Twig and ERB are the two front-end templating languages I use most when developing websites. Here I document the ways each write just about everything to build views: comments, conditionals, variables and undefined variables, interpolation, loops and the loop index, slicing, handling whitespace, retrieving an keyed values, and templating with blocks and partials. If you're familiar with one of Twig or ERB, use this as a cross-reference to translate your knowledge of the one language into the other. If you haven't used either, this should get you up and running quickly. Read on to learn more about ERB and Twig, or skip ahead to the code snippet reference section.
# What is Twig?
Twig is SensioLabs' Django- / Jinja-like templating language for PHP. The recommended extension for Twig files is .twig, .<compiled_extension>.twig is useful, and .html —though inaccurate— is common in front-end templating. It's used by SensioLabs' Symfony; by Drupal 8, which is built on Symfony; and by Craft.
Twig is a great language for building web front ends: it is full-featured without having more than one person could hope to learn, it reads fairly closely to English, and it has great official documentation. Twig is especially notable for its powerful support for complex inheritance across templates. Check out the use tag, the embed tag, and the block() function.
Twig even has Javascript implementations, making it easy to fit into projects built on the JS ecosystem. A quick overview to help you pick the one that best suits yours needs:
- Mozilla's Nunjucks is officially "jinja2 inspired" but it has [often followed Twig's lead](https://github.com/mozilla/nunjucks/issues?utf8=✓&q=is%3Aissue is%3Aclosed twig ) and is now close enough to Twig that Blendid, Viget's build tool for painless local development of static, Craft, Drupal, or Rails sites uses it as Twig proxy (Nunjucks notably does not support Twig's horizontal
embed inheritence). If you use Gulp in your build tools, you can use gulp-nunjucks.
- Twig.js is a popular JS port of Twig that sees more active development than Nunjucks does. It does not reach full parity with Twig (as of this writing Twig.js notably still has some bugs with Twig's
embed tag) but it currently comes closer than Nunjucks does and, since its goal is to duplicate Twig, it likely always will. The Twig.js Gulp plugin is gulp-twig.
- Twing is a Twig engine for Node.js written in TypeScript which aims to always maintain complete parity with Twig. It is described as "a maintainability-first engine that passes 100% of the TwigPHP integration tests, is as close as possible to its code structure and expose an as-close-as-possible API." Because Twing is able to essentially reuse much of Twig's codebase, adding new features as they are merged into Twig is straightforward. Twing is the youngest of these projects… Twig users, show it your love! gulp-twing lets you use Twing with Gulp.
To learn Twig, read through the official documentation, and try things out in twigfiddle.
# What is ERB?
ERB (Embedded Ruby) is a feature of Ruby that lets you —you guessed it!— embed Ruby in other files. ERB files have the extension .<compiled_extension>.erb. It is the language HAML and Slim are shorthand for. ERB is commonly used for templating Views in Rails apps — at Viget we use it when building large sites with custom CMSes. (If that's something you do, check out Colonel Kurtz, the block editor we often use for the client-facing admin area of Rails app sites.)
Because it can do anything Ruby can do, it's extremely powerful, has a much steeper learning curve than Twig, and can do a lot that isn't relevant to front-end templating. There's no cannonical ERB-for-front-end-developers documentation, and the Rails official documentation is immense and hard to dig through. Some resources if for learning ERB:
# ERB and Twig Snippets: Contents
- Delimiters
- Conditionals
- Defining variables
- Concatenation
- Iteration (loops)
- Inspecting data
- Slicing
- Trimming whitespace
- Keyed values
- Vertical inheritance
- Using partials
# Delimiters
-
ERB: <%# … %>
erb
-
Twig: {# … #}
twig
-
ERB: =begin…=end
the opening and closing tags must be at the start of the line
erb
<%
=begin %>
block comment
(both lines of both the begin and end tags must be at the start of their lines)
<%
=end %>
not
erb
<%
=begin %>
not a comment
<%
=end %>
-
Twig: {# … #}
twig
or
twig
not a comment {# block
comment #} not a comment
# Outputting values
# Execution (Control Code)
# Conditionals
# Single-statement conditionals
# Multi-statement conditionals
-
ERB: if…elsif…end
erb
<%# assuming x, y, z, and n are defined %>
<% if x %>
y
<% elsif z == n %> <%# note the spelling of elsif %>
0
<% else %>
1
<% end %>
-
Twig: if…elseif…endif
twig
{% if x %}
y
{% elseif z == n %}{# note the spelling of elseif #}
0
{% else %}
1
{% endif %}
# Conditionals with logical operators
Both ERB and Twig support "condition ? iftrue : iffalse", and "ifselftrue ?: otherwise".
-
ERB. Note that the "then" case : must be provided
erb
<%# assuming x, y, z, and n are defined %>
<%# if x then y %>
<%# omitting the "else" will throw an error #>
<%= x ? y : '' %>
<%# if x is true, y. otherwise, if z equals n then 0. otherwise 1 %>
<%= x ? y : z == n ? 0 : 1 %>
<%# ternary operator: x if x is true, otherwise y %>
<%= x ?: y %>
-
Twig
twig
{# assuming x, y, z, and n are defined and/or Twig's strict variables option is turned off #}
{# if x then y #}
{{ x ? y }}
{# if x is true, y. otherwise, if z equals n then 0. otherwise 1 #}
{{ x ? y : z == n ? 0 : 1 }}
{# ternary operator: x if x is true, otherwise y #}
{{ x ?: y }}
# Truth and falsity of zero in Boolean contexts
-
ERB: 0 is True in Boolean contexts
erb
<%= false ? 'truthy' : 'falsy' %> <%# output: `"falsy"` %>
<%= 0 ? 'truthy' : 'falsy' %> <%# output: `"truthy"` %>
-
Twig: as in PHP generally, 0 is False in Boolean contexts
twig
{{ false ? 'truthy' : 'falsy' }} {# output: `falsy` #}
{{ 0 ? 'truthy' : 'falsy' }} {# output: `falsy` #}
# Defining variables
-
ERB: =
erb
<% var = 1 %>
<% anotherVar = 0 %>
<% falseVar = false %>
<%= 2 if var %> <%# output: `2` %>
<%= 2 if anotherVar %> <%# output: `2` %>
<%= 2 if falseVar %> <%# output: `` %>
<%= 2 unless falseVar %> <%# output: `2` %>
-
Twig: set
twig
{% set var = 1 %}
{% set anotherVar = 0 %}
{% set falseVar = false %}
{{ var ? 2 }} {# output: `2` #}
{{ anotherVar ? 2 }} {# output: null - Twig, unlike PHP, equates 0 with falsehood #}
{{ falseVar ? '' : 2 }} {# output `2` #}
Twig can define multiple variables in a single call — just keep in mind that developers not used to this might overlook the multiple declarations!
twig
{% set x, y, z = 1, 2, 3 %}
(A value must be explicitly provided for each variable: {% set x, y = 1 %} will error.)
# Line breaks within a variable's value
-
ERB: multi-line blocks of markup can stored in an identifier with content_for x do…end
erb
<% content_for longVar do %>
<div>
…
</div>
<% end %>
<%= content_for(longVar) %>
Note: content_for is additive: each time you provide content for a given variable, that content is appeneded to what was there already. To use content_for to overwrite a global variable, use the flush: true option:
erb
<% content_for refreshedVar do %>
a
<% end %>
<% content_for refreshedVar, flush: true do %>
b
<% end %>
-
Twig: use the set tag's form set x…endset to capture chunks of text
twig
{% set longVar %}
<div>
…
</div>
{% endset %}
{{ longVar }}
# Dealing with undefined variables
-
ERB:
-
defined?()
erb
<%# output: the content if `var` is defined %>
<% if defined?(var) %>
…
<% end %>
<%# output: `var` if `var` is defined, otherwise `fallback` %>
<%= defined?(var) ? var : fallback %>
-
||=, the OR Equal operator
erb
<%# output: `var` if it is defined and not nil and not false, otherwise `fallback` %>
<% var ||= fallback %>
<%
=begin %> common front-end use cases:
1. output a variable only if it is defined
<%
=end %>
<% var ||= nil %>
<%# set a variable with a fallback %>
<% x = y ||= nil %>
-
Twig:
-
is defined
Especially useful when Twig's strict variables option is turned on, in which case referring to an undefined variable will throw an error.
twig
{# output: Twig_Error_Runtime: Variable "x" does not exist. #}
{{ x }}
{# output: the content if var is defined #}
{% if var is defined %}
…
{% endif %}
{# output: `advance` if var is defined, otherwise `fallback` #}
{{ var is defined ? advance : fallback }}
-
??, the null coalescing operator
twig
{# output: `var` if it is defined and not null, otherwise `fallback` #}
{{ var ?? fallback }}
{# common use cases:
1. output a variable only if it is defined #}
{{ var ?? null }}
{# set a variable with a fallback #}
{% set x = y ?? null %}
# Variable interpolation
# Concatenation
-
ERB: + (plus). Note that to concatenate a string and a number in Ruby, the number must be converted to a string.
erb
<% string_variable = 'world' %>
<% number_variable = 2 %>
<%= 'hello ' + string_variable %> <%# output: `"hello world"` %>
<%= "example #{number_variable}" %> <%# output: `"example 2"` %>
<%= 'example ' + 3.to_s %> <%# output: `"example 3"` %>
-
Twig: ~ (tilde). Note that strings and numbers can be freely concatenated.
twig
{% set string_variable = 'world' %}
{% set number_variable = 2 %}
{{ 'hello ' ~ string_variable }} {# output: `hello world` #}
{{ "example #{number_variable}" }} {# output: `example 2` #}
{{ 'example ' ~ 3 }} {# output: `example 3` #}
# Iteration (loops)
# Iterating over items
-
ERB: n.each do |i|…end
erb
<% items = ['a', 'b', 'c'] %>
<%# output: `...` %>
<% [0..items.length].each do %>.<% end %>
<%# output: `a b c ` %>
<% items.each do |item| %>
<%= item %>
<% end %>
-
Twig: for i in n…endfor
twig
{% set items = ['a','b','c'] %}
{# output: `...` #}
{% for i in 0..items.length %}.{% endfor %}
{# output: `a b c ` #}
{% for item in items %}
{{item}}
{% endfor %}
# Using the loop index, 0-indexed
# Using the loop index, 1-indexed
# Iterating a certain number of times
-
ERB: n.times do |i|…end
erb
<% n = 3 %>
<%# output: `...` %>
<% n.times do %>.<% end %>
<%# output: `1 2 3 ` %>
<% n.times do |i| %>
<%= i %>
<% end %>
-
Twig: for i in n…endfor
twig
{% set items = ['a','b','c'] %}
{# output: `...` #}
{% for i in 0..items.length %}.{% endfor %}
{# output: `a b c ` #}
{% for item in items %}
{{item}}
{% endfor %}
# Inspecting data
-
ERB: several options for formatting an object's data, notably: simply outputting, .inspecting, and debug()ing. For basic data-checking purposes in a view, the essential difference is debug() returns YAML while inspect and printing return strings.
erb
<%# for some object `posts` %>
<%= posts %>
<%= posts.inspect %>
<%= debug(posts) %>
-
Twig:
-
The |json_encode() filter formats an object's data.
twig
{# for some object `posts` #}
{{ posts|json_encode }}
-
The dump() function outputs information about a variable.
twig
{# for some object `posts` #}
{{ dump(posts) }}
Note: dump must be enabled. Some implementations make it available out of the box (for example, Craft in dev mode).
# Slicing
-
ERB: .slice(index), .slice(start,count)
erb
<%= [1,2,3,4].slice(1) %> <%# output: `2` %>
<%= [1,2,3,4].slice(1,2) %> <%# output: `[2,3]` %>
-
Twig: |slice(start,count) or [start:count]
twig
{{ [1,2,3,4]|slice(1) }} {# output: `Array` #}
{{ [1,2,3,4]|slice(1,2) }} {# output: `Array` #}
Note: The output of the above Twig examples is Array, because in Twig the output of {{ [anArray] }} is Array. If you need to print an array, use |json_encode:
twig
{{ [1,2,3,4]|slice(1)|json_encode() }} {# output: `[2,3,4]` #}
{{ [1,2,3,4]|slice(1,2)|json_encode() }} {# output: `[2,3]` #}
In execution, no special steps are necessary:
twig
{% set myArray = [1,2,3,4] %}
…
# Shorthand to slice the first count items
-
ERB: .take(count) or .first(count)
erb
<%= [1,2,3,4].take(2) %> <%# output: `[1,2]` %>
<%= [1,2,3,4].first(2) %> <%# output: `[1,2]` %>
-
Twig: [:count]
twig
{{ [1,2,3,4][:2]|json_encode() }} {# output: `[1,2]` #}
# Shorthand for everything after the start item
-
Twig: [start:]
twig
{{ [1,2,3,4][2:]|json_encode() }} {# output: `[3,4]` #}
# Trimming whitespace
-
ERB
If trim_mode is set to -, a - in the closing erb tag will trim trailing whitespace:
erb
<% something -%>
1
<%= something_else -%>
2
<% another_thing %>
is equivalent to
erb
<% something %>1
<%= something_else %>2
<% another_thing %>
-
Twig
Trim leading or trailing whitespace by adding a - inside in an opening or close delimiter, respectively:
twig
{% something -%}
1
{%- something_else -%}
2
{%- last_thing %}
is equivalent to
twig
{% something %}1{% something_else %}2{% last_thing %}
# Trimming space between HTML elements
-
Twig
Twig doesn't care what language you are compiling to, but it does provide a special spaceless tag for use with HTML.
twig
{% spaceless %}
<div>…</div>
<span>…</span>
{% endspaceless %}
is equivalent to
twig
<div>…</div><span>…</span>
Note that this spaceless has limited powers:
-
it isn't recursive
twig
{% spaceless %}
<div>
<div>
…
</div>
<div>
<span>…</span>
{% endspaceless %}
is equivalent to
twig
<div><div>
…
</div><div><span>…</span>
-
and content between HTML tags will disrupt it
twig
{% spaceless %}
<div>…</div>
sorry, spaceless
<span>…</span>
{% endspaceless %}
is equivalent to
twig
<div>…</div>
sorry, spaceless
<span>…</span>
# Keyed values
-
ERB:
Use a Symbol :property to look up an operation on a Hash:
erb
<% myHash = {hello: 'world'} %>
<%= myHash[:hello] %> <%# output: "world" %>
-
Twig:
Use dot notation or subscript syntax to access attributes of a variable:
twig
{% set myVar = {hello: 'world'} %}
{{ myVar.hello }} {# output: world #}
{{ myVar['hello'] }} {# output: world #}
# Vertical inheritance
For a layout file that pulls in page:
-
ERB: content_for in child, yield in parent
layouts/layout.html.erb
erb
views/page.html.erb
erb
<% content_for :myBlock do %>
the content
<% end %>
-
Twig: block + extends in child, block in parent.
layout.html.twig
twig
{% block myBlock '' %}
{# or #}
{% block myBlock %}{% endblock %}
{# or #}
{% block myBlock %}{% endblock myBlock %}
page.html.twig
twig
{% extends 'layout.html.twig' %}
{% block myBlock %}
the content
{% endblock %}
or if all the content is a variable x, page.html.twig
twig
{% extends 'layout.html.twig' %}
{% block myBlock x %}
or if all the content is a single string, page.html.twig
twig
{% extends 'layout.html.twig' %}
{% block myBlock "#{x} content" %}
{# or #}
{% extends 'layout.html.twig' %}
{% block myBlock x ~ "content" %}
or if all the content is a single literal string, page.html.twig
twig
{% extends 'layout.html.twig' %}
{% block myBlock 'the content' %}
{# or #}
{% block myBlock "the content" %}
# Vertical inheritance with default content in the parent
-
ERB
layouts/layout.html.erb
erb
<% if content_for?(:my_content) %>
<%= yield :my_content %>
<% else %>
default content
<% end %>
views/page.html.erb
erb
<% content_for :my_content do %>
the content
<% end %>
-
Twig
main.html.twig
twig
{% block content %}
default content
{% block sub_content '' %}
{% endblock %}
override-content.html.twig
twig
{% extends 'main.html.twig' %}
{% block content %}
the content
{% endblock %}
Result of override-content.html.twig:
override-subcontent.html.twig
twig
{% extends 'main.html.twig' %}
{% block subcontent %}
the sub-content
{% endblock %}
Result of override-subcontent.html.twig:
default content
the sub-content
# Using partials
-
ERB:
render will output the contents of another file
erb
<%= render 'path/to/x' %>
To pass values to the rendered file, define them:
erb
<% a = 1 %>
<% b = 2 %>
<%= render 'path/to/x', a:a, b:b %> <%# in path/to/x a=1 and b=2 %>
If the rendered file expects different variable names, use those:
erb
<% a = 1 %>
<% b = 2 %>
<%= render 'path/to/x', y:a, z:b %> <%# in path/to/x y=1 and z=2 %>
-
Twig:
The include tag passes the entire parent context to the included file by default:
twig
{% set a = 1 %}
{% set b = 2 %}
{% include 'path/to/x' %} {# in path/to/x a=1 and b=2 #}
To pass only certain data, use include with only:
twig
{% set a = 1 %}
{% set b = 2 %}
{% include 'path/to/x' with {a:a} only %}
{# in path/to/x a=1 and b does not exist #}
Rename variables in the with (can be combined with only):
twig
{% set a = 1 %}
{% include 'path/to/x' with {y:a} %} {# in path/to/x a=1 and y=1 #}
{% include 'path/to/z' with {y:a} only %}
{# in path/to/z y=1 and a does not exist #}
