Fundamental Twig for Front-End Development

Henry Bley-Vroman

Posted

A version of this post appeared on viget.com

Daffodil (Narcissus species): flowering plants in grass. Watercolour. Wellcome Collection. Public Domain Mark.

Twig is the view templating languages Viget's front-end developers use with Craft CMS sites and (with Timber) on Wordpress. This primer is designed to bring new developers up to speed writing it.

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This post is an iteration on ERB and Twig Cross-Reference for Front-End Development. That post is geared towards developers who want to translate their Twig knowledge to ERB, or vice versa. You may also be interested in Fundamental ERB for Front-End Development.

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.

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:

Reference

Delimiters

Comments

Inline comments

{# … #}

twig
{# comment #}
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Block comments

{# … #}

twig
{#
block comment
#}
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or

twig
not a comment {# block
comment #} not a comment
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Outputting values

{{ }}

twig
{{ "print this" }} {# output: `print this` #}
{{ 1 + 2 }}        {# output: `3` #}
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Execution (Control Code)

{% … %}

twig
{% if … %} … {% endif %}
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Conditionals

if…elseif…endif

twig
{% if x %}
    y
{% elseif z == n %}{# note the spelling of elseif #}
    0
{% else %}
    1
{% endif %}
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With logical operators

Twig supports "condition ? iftrue : iffalse", and "ifselftrue ?: otherwise".

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 }}
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Truth and falsity of zero in Boolean contexts

As in PHP, 0 is False in Boolean contexts

twig
{{ false ? 'truthy' : 'falsy' }} {# output: `falsy` #}
{{ 0 ? 'truthy' : 'falsy' }}     {# output: `falsy` #}
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Defining variables

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` #}
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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 %}
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(A value must be explicitly provided for each variable: {% set x, y = 1 %} will error.)

Line breaks within a variable's value

Use the set tag's form set x…endset to capture chunks of text

twig
{% set longVar %}
  <div>
    …
  </div>
{% endset %}
{{ longVar }}
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Dealing with undefined variables

  • 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 }}
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  • ??, 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 %}
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Variable interpolation

#{var}

twig
{% set x = 1 %}
{{ "this is interpolated #{x}" }}{# output: `this is interpolated: 1` #}
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Concatenation

~ (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` #}
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Iteration (loops)

Iterating over items

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 %}
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Using the loop index, 0-indexed

loop.index0

twig
{% for item in items %}
    {{loop.index0}}. {{item}}
{% endfor %}
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Using the loop index, 1-indexed

loop.index

twig
{% for item in items %}
    {{loop.index}}. {{item}}
{% endfor %}
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Iterating a certain number of times

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 %}
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Inspecting data

  • The |json_encode() filter formats an object's data.

    twig
    {# for some object `posts` #}
    {{ posts|json_encode }}
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  • The dump() function outputs information about a variable.

    twig
    {# for some object `posts` #}
    {{ dump(posts) }}
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    Note: dump must be enabled. Some implementations make it available out of the box (for example, Craft in dev mode).

Slicing

|slice(start,count) or [start:count]

twig
{{ [1,2,3,4]|slice(1) }}   {# output: `Array` #}
{{ [1,2,3,4]|slice(1,2) }} {# output: `Array` #}
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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]` #}
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In execution, no special steps are necessary:

twig
{% set myArray = [1,2,3,4] %}
…
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Shorthand to slice the first count items

[:count]

twig
{{ [1,2,3,4][:2]|json_encode() }} {# output: `[1,2]` #}
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Shorthand for everything after the start item

[start:]

twig
{{ [1,2,3,4][2:]|json_encode() }} {# output: `[3,4]` #}
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Trimming whitespace

Trim leading or trailing whitespace by adding a - inside in an opening or close delimiter, respectively:

twig
{% something -%}
1
{%- something_else -%}
2
{%- last_thing %}
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is equivalent to

twig
{% something %}1{% something_else %}2{% last_thing %}
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Trimming space between HTML elements

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 %}
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is equivalent to

twig
<div>…</div><span>…</span>
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Note that this spaceless has limited powers:

  • it isn't recursive

    twig
    {% spaceless %}
      <div>
        <div>
          …
        </div>
      <div>
      <span>…</span>
    {% endspaceless %}
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    is equivalent to

    twig
    <div><div>
        …
      </div><div><span>…</span>
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  • and content between HTML tags will disrupt it

    twig
    {% spaceless %}
      <div>…</div>
      sorry, spaceless
      <span>…</span>
    {% endspaceless %}
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    is equivalent to

    twig
    <div>…</div>
      sorry, spaceless
      <span>…</span>
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Keyed values

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 #}
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Vertical inheritance

For a layout file that pulls in page:

block + extends in child, block in parent.

layout.html.twig

twig
{% block myBlock '' %}
{# or #}
{% block myBlock %}{% endblock %}
{# or #}
{% block myBlock %}{% endblock myBlock %}
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page.html.twig

twig
{% extends 'layout.html.twig' %}
{% block myBlock %}
    the content
{% endblock %}
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or if all the content is a variable x, page.html.twig

twig
{% extends 'layout.html.twig' %}
{% block myBlock x %}
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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" %}
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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" %}
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Vertical inheritance with default content in the parent

main.html.twig

twig
{% block content %}
    default content
    {% block sub_content '' %}
{% endblock %}
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override-content.html.twig

twig
{% extends 'main.html.twig' %}
{% block content %}
    the content
{% endblock %}
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Result of override-content.html.twig:

default content
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override-subcontent.html.twig

twig
{% extends 'main.html.twig' %}
{% block subcontent %}
    the sub-content
{% endblock %}
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Result of override-subcontent.html.twig:

default content
  the sub-content
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Using partials

  • include tag

    twig
    {% include 'path/to/x' %}
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  • include function

    twig
    {{ include('path/to/x') }}
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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 #}
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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 #}
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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  #}
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