What is the difference between <%, <%=, <%# and -%> in ERB in Rails?

<% %>

Executes the ruby code within the brackets.

<%= %>

Prints something into erb file.

<%== %>

Equivalent to <%= raw %>. Prints something verbatim (i.e. w/o escaping) into erb file. (Taken from Ruby on Rails Guides.)

<% -%>

Avoids line break after expression.

<%# %>

Comments out code within brackets; not sent to client (as opposed to HTML comments).

Visit Ruby Doc for more infos about ERB.

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<% %> and <%- and -%> are for any Ruby code, but doesn't output the results (e.g. if statements). the two are the same.

<%= %> is for outputting the results of Ruby code

<%# %> is an ERB comment

Here's a good guide: http://api.rubyonrails.org/classes/ActionView/Base.html

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Rails does not use the stdlib's ERB by default, it uses erubis. Sources: this dev's comment, ActionView's gemspec, accepted merge request I did while writing this.

There are behavior differences between them, in particular on how the hyphen operators %- and -% work.

Documentation is scarce, Where is Ruby's ERB format "officially" defined? so what follows are empirical conclusions.

All tests suppose:

require 'erb'
require 'erubis'

When you can use -

ERB: you must pass - to trim_mode option of ERB.new to use it.

erubis: enabled by default.

Examples:

begin ERB.new("<%= 'a' -%>\nb").result; rescue SyntaxError ; else raise; end
ERB.new("<%= 'a' -%>\nb"  , nil, '-') .result == 'ab'  or raise
Erubis::Eruby.new("<%= 'a' -%>  \n b").result == 'a b' or raise

What -% does:

ERB: remove the next character if it is a newline.

erubis: in <% %> (without =), - is useless because <% %> and <% -%> are the same. <% %> removes the current line if it only contains whitespaces, and does nothing otherwise. in <%= -%> (with =): remove the entire line if it only contains whitespaces else, if there is a non-space before the tag, and only whitesapces after, remove the whitespces that come after else, there is a non-space after the tag: do nothing

in <% %> (without =), - is useless because <% %> and <% -%> are the same. <% %> removes the current line if it only contains whitespaces, and does nothing otherwise.

in <%= -%> (with =): remove the entire line if it only contains whitespaces else, if there is a non-space before the tag, and only whitesapces after, remove the whitespces that come after else, there is a non-space after the tag: do nothing

remove the entire line if it only contains whitespaces

else, if there is a non-space before the tag, and only whitesapces after, remove the whitespces that come after

else, there is a non-space after the tag: do nothing

Examples:

# Remove
ERB.new("a \nb <% 0 -%>\n c", nil, '-').result == "a \nb  c" or raise

# Don't do anything: not followed by newline, but by space:
ERB.new("a\n<% 0 -%> \nc", nil, '-').result == "a\nb \nc" or raise

# Remove the current line because only whitesapaces:
Erubis::Eruby.new(" <% 0 %> \nb").result == 'b' or raise

# Same as above, thus useless because longer.
Erubis::Eruby.new(" <% 0 -%> \nb").result == 'b' or raise

# Don't do anything because line not empty.
Erubis::Eruby.new("a <% 0 %> \nb").result == "a  \nb" or raise
Erubis::Eruby.new(" <% 0 %> a\nb").result == "  a\nb" or raise
Erubis::Eruby.new(" <% 0 -%> a\nb").result == "  a\nb" or raise

# Don't remove the current line because of `=`:
Erubis::Eruby.new(" <%= 0 %> \nb").result == " 0 \nb" or raise

# Remove the current line even with `=`:
Erubis::Eruby.new(" <%= 0 -%> \nb").result == " 0b"   or raise

# Remove forward only because of `-` and non space before:
Erubis::Eruby.new("a <%= 0 -%> \nb").result == "a 0b"   or raise

# Don't do anything because non-whitespace forward:
Erubis::Eruby.new(" <%= 0 -%> a\nb").result == " 0 a\nb"   or raise

What %- does:

ERB: remove whitespaces before tag and after previous newlines, but only if there are only whitespaces before.

erubis: useless because <%- %> is the same as <% %> (without =), and this cannot be used with = which is the only case where -% can be useful. So never use this.

Examples:

# Remove
ERB.new("a \n  <%- 0 %> b\n c", nil, '-').result == "a \n b\n c" or raise

# b is not whitespace: do nothing:
ERB.new("a \nb  <%- 0 %> c\n d", nil, '-').result == "a \nb   c\n d" or raise

What %- and -% do together

The exact combination of both effects separately.

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<% %> : Executes the ruby code

<%= %> : Prints into Erb file. Or browser

<% -%> : Avoids line break after expression.

<%# %> : ERB comment

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I've added the <%% literal tag delimiter as an answer to this because of its obscurity. This will tell erb not to interpret the <% part of the tag which is necessary for js apps like displaying chart.js tooltips etc.

Update (Fixed broken link)

Everything about ERB can now be found here: https://puppet.com/docs/puppet/5.3/lang_template_erb.html#tags

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These are use in ruby on rails :-

<% %> :-

The <% %> tags are used to execute Ruby code that does not return anything, such as conditions, loops or blocks. Eg :-

<h1>Names of all the people</h1>
<% @people.each do |person| %>
  Name: <%= person.name %><br>
<% end %>

<%= %> :-

use to display the content .

Name: <%= person.name %><br>

<% -%>:-

Rails extends ERB, so that you can suppress the newline simply by adding a trailing hyphen to tags in Rails templates

<%# %>:-

comment out the code

<%# WRONG %>
Hi, Mr. <% puts "Frodo" %>

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<% %> executes the code in there but does not print the result, for eg:
We can use it for if else in an erb file.

<% temp = 1 %>
<% if temp == 1%>
  temp is 1
<% else %>
  temp is not 1
<%end%>  

Will print temp is 1

<%= %> executes the code and also prints the output, for eg:
We can print the value of a rails variable.

<% temp = 1 %>
<%= temp %>  

Will print 1

<% -%> It makes no difference as it does not print anything, -%> only makes sense with <%= -%>, this will avoid a new line.

<%# %> will comment out the code written within this.

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