Fepper: a Front End Prototyper tool for rapid prototyping of web sites
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A frontend prototyper tool for rapid prototyping of websites

Downstream projects

  • Fepper Base - no unnecessary assets, styles, Pattern Lab demo, or fp-stylus extension.
  • Fepper for Drupal - templates configured for Drupal 8, along with a Drupal theme built to accommodate those templates.
  • Fepper for Windows - scripted to run on Windows.
  • Fepper for Wordpress - templates configured for WordPress, along with a WordPress theme built to accommodate those templates.

Table of contents


System requirements

  • Unix-like or Windows OS.
  • Minimum supported Node.js version 8.5.0.

Simplest way to get started

Main install

  • In macOS Finder:
    • Double-click fepper.command.
    • Among other things, this will install the fepper-cli, which will give you the fp command.
    • If opening for the first time, macOS may warn that it can't be opened because it is from an unidentified developer.
      • In this case, ctrl+click fepper.command and click "Open".
      • In the following prompt, click "Open" to confirm that you're sure you want to open it.
    • Enter your password to allow installation.
    • After installation, Fepper should automatically open in a browser.
    • Open http://localhost:3000 if it doesn't open automatically.
  • On other Unix-like OSs (or if you prefer working on a BASH-like command line):
    • Install Node.js if it isn't installed already.
    • npm install -g fepper-cli
    • npm install
    • fp
  • To stop Fepper, go to the command line where Fepper is running and press ctrl+c.
  • To restart Fepper:
    • Double-click fepper.command again.
    • Or enter fp on the command line.
  • Consult the Pattern Lab docs for instructions on using Pattern Lab.
  • Start editing files in source. Changes should automatically appear in the browser.

Base install

  • Comes with no unnecessary assets, styles, Pattern Lab demo, or fp-stylus extension.
  • Node.js must be installed beforehand.
  • npm install -g fepper-cli
  • npm run install-base
  • fp

Windows install

  • Assumes you haven't checked out the Fepper for Windows project and just need to add the Windows scripts to your project.
  • Also assumes you have Node.js installed.
  • PowerShell >= 3.0 required.
  • Open PowerShell and enter npm run install-windows
  • In File Explorer, double-click fepper.vbs to launch the UI.
  • In PowerShell, enter cscript .\fepper.vbs [task] to run Fepper tasks.
    • If you Set-ExecutionPolicy to allow ps1 scripts, you may also enter .\fepper.ps1 [task]


Edit the pref.yml file to customize preferences and to view further documentation in the comments. If you wish to use the syncback, frontend-copy, or template tasks, you must supply values for the backend.synced_dirs preferences in order for those directories to get processed and copied to the backend.


  • To launch from the macOS Finder:
    • Double-click fepper.command.
  • To launch from the command line:
    • fp
  • These other utility tasks are runnable on the command line:
    • fp data - build data.json from underscore-prefixed .json files.
    • fp frontend-copy - copy assets, scripts, and styles to the backend.
    • fp help - print documentation of Fepper tasks.
    • fp once - do a one-off Fepper build to the public directory.
    • fp restart - restart after shutdown, but without opening the browser.
    • fp static - generate a static site from the 04-pages directory.
    • fp syncback - combine frontend-copy and template tasks.
    • fp template - translate templates in 03-templates for the backend and copy them there.
    • fp ui:help - print documentation of Fepper UI tasks. Any one of those tasks is also runnable.
    • fp version - print versions of Fepper NPM, Fepper CLI, and Fepper UI.
  • Enter a -d or --debug switch to run the command in debug mode.
  • If using Git for version control, directories named "ignore" will be ignored.


Run fp update to download and install the latest updates.

Global Data

Edit source/_data/_data.json to globally populate Feplet (.mustache) templates with data. Manual edits to source/_data/data.json will get overwritten on each build.

Partial Data

Underscore-prefixed .json files within source/_patterns will be concatenated to the output of _data.json, the whole in turn getting built into data.json, the final source of globally scoped data.

Partial data is distinct from pattern data. For example, 00-homepage.json is pattern data and specific to the 00-homepage pattern. No other pattern will pick up 00-homepage.json, even if 00-homepage.mustache is included in another pattern. However, _00-homepage.json is partial data and will get concatenated to the global data outputted to data.json. _00-homepage.json will be picked up by all patterns.

  • DO NOT EDIT source/_data/data.json
  • DO PUT GLOBAL DATA IN source/_data/_data.json

Static Site Generation

Running fp static will generate a complete static site based on the files in source/_patterns/04-pages. The site will be viewable at http://localhost:3000/static/. An index.html will be generated based on 04-pages-00-homepage, or whatever is defined as the homepage in _data.json. If links to other pages in the 04-pages directory work correctly in the Fepper UI, they will work correctly in the static site, even if the public/static directory is copied and renamed.

The Backend

Fepper can very easily work with a CMS backend such as Drupal or WordPress, while not requiring Apache, MySQL, or PHP. Put the actual backend codebase or even just a symbolic link to the codebase into the backend directory. Then, enter the relative paths to the appropriate backend directories in pref.yml. (Do not include "backend" or a leading slash.) You will then be able to run fp syncback or fp frontend-copy to export your frontend data into your backend web application.

  • Be sure that backend.synced_dirs.assets_dir, backend.synced_dirs.scripts_dir, and backend.synced_dirs.styles_dir are set in pref.yml.
  • The above values set in pref.yml can be overridden on a per-file basis by similarly named YAML files with similarly named settings.
    • These YAML files must match the source file's name with the exception of the extension.
    • The extension must be .yml
    • The overriding property must only contain the lowest level key-value, not the entire hierarchy, i.e., only assets_dir, not backend.synced_dirs.assets_dir.
  • Asset, script, and style files prefixed by "__" will be ignored by fp syncback and fp frontend-copy as will files in the _nosync directory at the root of the source directories.


Fepper's Feplet (.mustache) templates can be translated into templates compatible with your backend. Feplet tags just need to be replaced with tags the backend can use. Put these translations into YAML files named similarly to the .mustache files in source/_patterns/03-templates. Follow this example for the correct YAML syntax.

Follow these rules for setting up keys and values:

  • Delete the Feplet curly braces for keys.
  • Trim any exterior whitespace.
  • Leave other control structures within the key, e.g., !#/>^
  • Escape parentheses, carets, and question marks with a backslash.
  • Wrap the key in single quotes.
  • Follow the closing quote with a colon, space, pipe, the numeral 2, and a newline : |2
  • Indent each line of the value by at least two spaces.
  • When translating to a language with double-curly braces for tags (as per the example), the double-curly braces must be escaped with a backslash per curly brace.

Run fp syncback or fp template to execute the Templater.

  • Be sure that backend.synced_dirs.templates_dir and backend.synced_dirs.templates_ext are set in pref.yml.
  • The default templates_dir and templates_ext settings in pref.yml can be overridden by similarly named settings in the template-specific YAML files.
  • Templates prefixed by "__" will be ignored by the Templater as will files in the _nosync directory.
  • The Templater will recurse through nested Feplet templates if the tags are written in the verbose syntax and have the .mustache extension, e.g., {{> 02-components/00-global/00-header.mustache }}
  • However, the more common inclusion use-case is to leave off the extension, and not recurse.
Fepper for Drupal and Fepper for WordPress have working examples of templates compatible with the Templater.

Webserved Directories

When using a backend, assets generally need to be shared with the Fepper frontend. The syncback and frontend-copy tasks copy files from Fepper to the backend, but not the other way. Instead of providing a task to copy in the reverse direction, Fepper serves backend files if their directories are entered into the webserved_dirs block in pref.yml. Be sure these values are formatted as YAML array elements.


Mustache Browser

Feplet (.mustache) code can be viewed in the Fepper UI by clicking the eyeball icon in the upper right, then clicking Code, and then clicking the Mustache tab in the bottom pane. Clicking the hot-linked area will open the pattern's .mustache file and display its code in the Fepper UI, with its partial tags hot-linked as well.

Fepper's implementation of Mustache is powered by the Feplet template engine. It extends Mustache functionality by allowing the passing of data parameters as follows:

{{> 02-components/00-global/99-example(greeting: 'Hello', place: 'World') }}


{{ greeting }} {{ place }}

So long as the path is navigable, the Mustache Browser will link such parameterized tags.

HTML Scraper

Fepper can scrape and import the HTML of any valid web page into a reusable pattern. A common use-case is to scrape pages from a backend populated with CMS content in order to auto-generate data files, and to replicate the CMS's HTML structure. To open the Scraper, click Scrape in the Fepper UI, and then click HTML Scraper. Enter the URL of the page you wish to scrape. Then, enter the CSS selector you wish to target (prepended with "#" for IDs and "." for classes). Classnames and tagnames may be appended with array index notation ([n]). Otherwise, the Scraper will scrape all elements of that class or tag sequentially. Such a loosely targeted scrape will save many of the targeted fields to a JSON file, but will only save the first instance of the target to a Feplet (.mustache) file.

Upon submission, you should be able to review the scraped output on the subsequent page. If the output looks correct, enter a filename and submit again. The Scraper will save .mustache and .json files by that name in the 98-scrape directory, also viewable under the Scrape menu of the toolbar. The Scraper will also correctly indent the Feplet code.


source/_scripts/src/variables.styl is a file containing variables that can be shared across the Stylus CSS preprocessor, browser JavaScripts, and PHP backends (and possibly other language backends as well). It ships with these values:

bp_lg_max = -1
bp_md_max = 1024
bp_sm_max = 767
bp_xs_max = 480
bp_xx_max = 320
bp_xx_min = 0

It cannot contain comments, semi-colons, curly braces, etc. It is straightforward to import and use these variables in Stylus and JavaScript. PHP must import them with parse_ini_file(). Fepper tries to be agnostic about CSS processors and tries to keep the amount of NPMs to download to a minimum. However, since Stylus allows for this easy sharing of variables, most Fepper distros ship with the fp-stylus extension and a fully-populated source/_styles/src/stylus directory. The Stylus files are written in the terse, Python-like, indentation-based syntax. However, the more verbose, CSS-like syntax (with curly braces and semi-colons) is perfectly valid as well.

The UI's viewport resizer buttons are dependent on the values in this file. The default values will configure the XX, XS, SM, and MD buttons to resize the viewport to each breakpoint's assigned maximum width. The LG button will resize the viewport to a width that is greater than bp_md_max by the distance between bp_sm_max and bp_md_max.

Users have the ability to add, modify, or delete values in this file. The UI will respect these changes; but keep in mind that additions must be prefixed by bp_ and suffixed by _max in order for them to appear as viewport resizer buttons. A -1 value translates to Number.MAX_SAFE_INTEGER, and effectively means infinity.

UI Customization

All aspects of the UI are available for customization. For example, the toolbar can accept additions, modifications, and deletions per the needs of end users. The UI markup is compiled by recursive, functional React calls. The recursion tree is reflected by the directory structure containing the modules which compose the UI. To override any given module, copy the directory structure leading to the module from https://github.com/electric-eloquence/fepper-npm/tree/dev/ui/core/styleguide/index/html to source/_ui/index/html, respective to your implementation. Modifications to modules in that directory will override the corresponding modules in core. Additions (so long as they are correctly nested) will also be recognized.

A working example of UI customization can be found at https://github.com/electric-eloquence/fepper-drupal/blob/dev/source/_ui/index/html/00-head/head.component.js. The Fepper for Drupal project overrides its HTML title to read "Fepper D8" instead of "Fepper". In order to do so, it has the head.component.js module nested in directories that correspond to the tags that nest the head HTML element. Both head.component.js and its nesting directories must be named similarly their corresponding elements. .component.js indicates that the file is a module to be rendered by React. It must export properties that React.createElement() understands. The numeric prefix to 00-head orders it to precede 01-foot, even though "foot" precedes "head" alphabetically.

In this example, by allowing customizations in the 00-head directory separate from the core components, core updates will be respected for all components except for the HTML head.

Browser JavaScript and CSS customizations can (and should) be componentized this way as well. While a head element is unlikely to have associated scripts or styles, the UI's main element does have its scripts and styles componentized as main.js and main.css in index/html/01-body/40-main. A big advantage for this type of componentization comes when elements are renamed or deleted. When you rename or delete the element, are you absolutely sure you'll rename or delete accordingly in some far-flung, monolithic script or style file?

Alas, no one should be forced to componentize this way. Generic modifications to UI scripts can be added to source/_scripts/ui-extender.js.

Similarly, generic modifications to UI styles can be added to source/_styles/pattern-scaffolding.css. (The file is named this way to adhere to the Pattern Lab documentation on pattern states. It should not be relied on for pattern scaffolding.)

View All markup can also be overridden by copying the .mustache files in https://github.com/electric-eloquence/fepper-npm/tree/dev/ui/core/styleguide/viewall and pasting them to source/_ui/viewall (nested correctly). Modifications will then be recognized and displayed in the UI. (No additions are allowed.) Custom View All styles can be added to regular pattern styles in source/_styles.

You will need to compile the UI in order for the browser to pick up custom changes to the UI:

fp ui:compile

New UI customizations will not be picked up simply by restarting Fepper.

The UI exposes these tasks for extension purposes:

  • fp ui:build - build the patterns, outputting to the public directory.
  • fp ui:clean - delete all patterns in the public directory.
  • fp ui:compile - compile the user interface from its component parts.
  • fp ui:copy - copy frontend files (_assets, _scripts, _styles) to the public directory.
  • fp ui:copy-styles - copy _styles to the public directory (for injection into browser without refresh.
  • fp ui:help - get more information about Fepper UI CLI commands.


The extend directory is purposed for extending Fepper's functionality. Extensions can be contributed or custom. The extend directory will not be modified when updating Fepper.

Contributed extensions:

  • Install and update contributed extensions with NPM in the extend directory.
  • Add the tasks to extend/contrib.js (and extend/auxiliary/auxiliary_contrib.js if necessary) in order for Fepper to run them.
  • Contributed Fepper extensions can be found at https://www.npmjs.com/ by searching for "Fepper extension".

Custom extensions:

  • Write custom extensions in the extend/custom directory.
  • Extensions require a file ending in "~extend.js" in order for Fepper to recognize their tasks.
  • The "*~extend.js" file can be directly under extend/custom, or nested one directory deep, but no deeper.
  • Add the tasks to extend/custom.js (and extend/auxiliary/auxiliary_custom.js if necessary) in order for Fepper to run them.
  • Fepper runs a self-contained instance of gulp to manage tasks. This gulp instance will be independent of any other gulp instance on your system.
  • The fp command is an alias for gulp (among other things). Any fp task can be included in a custom task.
  • Fepper only supports gulp 3 syntax.

Confs and prefs:

You might need to access the values in the conf.yml and pref.yml files in order to write custom tasks. They are exposed through global.conf and global.pref (on the global Node object).

The values in patternlab-config.json are exposed through global.conf.ui. Please note that all paths in patternlab-config.json will be converted to absolute paths in global.conf.ui.

gulp.watch will not work correctly with absolute paths. There are two workarounds for this:

  • Hard-code a relative path as the first gulp.watch parameter. Pass an absolute path (from global.conf.ui.paths or otherwise) as the options.cwd value for the second parameter.
  • Pass a value from global.conf.ui.pathsRelative as the first parameter.
    • global.conf.ui.pathsRelative stores relative versions of the values in global.conf.ui.paths.
    • This will still probably require options.cwd in the second parameter.

Common utilty functions for custom extensions are available from the Fepper Utils NPM. Its API documentation can be viewed by following the link.

Express App

Fepper exposes its Express application through the global.expressApp object. This object can be overridden with custom routes and middleware via the custom:tcp-ip (or contrib:tcp-ip) extension task. Initialization of global.expressApp occurs before this task, and listening occurs afterward.

Mobile Devices

The best way to browse the Fepper UI on a mobile device is through the wireless connection on your development machine. These are the instructions for doing this on a Mac:

If your Mac is connected to the Internet wirelessly:

  • Open System Preferences
  • Click Network
  • In the left pane, select Wi-Fi
  • In the right pane, underneath Status, the IP address will be displayed
  • On your mobile device, connect to the same wireless network that your Mac is connected to
  • Browse http://[IP address]:3000
  • Change the port number if Fepper is listening on a different port

If your Mac is connected to the Internet through a wire:

  • In the top menu bar, turn Wi-Fi off
  • Open System Preferences
  • Click Sharing
  • In the left pane, select Internet Sharing
  • In the right pane, on "Share your connection from:", select the interface which is connected to the Internet
  • On "To computers using:", check Wi-Fi
  • Click "Wi-Fi Options..."
  • This will show your Mac's name as the wireless network name
  • Add security if you are in a public space
  • Click OK
  • Back in the System Preferences main window, in the left pane, check to activate Internet Sharing
  • In the dialog that appears, click "Turn Wi-Fi On"
  • In the next dialog, click Start
  • The Wi-Fi icon in the top menu bar should now be gray with a white up-arrow
  • Back in the System Preferences main window, click Network
  • In the left pane, select your wired connection
  • In the right pane, underneath Status, the IP address will be displayed
  • On your mobile device, connect to the wireless network that is the same name as your Mac
  • Browse http://[IP address]:3000
  • Change the port number if Fepper is listening on a different port


The Elements, Compounds, and Components directories are optional and can be renamed to anything your charset and file system allow without further configuration. The Templates, Pages, and Scrape directories can be renamed, but must also be reconfigured in patternlab-config.json. Just about all the text in the UI can be changed via UI customization.

If a need arises for documentation in other languages, you are strongly encouraged to make the translations, and use the options that Open Source offers to distribute them to the rest of the world.

Keyboard Shortcuts

  • ctrl+alt+0: set the viewport to "extra extra small"
  • ctrl+shift+x: set the viewport to "extra small"
  • ctrl+shift+s: set the viewport to "small"
  • ctrl+shift+m: set the viewport to "medium"
  • ctrl+shift+l: set the viewport to "large"
  • ctrl+alt+w: set the viewport to "whole"
  • ctrl+alt+r: set the viewport to a random width
  • ctrl+alt+g: start/stop "grow" animation
  • ctrl+shift+a: open/close annotations panel
  • ctrl+shift+c: open/close code panel
  • cmd+a/ctrl+a: select the content of the current open tab in code panel
  • ctrl+alt+m: make the Mustache tab active in code panel
  • ctrl+alt+h: make the HTML tab active in code panel
  • ctrl+shift+f: open/close the pattern search

As a reminder, the viewport sizes can be customized in source/_scripts/src/variables.styl.

More Documentation