ERB template syntax in Ruby on Rails - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the ERB tag <%= %> do in a Rails view?

It evaluates Ruby code inside the tag and inserts the result into the HTML output.

beginner

What is the difference between <% %> and <%# %> in ERB?

<% %> runs Ruby code without outputting it, while <%# %> is a comment and does not run or output anything.

beginner

How do you write a Ruby if statement in ERB to conditionally show content?

Use <% if condition %> to start, then HTML or ERB code, and close with <% end %>.

intermediate

What does <%== %> do in ERB templates?

It outputs raw HTML without escaping it, so be careful to avoid XSS security risks.

beginner

How can you embed Ruby variables inside HTML using ERB?

Use <%= variable_name %> to insert the variable's value into the HTML output.

Which ERB tag outputs the result of Ruby code into the HTML?

A <%= %>

B <% %>

C <%# %>

D <%== %>

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

What does the ERB tag <%# %> do?

A Comments out code, no execution or output

B Runs Ruby code without output

C Runs Ruby code and outputs result

D Outputs raw HTML without escaping

The <%# %> tag is a comment in ERB and does not run or output anything.

How do you write a loop in ERB to repeat HTML content?

A <% if item in list %> ... <% end %>

B <%= for item in list %> ... <%= end %>

C <% for item in list %> ... <% end %>

D <%# for item in list %> ... <%# end %>

Use <% for item in list %> to start the loop and <% end %> to close it, running Ruby code without outputting the loop itself.

Which ERB tag should you use to insert unescaped HTML safely?

A <%= %>

B <%== %>

C <%# %>

D <% %>

The <%== %> tag outputs raw HTML without escaping, but use it carefully to avoid security risks.

What happens if you use <% %> in ERB?

A Raw HTML is output

B Ruby code runs and output is inserted

C Code is commented out

D Ruby code runs but no output is inserted

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

Explain how to use ERB tags to insert Ruby code and output into an HTML template.

Think about which tags run code, output, or comment.

Describe how to write a conditional statement in ERB to show content only when a condition is true.

Remember ERB uses Ruby control flow syntax inside tags.

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)