ERB Tags in Rails: Difference Between <% %>, <%= %>, <%# %>, and -%>

By the end of this page, you will understand how ERB tags work in Rails templates, which tags produce output, which tags only run Ruby code, how ERB comments behave, and what -%> does when trimming whitespace or newlines.

ERB stands for Embedded Ruby. It lets you write Ruby inside a text template, usually HTML in a Rails view.

The main idea is simple:

• some ERB tags run Ruby code without showing anything
• some tags run Ruby code and insert the result into the page
• some tags are comments
• some closing forms affect whitespace and newlines

Here are the core forms:

• <% ... %>: execute Ruby code, but do not print the result
• <%= ... %>: execute Ruby code and output the result into the template
• <%# ... %>: ERB comment, ignored by ERB
• -%>: a variation of the closing tag used to trim a newline or surrounding whitespace depending on ERB configuration and usage

This matters because Rails views are not just static HTML. They often contain:

• loops
• conditionals
• helper calls
• partial rendering
• dynamic content

Choosing the right tag controls whether your Ruby logic affects the final HTML output or only the template flow.

Think of an ERB template like a theater script:

• <% ... %> is a stage direction: it tells the actors what to do, but the audience does not hear it.
• <%= ... %> is a spoken line: it becomes part of what the audience sees or hears.
• <%# ... %> is a note in the script: only for the writer, not used in the performance.
• -%> is a cleanup instruction: it helps remove an extra blank line or unwanted spacing after the tag.

So the biggest question is always: Do I want this Ruby code to appear in the final HTML, or just control the template?

Core syntax

<% ruby_code %>
<%= ruby_expression %>
<%# comment %>
<% ruby_code -%>

1. <% %> — run Ruby without output

Use this for logic such as if, each, variable assignment, or setup.

<% items = ["Apple", "Banana", "Cherry"] %>
<% items.each do |item| %>
  <li><%= item %></li>
<% end %>

What happens?

• items = [...] runs, but does not print anything
• each do and end control the loop, but do not print anything
• <%= item %> prints each item inside the <li>

Rendered output:

<li>Apple</li>
<li>Banana</li>
<li>Cherry</li>

2. — run Ruby and output the result

Consider this ERB template:

<% name = "Sam" %>
<p><%= name %></p>
<%# This is hidden %>

Step-by-step

Line 1

<% name = "Sam" %>

• Ruby assigns the string "Sam" to name
• Nothing is printed into the HTML

Line 2

<p><%= name %></p>

• ERB evaluates name
• The value Sam is inserted into the template

So this becomes:

<p>Sam</p>

Line 3

<%# This is hidden %>

• ERB treats this as a comment
• It is ignored and does not appear in the output

Final rendered result

<>Sam

ERB tags are used constantly in Rails views.

Common scenarios

Showing dynamic data

<h1><%= @post.title %></h1>
<p><%= @post.body %></p>

Use <%= %> because the values must appear in the page.

Running loops

<% @posts.each do |post| %>
  <h2><%= post.title %></h2>
<% end %>

Use <% %> for the loop itself, and <%= %> for what should be shown.

Conditional display

<% if current_user.admin? %>
  <a href="/admin">Admin Panel</a>
<% end %>

Use <% %> because the if controls whether content is included.

Developer notes in templates

<%# Replace this with a reusable partial later %>

Use <%# %> for notes that should not appear in HTML.

Avoiding extra blank lines in generated text

This is more common in text-based templates such as emails, config files, or XML:

In real Rails codebases, developers use ERB tags in a few repeatable patterns.

1. Guarded rendering

<% if @order.present? %>
  <p>Order total: <%= @order.total %></p>
<% end %>

• logic uses <% %>
• visible content uses <%= %>

2. Iteration with helpers

<% @users.each do |user| %>
  <li><%= link_to user.name, user_path(user) %></li>
<% end %>

Helpers like link_to are often placed inside <%= %> because they return HTML to render.

3. Partial rendering

<%= render "form", product: @product %>

render returns content, so it usually appears inside <%= %>.

4. Simple setup logic

<% css_class = @product.in_stock? ? "available" : "sold-out" %>
<div class="<%= css_class %>">
  <%= @product.name %>
</div>

Assignment uses <% %>; output uses <%= %>.

1. Using <% %> when you meant to print something

Broken example:

<p><% @user.name %></p>

Problem:

• the code runs, but nothing is output

Correct version:

<p><%= @user.name %></p>

2. Using <%= %> for control flow keywords

Broken example:

<%= if @user.admin? %>
  <p>Admin</p>
<%= end %>

Problem:

• if and end are control-flow structure, not content to print
• this is invalid or confusing

Correct version:

<% if @user.admin? %>
  <p>Admin</p>
<% end %>

3. Forgetting that comments differ

An HTML comment is still sent to the browser:

<!-- secret note -->

If you do not want it in the rendered HTML, use an ERB comment:

<%# secret note %>

<% ruby_code %>

• Runs Ruby
• Does not print anything
• Use for if, each, variable assignment, end

<%= expression %>

• Runs Ruby
• Prints the result into the template
• Use for variables, helper output, render, formatted values

<%# comment %>

• ERB comment
• Not rendered
• Good for developer notes inside templates

<% code -%>

• Same as normal ERB code, but trims newline/whitespace in supported contexts
• Mainly useful for formatting clean output

Quick rule

• logic only: <% %>
• show result: <%= %>
• leave note: <%# %>
• control whitespace: -%>

Example

What does <% %> do in Rails ERB?

It executes Ruby code without inserting the result into the rendered HTML.

What does <%= %> do in ERB?

It executes Ruby and outputs the result into the template.

What is <%# %> used for in ERB?

It creates an ERB comment that is ignored during rendering and does not appear in the final HTML.

What does -%> mean in ERB?

It is a trimming form of the closing tag, used to remove an extra newline or whitespace in some ERB contexts.

Why is nothing showing when I use <% @name %>?

Because <% %> runs code without output. Use <%= @name %> if you want the value displayed.

Should I use ERB comments or HTML comments?

Use ERB comments for developer-only notes. Use HTML comments only if the comment should remain in the rendered page source.

Can I use if with <%= %>?

Usually no. Use <% if condition %> for control flow, then use <%= %> inside the block for visible content.

Does Rails still use ERB often?

• Rails views — ERB tags are mainly used inside Rails view templates.
• Ruby expressions — <%= %> outputs the result of a Ruby expression.
• Conditionals in Ruby — if, elsif, and else are commonly used inside <% %> tags.
• Loops in Ruby — iteration such as each is a common ERB pattern.
• Rails helpers — helpers often return HTML or text that is printed with <%= %>.
• Partials in Rails — partials are frequently rendered with <%= render ... %>.
• HTML escaping in Rails — important when outputting dynamic content in templates.
• Template whitespace control — related to how -%> affects output formatting.