What is the difference between <%, <%=, <%# and -%> in ERB templates (Ruby on Rails)?

May 22, '23 16:26

2 min. read

This content has been automatically translated from Ukrainian.

In ERB (Embedded Ruby, *.erb files) templates used in Ruby on Rails, there are tags that define different types of embedded code:

<%
<%=
<%#
-%>

They have the following meanings:

<% - this tag is used to insert Ruby code without outputting the result. It is used for logic and conditional expressions. For example, you can use it to create a loop or a conditional statement without directly outputting the result.
<%= - this tag is used to insert Ruby code with outputting the result. It is used to insert the value of a variable or the result of an expression directly into the output HTML code. For example, you can use it to display the value of a variable on the page.
<%# - this tag is used to ignore Ruby code (ERB comment) during the execution of the template. Everything between <%# and %> will be ignored and not outputted in the resulting HTML code. This is useful when you want to leave comments or temporarily disable a certain part of the code (during debugging).

For example:

<% if condition %>
  <%= variable %>
<% else %>
  <%# commented part %>
<% end -%>

-%> - This tag is used to control the whitespace after the closing tag. It is used to remove extra spaces or newline characters that are usually added after the tag %>.

For example:

 <% 1 + 1 -%>

will output `2` without adding a space or newline character after it.

Tips:

<% %>  : Executes Ruby code
<%= %> : Outputs the given value in the ERB template
<% -%> : Removes spaces or newline characters after the expression
<%# %> : Commenting code