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Jan 20, 2016
Forrest Phillips

BEM (Block Element Modifier) is a naming convention created by Yandex that we have adopted to help organize our CSS in a meaningful way.

Problems we’ve solved using BEM

The DOM is an indiscernible mess

BEM can be a bit verbose. Something to remember is that the class names do not necessarily reflect the structure of the DOM. If you get caught, it’s really easy to make a bit of a mess. However, even a BEM mess is still better than a meaningless pile of unrelated class names. This will make your DOM ugly and make it difficult to reuse components. It also means that all of the styles for the header, navigation and links would be contained in the header.coffee and header.sass files.

div { className: 'header' },
  div { className: 'header__logo header__logo--bigger'}
    div { className: 'header__navigation' },
      div { className: 'header__navigation' },
        div { className: 'header__navigation-item' },
          a
            className: 'header__navigation-item-link'
            href: "#"
            target: '_blank'

One of our developers (Alex Barry) went so far as to code a library to help enforce the BEM naming conventions when writing javascript. Simply using the library makes class naming much simpler and consistent.

In the following example, you can see our preferred approach to the naming of DOM elements. You can see that the menu system rests inside the header but it’s named in a way that makes it independent from the header. That also applies to the link inside the navigation, since in this case we don’t want it styled differently from other links on the site. All of the basic links on our site will use the class name “default-link” and share all of the declared styles. The independent blocks are also good indicators of reusable components.

div { className: 'header' },
  div { className: 'header__logo header__logo--bigger'}
  div { className: 'header__navigation' },
      div { className: 'navigation' },
        div { className: 'navigation__item' },
          a
            className: 'default-link'
            href: "#"
            target: '_blank'

File folders are disorganized and it’s difficult to find what you’re looking for

By following the BEM naming convention we found it that our file names, CSS and DOM are more closely related. For example: Here we want to create a header and add a logo. So we create a component file and let’s say in this case it’s coffeescript. We will call the file header.coffee and store it in the components folder.

Then we create a SASS file in the stylesheets/components folder also called header.sass. In this header we also want to include a menu, but we know we’ll be reusing the component in other places on the site. So we create two files for it as well, navigation.coffee and navigation.sass in their appropriate folders.

scripts/coffee/main.coffee
scripts/coffee/components/header.coffee
scripts/coffee/components/navigation.coffee

stylesheets/sass/base.sass
stylesheets/sass/components/header.sass
stylesheets/sass/components/navigation.sass

We now have a DOM that directly reflects the file structure. This makes our codebase much easier to maintain and reduces the amount of time spent hunting for files. It also helps developers consider how the components they are creating might be reused or extended.

CSS is an unmaintainable nightmare of spiderwebs

BEM and SASS play very nice together. In our header.sass file we can declare a few things; First off some basic styles for the header layout and logo. The styles for the navigation will go in their own file. SASS supports inline media queries which are just fantastic. When styling with BEM one should readily avoid using the !important tag and nested styles. When BEM is employed correctly, the need for an !important flag can be an indicator that something is wrong.

We also approach our CSS mobile first, so that all of our media queries are based on the min-width of tablet and desktop breakpoints. It’s important to remember to declare the mobile styles first, then your tablet media query, followed by your desktop. Cascading means you don’t always need to declare a max-width on your media queries. We even organize all of our CSS styles to improve legibility. It seems tedious at first but we found that it became second nature.

You can find a basic guideline we follow here.

.header
  position: relative
  width: 100%
  &__logo
    position: absolute
    width: 80px
    height: 80px
    color: red
    @media (min-width: 1024px)
      width: 120px
      height: 120px
    &--bigger
      width: 240px
      height: 240px

Conclusion

Our experience with BEM has been a positive one. For us BEM has been easy to adopt because our company as a whole follows it. BEM is great for encouraging consistency and meaningful naming conventions. It is also great at helping developers directly relate their file structure and CSS to the front end DOM.