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A JavaScript workflow designed to progressively enhance sites in a qualified manner.
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AUTHORS.txt Update to GPLv3 License
enhance.js Strict mode


Enhance.js is a small JavaScript workflow designed to help developers progressively enhance a page in a qualified manner.

  • Copyright 2010-2014 @scottjehl, @beep, @wilto, @maggiewachs, and Filament Group, Inc.
  • License: MIT


As of version 2, Enhance.js is no longer meant to be a library or framework, but rather an editable boilerplate file. It is small, delete-key-friendly, and meant to be included "inline" in the head of a page and edited to the needs of your project.

We typically use Enhance.js to run brief browser diagnostics before deciding whether to enhance a basic, but already-functional page further by adding classes and requesting additional scripts and stylesheets.

Enhancement workflow

The logic within Enhance.js can and should vary from project to project - edit it to your needs.

By default though, it is set up to follow these steps:

  1. Define some variables and functions for loading assets and accessing information about the browser and markup
  2. Run one or more tests to see if a browser is qualified to load and render additional enhancements

    • A. If it's not qualified, exit early and do nothing more
    • B. If it is qualified, proceed on to load additional assets, add classes to the document, etc.

How to use

We suggest keeping enhance.js in a folder alongside other editable JavaScript files used in your site.

It should be included in your page inline, rather than referencing it externally, through a server-side include or build process:

<!doctype html>
    <script><!--#include virtual="/path/to/enhance.js" --></script>

Including this file inline is in keeping with the overall goal of reducing the number of blocking, external references found in the head of the page (to allow the page to render as quickly as possible). Note: In a production setting, this file should be minified before being included in a page to ensure it's as small as possible. Tools like Uglify.js can help with this.

Enhancing a page EnhanceJS

TODO: A lot of this information should move to the SouthStreet Project, to explain how the pieces above come together.

For EnhanceJS to work best, a careful setup in the head of a page is necessary.

First, we recommend including the critical CSS for each template of a site directly in a style element in the head of the page, and requesting the rest of the site's CSS asynchronously. Once we've fetched the full CSS for the site, we like to set a cookie to tell our server-side templates to reference that CSS file directly with a link element on subsequent visits, instead of any inline styles.

We also recommend loading any non-critical JavaScript asynchronously as well, and the loadJS function in EnhanceJS allows us to do that in a qualified manner, so older and underfeatured browsers don't need to be bothered by DOM Frameworks and such.

To define URLs for files that may be requested by EnhanceJS, such as a file containing JavaScript enhancements, we like to use meta tags in the head of our page.

A fully-configured head setup for EnhanceJS

Here's an example configuration for the head of a page. It uses SSI to detect cookies and decide whether to inline CSS or request it directly.

<!doctype html>
    <meta name="fullcss"  content="/path/to/full.css">
    <meta name="fulljs"  content="/path/to/enhancements.js">
<!--#if expr="$HTTP_COOKIE=/fullcss\=true/" -->
    <link rel="stylesheet" href="/path/to/full.css">
<!--#else -->
        /* critical CSS styles for this template go here... */
<!--#endif -->  
        <!--#include virtual="/path/to/enhance.js" -->
    <noscript><link rel="stylesheet" href="/path/to/full.css"></noscript>


Enhance.js exposes an object called window.enhance or just enhance. You can place whatever enhancement-related properties and methods on this object that you might want to use elsewhere in your codebase, or, none at all.

By default, we typically expose functions like loadCSS, loadJS, getMeta, and cookie, so that we can use these in other scripts in our codebase.

You might choose to expose feature support information, or even user agent info if you really need that.

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