ERB template syntax in Ruby on Rails - Step-by-Step Execution

Concept Flow - ERB template syntax

Start ERB Template

↓

Read line

↓

Check for ERB tags

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Run Ruby

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No Output

↓

Next line or End

The ERB template reads each line, detects Ruby code tags, executes code, and inserts output if needed.

Execution Sample

Ruby on Rails

<h1>Welcome</h1>
<% name = "Alice" %>
<p>Hello, <%= name %>!</p>

This template sets a Ruby variable and inserts its value into HTML.

Execution Table

Step	Line Read	ERB Tag	Action	Output Produced
1	<h1>Welcome</h1>	None	Output HTML as is	<h1>Welcome</h1>
2	<% name = "Alice" %>	<% %>	Run Ruby code, no output
3	<p>Hello, <%= name %>!</p>	<%= %>	Run Ruby, insert output	<p>Hello, Alice!</p>
4	End of template	None	Stop processing

💡 Reached end of template, all lines processed

Variable Tracker

Variable	Start	After Step 2	Final
name	undefined	"Alice"	"Alice"

Key Moments - 2 Insights

Why does <% %> not produce visible output but <%= %> does?

Because <% %> runs Ruby code without inserting anything into HTML, while <%= %> runs Ruby and inserts the result into the output, as shown in steps 2 and 3 of the execution_table.

What happens if you put Ruby code outside ERB tags?

Ruby code outside ERB tags is treated as plain text and output as is, not executed. ERB only runs code inside <% %> or <%= %> tags.

Visual Quiz - 3 Questions

Look at the execution_table, what is the output after step 2?

A "" (empty string)

B "Alice"

C <p>Hello, Alice!</p>

D <h1>Welcome</h1>

💡 Hint

Check the Output Produced column for step 2 in execution_table

At which step does the variable 'name' get assigned a value?

A Step 1

B Step 3

C Step 2

D Step 4

💡 Hint

Look at the Action column in execution_table where Ruby code runs

If you change <%= name %> to <% name %> in line 3, what changes in the output?

A The name 'Alice' will still appear in the output

B Nothing will be output for that part

C An error will occur

D The whole line will be removed

💡 Hint

Compare the difference between <%= %> and <% %> tags in execution_table

Concept Snapshot

ERB template syntax:
- <% code %> runs Ruby code, no output
- <%= code %> runs Ruby and inserts output
- Lines without tags output as HTML
- Used to mix Ruby logic with HTML
- Output builds line by line as template processes

Full Transcript

ERB templates mix Ruby code with HTML. The template reads each line. If it finds <% %> tags, it runs Ruby code without output. If it finds <%= %> tags, it runs Ruby and inserts the result into the HTML output. Lines without tags are output as plain HTML. Variables set inside <% %> can be used later in <%= %> to show values. This process continues line by line until the template ends.