(data) call with actually putting the response into the body of our post.

If we look back at _post.html.erb, we see that our body is in a <div>. All we need to do is replace the inner text of that <div>. To do that, we need to identify it with the post.id:

# _post.html.erb
<h1><%= post.id %>: <%= post.title %></h2>
<div id="body-<%= post.id %>"><%= truncate post.description %></div>

<button class="js-more" data-id="<%= post.id %>">More...</button>

Now, we've identified the element as body-id, so each div will have a predictable id, like body-1, body-2, and so on.

With that in place, we just need to update our JavaScript to find the right div by id and replace the text:

# posts/index.html.erb
# ...
<script type="text/javascript" charset="utf-8">
$(function () {
  $(".js-more").on('click', function() {
    let id = $(this).data("id");
    $.get("/posts/" + id + "/body", function(data) {
      // Replace text of body-id div
      $("#body-" + id).text(data);
    });
  });
});
</script>

We simply use the # selector to find the right body-id, and call .text(data) to replace the text of the div with what we got back from our API call.

If we reload the page now, and click "More..." on our posts, we should see the truncated text replaced with the full text!

Congratulations, you've just implemented and consumed your first internal API endpoint!

We've reminded ourselves that the Internet is run by passing strings around, and that we can use Rails to return plain-text strings without markup from our API endpoints.

We've also seen how we can easily request non-HTML data from the client to the server and use the response on the page without needing to render an entire page or redirect.

Cliffhanger:

What if we wanted all of the data from a post, and not just the body, from that API endpoint? What if we wanted to display the show page without doing a page refresh? Certainly we wouldn't want to make a bunch of separate requests to endpoints like /post/:id/body and /post/:id/title and /post/:id/author, would we? There has to be a better way!