Why ERB template syntax in Ruby on Rails? - Purpose & Use Cases

The Big Idea

Discover how ERB turns tangled code into clean, dynamic web pages effortlessly!

The Scenario

Imagine writing a web page where you have to mix HTML and Ruby code by hand, inserting Ruby output manually into the HTML file.

The Problem

Manually mixing Ruby and HTML is confusing, easy to break, and hard to maintain because you must remember where to switch between code and markup.

The Solution

ERB template syntax lets you embed Ruby code directly inside HTML using simple tags, so the server can generate dynamic pages cleanly and safely.

Before vs After

✗ Before

<html>
  <body>
    <p>Hello, #{user.name}!</p>
  </body>
</html>

✓ After

<p>Hello, <%= user.name %>!</p>

What It Enables

It enables creating dynamic web pages that update content based on Ruby code without messy manual string building.

Real Life Example

Showing a personalized greeting on a website that changes depending on who is logged in.

Key Takeaways

ERB cleanly mixes Ruby and HTML.

It avoids manual string concatenation.

It makes dynamic web pages easy to build and maintain.

Practice

1. Which ERB tag should you use to display the result of a Ruby expression inside HTML?

easy

A. <% %>

B. <%# %>

C. <%= %>

D. <%== %>

Solution

1. Step 1: Understand ERB tags for output

   The <%= %> tag evaluates Ruby code and inserts the result into the HTML.

2. Step 2: Compare with other tags

   <% %> runs Ruby code without output, <%# %> is a comment, and <%== %> is not standard ERB syntax.

3. Final Answer:

   <%= %> -> Option C

4. Quick Check:

   Output Ruby result = <%= %> [OK]

Hint: Use <%= %> to show Ruby results in HTML [OK]

Common Mistakes:

• Using <% %> which does not output anything
• Confusing comment tag <%# %> with output
• Trying to use <%== %> which is invalid

2. Which of the following is the correct syntax to run Ruby code without showing output in an ERB template?

easy

A. <%== ruby_code %>

B. <%= ruby_code %>

C. <%# ruby_code %>

D. <% ruby_code %>

Solution

1. Step 1: Identify ERB tags for code execution

   The <% %> tag runs Ruby code but does not insert output into HTML.

2. Step 2: Differentiate from output tags

   <%= %> outputs result, <%# %> is a comment, and <%== %> is invalid syntax.

3. Final Answer:

   <% ruby_code %> -> Option D

4. Quick Check:

   Run code without output = <% %> [OK]

Hint: Use <% %> for Ruby logic without output [OK]

Common Mistakes:

• Using <%= %> which outputs result
• Using <%# %> which comments out code
• Trying invalid <%== %> syntax

3. Given this ERB snippet:

<ul>
  <% items.each do |item| %>
    <li><%= item.upcase %></li>
  <% end %>
</ul>

What will be the rendered HTML if items = ['apple', 'banana']?

medium

A. APPLEBANANA

B. item.upcaseitem.upcase

C. applebanana

D.

Solution

1. Step 1: Understand the loop and method call

   The code loops over each item in the array and outputs the uppercase version using item.upcase.

2. Step 2: Predict the HTML output

   Each item is wrapped in <li> tags with uppercase text, so the list shows APPLE and BANANA.

3. Final Answer:

   <ul><li>APPLE</li><li>BANANA</li></ul> -> Option A

4. Quick Check:

   Loop with upcase output = <ul><li>APPLE</li><li>BANANA</li></ul> [OK]

Hint: Remember <%= %> outputs evaluated Ruby code [OK]

Common Mistakes:

• Expecting original lowercase instead of upcase
• Thinking <% %> outputs text
• Confusing string literal with method call

4. Identify the error in this ERB snippet:

<ul>
  <% items.each do |item| %>
    <li><% item.upcase %></li>
  <% end %>
</ul>

medium

A. Syntax error in the loop declaration

B. Missing output tag; item.upcase won't display

C. Extra closing tag <% end %> is invalid

D. Using <%# %> instead of <% %> for code

Solution

1. Step 1: Check the ERB tags around item.upcase

   The code uses <% item.upcase %> which runs Ruby but does not output anything.

2. Step 2: Understand output requirement

   To show the uppercase text, it should use <%= item.upcase %> instead.

3. Final Answer:

   Missing output tag; item.upcase won't display -> Option B

4. Quick Check:

   Use <%= %> to output values [OK]

Hint: Use <%= %> to output, <% %> only runs code [OK]

Common Mistakes:

• Confusing <% %> with output tag
• Thinking loop syntax is wrong
• Believing extra end tag causes error

5. You want to conditionally display a message in ERB only if user.admin? is true. Which snippet correctly does this?

hard

A. <% if user.admin? %>Welcome, admin!<% end %>

B. <%= if user.admin? %>Welcome, admin!<% end %>

C. <% if user.admin? %><% 'Welcome, admin!' %><% end %>

D. <%= user.admin? ? 'Welcome, admin!' %>

Solution

1. Step 1: Understand conditional rendering in ERB

   <% if user.admin? %> starts an if block where literal text like "Welcome, admin!" is rendered only if true, ended by <% end %>.

2. Step 2: Why others fail

   B: Invalid syntax (<%= if %> can't start control flow like that). C: Inner <% %> runs code but outputs nothing. D: Incomplete ternary (missing : '').

3. Final Answer:

   <% if user.admin? %>Welcome, admin!<% end %> -> Option A

4. Quick Check:

   Conditional literal text output = <% if %>...<% end %> [OK]

Hint: Use <% if cond %> text <% end %> for conditional display [OK]

Common Mistakes:

• Using <% %> for text inside conditional (no output)
• Incomplete ternary missing : false_expr (syntax error)
• Using <%= if cond %> (invalid Ruby syntax)