Expressions Plugin

Install

npm i -D posthtml-expressions

Usage

const { readFileSync } = require('fs')

const posthtml = require('posthtml')
const expressions = require('posthtml-expressions')

posthtml(expressions({ locals: { foo: 'bar' } }))
  .process(readFileSync('index.html', 'utf8'))
  .then((result) => console.log(result.html))

This plugin provides a syntax for including local variables and expressions in your templates, and also extends custom tags to act as helpers for conditionals and looping.

You have full control over the delimiters used for injecting locals, as well as the tag names for the conditional and loop helpers, if you need them. All options that can be passed to the expressions plugin are shown below:

Options

Option	Default	Description
delimiters	['{{', '}}']	Array containing beginning and ending delimiters for escaped locals os expressions
unescapeDelimiters	['{{{', '}}}']	Array containing beginning and ending delimiters for unescaped locals
locals	{}	Object containing any local variables you want to be available inside your expressions
localsAttr	locals	Attribute name for the tag script which contains locals
removeScriptLocals	false	Will remove tag script which contains locals
conditionalTags	['if', 'elseif', 'else']	Array containing names for tags used for if/else statements
switchTags	['switch', 'case', 'default']	Array containing names for tags used for switch/case/default statements
loopTags	['each']	Array containing names for for loops
scopeTags	['scope']	Array containing names for scopes
ignoredTag	'raw'	String containing name of tag inside which parsing is disabled
strictMode	true	Boolean value set to false will not throw an exception if a value in locals not found or expression could not be evaluated
missingLocal	undefined	string defining the replacement value in case value not found in locals. May contain {expression} placeholder

Locals

You can inject locals into any piece of content in your html templates, other than overwriting tag names. For example, if you passed the following config to the expressions plugin:

locals: { className: 'intro', name: 'Marlo', 'status': 'checked' }

<div class="{{ className }}">
  <input type="radio" {{ status }}>
  My name is {{ name }}
</div>

<div class="intro">
  <input type="radio" checked="">
  My name is Marlo
</div>

You can also use the script tag with the attribute locals or you custome attribute containing data to interpolate in the template.

<script locals>
  module.exports = {
    name: 'Scrum'
  }
</script>

<div>My name: {{name}}</div>

<script locals>
  module.exports = {
    name: 'Scrum'
  }
</script>

<div>My name: Scrum</div>

In addition, the use of script tag allow you to use locals defined globally to assign data to variables.

posthtml(expressions({ locals: { foo: 'bar' } }))
  .process(readFileSync('index.html', 'utf8'))
  .then((result) => console.log(result.html))

<script locals>
  module.exports = {
    name: 'Scrum',
    foo: locals.foo || 'empty'
  }
</script>

<div>My name: {{name}}</div>
<div>Foo: {{foo}}</div>

<script locals>
  module.exports = {
    name: 'Scrum',
    foo: locals.foo || 'empty'
  }
</script>

<div>My name: {{name}}</div>
<div>Foo: bar</div>

Missing locals

What to produce in case of referencing a value not in locals can be configured by the missingLocal and strictMode options.

When strictMode is true (default) and leaving the missingLocal option undefined, then "'foo' is not defined" exception is thrown.

Setting strictMode false and leaving the missingLocal option undefined results the string undefined in the output

Setting the option missingLocal to a string will produce that string in the output regardless the value of option strictMode. missingLocal can contain the placeholder {local} which will be replaced with the name of the missing local in the output. This solution allows to:

1. Silently ignore missing locals by setting missingLocal to ""
2. Include the name of the missing local in the output to help detect the which value is missing in locals like "#Missing value: {local}"

missingLocal	strictMode	output
undefined (default)	true (default)	Error is thrown
undefined (default)	false	'undefined'
''	false/true	'' (not output)
{local}	false/true	original reference like {{foo}}

Unescaped Locals

By default, special characters will be escaped so that they show up as text, rather than html code. For example, if you had a local containing valid html as such:

locals: { statement: '<strong>wow!</strong>' }

<p>The fox said, {{ statement }}</p>

<p>The fox said, &lt;strong&gt;wow!&lt;strong&gt;</p>

In your browser, you would see the angle brackets, and it would appear as intended. However, if you wanted it instead to be parsed as html, you would need to use the unescapeDelimiters, which by default are three curly brackets, like this:

<p>The fox said, {{{ strongStatement }}}</p>

In this case, your code would render as html:

<p>The fox said, <strong>wow!<strong></p>

Expressions

You are not limited to just directly rendering local variables either, you can include any type of javascript expressions and it will be evaluated, with the result rendered. For example:

<p class="{{ env === 'production' ? 'active' : 'hidden' }}">in production!</p>

With this in mind, it is strongly recommended to limit the number and complexity of expressions that are run directly in your template. You can always move the logic back to your config file and provide a function to the locals object for a smoother and easier result. For example:

locals: {
  isProduction: (env) => env === 'production' ? 'active' : 'hidden'
}

<p class="{{ isProduction(env) }}">in production!</p>

Ignoring Expressions

Many JavaScript frameworks use {{ and }} as expression delimiters. It can even happen that another framework uses the same custom delimiters you have defined in this plugin.

You can tell the plugin to completely ignore an expression by prepending @ to the delimiters:

<p>The @{{ foo }} is strong with this one.</p>

Result:

<p>The {{ foo }} is strong with this one.</p>

Conditionals

Conditional logic uses normal html tags, and modifies/replaces them with the results of the logic. If there is any chance of a conflict with other custom tag names, you are welcome to change the tag names this plugin looks for in the options. For example, given the following config:

locals: { foo: 'foo' }

<if condition="foo === 'bar'">
  <p>Foo really is bar! Revolutionary!</p>
</if>

<elseif condition="foo === 'wow'">
  <p>Foo is wow, oh man.</p>
</elseif>

<else>
  <p>Foo is probably just foo in the end.</p>
</else>

<p>Foo is probably just foo in the end.</p>

Anything in the condition attribute is evaluated directly as an expressions.

It should be noted that this is slightly cleaner-looking if you are using the SugarML parser. But then again so is every other part of html.

if(condition="foo === 'bar'")
  p Foo really is bar! Revolutionary!

elseif(condition="foo === 'wow'")
  p Foo is wow, oh man.

else
  p Foo is probably just foo in the end.

conditionalTags

Type: array
Default: ['if', 'elseif', 'else']

You can define custom tag names to use for creating a conditional.

Example:

conditionalTags: ['when', 'elsewhen', 'otherwise']

<when condition="foo === 'bar'">
  <p>Foo really is bar! Revolutionary!</p>
</when>

<elsewhen condition="foo === 'wow'">
  <p>Foo is wow, oh man.</p>
</elsewhen>

<otherwise>
  <p>Foo is probably just foo in the end.</p>
</otherwise>

Note: tag names must be in the exact order as the default ones.

Switch statement

Switch statements act like streamline conditionals. They are useful for when you want to compare a single variable against a series of constants.

locals: { foo: 'foo' }

<switch expression="foo">
  <case n="'bar'">
    <p>Foo really is bar! Revolutionary!</p>
  </case>
  <case n="'wow'">
    <p>Foo is wow, oh man.</p>
  </case>
  <default>
    <p>Foo is probably just foo in the end.</p>
  </default>
</switch>

<p>Foo is probably just foo in the end.</p>

Anything in the expression attribute is evaluated directly as an expressions.

switchTags

Type: array
Default: ['switch', 'case', 'default']

You can define custom tag names to use when creating a switch.

Example:

switchTags: ['clause', 'when', 'fallback']

<clause expression="foo">
  <when n="'bar'">
    <p>Foo really is bar! Revolutionary!</p>
  </when>
  <when n="'wow'">
    <p>Foo is wow, oh man.</p>
  </when>
  <fallback>
    <p>Foo is probably just foo in the end.</p>
  </fallback>
</clause>

Note: tag names must be in the exact order as the default ones.

Loops

You can use the each tag to build loops. It works with both arrays and objects. For example:

locals: {
  array: ['foo', 'bar'],
  object: { foo: 'bar' }
}

Array

<each loop="item, index in array">
  <p>{{ index }}: {{ item }}</p>
</each>

<p>1: foo</p>
<p>2: bar</p>

Object

<each loop="value, key in anObject">
  <p>{{ key }}: {{ value }}</p>
</each>

<p>foo: bar</p>

The value of the loop attribute is not a pure expressions evaluation, and it does have a tiny and simple custom parser. Essentially, it starts with one or more variable declarations, comma-separated, followed by the word in, followed by an expressions.

<each loop="item in [1,2,3]">
  <p>{{ item }}</p>
</each>

So you don't need to declare all the available variables (in this case, the index is skipped), and the expressions after in doesn't need to be a local variable, it can be any expressions.

loopTags

Type: array
Default: ['each']

You can define custom tag names to use for creating loops:

Example:

loopTags: ['each', 'for']

You can now also use the <for> tag when writing a loop:

<for loop="item in [1,2,3]">
  <p>{{ item }}</p>
</for>

Loop meta

Inside a loop, you have access to a special loop object, which contains information about the loop currently being executed:

• loop.index - the current iteration of the loop (0 indexed)
• loop.remaining - number of iterations until the end (0 indexed)
• loop.first - boolean indicating if it's the first iteration
• loop.last - boolean indicating if it's the last iteration
• loop.length - total number of items

Example:

<each loop='item in [1,2,3]'>
  <li>Item value: {{ item }}</li>
  <li>Current iteration of the loop: {{ loop.index }}</li>
  <li>Number of iterations until the end: {{ loop.remaining }} </li>
  <li>This {{ loop.first ? 'is' : 'is not' }} the first iteration</li>
  <li>This {{ loop.last ? 'is' : 'is not' }} the last iteration</li>
  <li>Total number of items: {{ loop.length }}</li>
</each>

Scopes

You can replace locals inside certain area wrapped in a <scope> tag. For example you can use it after posthtml-include

locals: {
  author: { name: 'John'},
  editor: { name: 'Jeff'}
}

<scope with="author">
  <include src="components/profile.html"></include>
</scope>
<scope with="editor">
  <include src="components/profile.html"></include>
</scope>

<div class="profile">
  <div class="profile__name">{{ name }}</div>
  <img class="profile__avatar" src="{{ image }}" alt="{{ name }}'s avatar" />
  <a class="profile__link" href="{{ link }}">more info</a>
</div>

scopeTags

Type: array
Default: ['scope']

You can define a custom tag name to use for creating scopes:

Example:

scopeTags: ['context']

You can now also use the <context> tag when writing a scope:

<context with="author">
  <include src="components/profile.html"></include>
</context>

Ignored tag

Anything inside this tag will not be parsed, allowing you to output delimiters and anything the plugin would normally parse, in their original form.

<raw>
  <if condition="foo === 'bar'">
    <p>Output {{ foo }} as-is</p>
  </if>
</raw>

<if condition="foo === 'bar'">
  <p>Output {{ foo }} as-is</p>
</if>

You can customize the name of the tag:

var opts = {
  ignoredTag: 'verbatim',
  locals: { foo: 'bar' } }
}

posthtml(expressions(opts))
  .process(readFileSync('index.html', 'utf8'))
  .then((result) => console.log(result.html))

<verbatim>
  <if condition="foo === 'bar'">
    <p>Output {{ foo }} as-is</p>
  </if>
</verbatim>

<if condition="foo === 'bar'">
  <p>Output {{ foo }} as-is</p>
</if>

Maintainers

Jeff Escalante

Denis Malinochkin

Contributors

Michael Ciniawsky

Krillin

Cosmin Popovici