This presentation was given internally for the frontend developers of Booking.com

1. by Varya Stepanova
@varya_en http://varya.me
BEM it!
Introduction to BEM Methodology

2. Why bother?

3. There is no unified semantic model
across different FE technologies
● HTML stands for hypertext
I've heard we mostly do web apps...
● CSS offers no structure out of the box
Usually a pile of rules put together. Sorry.
● JavaScript uses its own approaches.
...a new one comes with every framework.

4. ● ≈ 8,500 packages in Bower registry
● JavaScript:
the most popular language on GitHub
Repositories created:
≈ 264,000 in 2013
≈ 296,000 in 2012
Frameworks are not enough

5. BEM to the rescue

6. What is BEM?
BEM claims that simple semantic model
(Blocks, Elements, and Modifiers)
is enough to define the way you author
HTML / CSS / JavaScript, structure code
and components, set up interaction
and scale your project to build
an industry-leading service.

7. What is BEM?
● BEM is a methodology, not a framework
Semantic model + best practices
for all things frontend
● BEM is a fix for web app semantics
...the same as jQuery is a fix for DOM APIs
● Originally introduced by Yandex
— 19 million daily audience
— 200+ web services
— tools, code, tutorials, conferences
— open source

8. Some theory

9. What is BEM?
BLOCK
– Standalone part of an interface:
● button
● text field
● flyout
● heading
● menu

10. What is BEM?
BLOCK
– Standalone part of an interface:
● button
● text field
● flyout
● heading
● menu
– Re-usable in different contexts
– Self-sufficient

11. What is BEM?
ELEMENT
– An integral part of a block:
● button
● text field
● flyout
● heading
● menu

12. What is BEM?
ELEMENT
– An integral part of a block:
● button — contains no elements
● text field label
● flyout title
● heading logo
● menu item

13. What is BEM?
ELEMENT
– An integral part of a block:
● button — contains no elements
● text field label
● flyout title
● heading logo
● menu item
– No standalone meaning outside of a block
– Some blocks have no elements

14. What is BEM?
MODIFIER
– Defines property or state on a block or
element:
● button
● text field
● flyout
● heading
● menu item

15. What is BEM?
MODIFIER
– Defines property or state on a block or
element:
● button theme
● text field editable state
● flyout alignment
● heading level
● menu item bullet type

16. What is BEM?
MODIFIER
– Defines property or state on a block or
element:
● button theme
● text field editable state
● flyout alignment
● heading level
● menu item bullet type
– Multiple modifiers may co-exist
on a single block/element

17. BEM forms a semantic overlay over
the existing DOM structure.
This overlay is called a BEM tree.

18. DOM tree → BEM tree

19. How does BEM map to DOM?
● Blocks/elems/mods are denoted
with CSS classes using a naming
convention.
● DOM nodes can be shared:
— block1 + block2 may occupy the same
container;
— element1 + block2 may co-exist on
the same node.
● DOM is encapsulated:
— complex DOM structure may constitute
a single element

20. BEM HOWTO
for your beloved project
with benefits explained

21. HOWTO: HTML / CSS

22. CSS naming conventions
“BEM uses CSS class names to
denote blocks, elements and
modifiers.”

23. CSS naming conventions
BLOCK
.button
.text-field
.flyout
.heading
.menu
or with prefix
.b-button
.b-text-field
.b-flyout
.b-heading
.b-menu

24. CSS naming conventions
<ul class=”menu”>
<li>
<a href=”/more”>Read More</a>
</li>
<li>
<a href=”/buy”>Buy Online</a>
</li>
<li>
<a href=”/buy”>Contact</a>
</li>
</ul>

25. CSS naming conventions
ELEMENT
.button__icon
.text-field__label
.flyout__title
.heading__logo
.menu__item

26. CSS naming conventions
<ul class=”menu”>
<li class=”menu__item”>
<a href=”/more”>Read More</a>
</li>
<li class=”menu__item”>
<a href=”/buy”>Buy Online</a>
</li>
<li class=”menu__item”>
<a href=”/buy”>Contact</a>
</li>
</ul>

27. CSS naming conventions
MODIFIER
.button_theme_dark
.text-field_editable
.flyout_align_top
.heading_level_alpha
.menu__item_promo

28. CSS naming conventions
MODIFIER
.button--theme--dark
.text-field--editable
.flyout--align--top
.heading--level--alpha
.menu__item--promo
as you wish

29. CSS naming conventions
<ul class=”menu”>
<li class=”menu__item”>
<a href=”/more”>Read More</a>
</li>
<li class=”menu__item”>
<a href=”/buy”>Buy Online</a>
</li>
<li class=”menu__item”>
<a href=”/buy”>Contact</a>
</li>
</ul>

30. CSS naming conventions
<ul class=”menu”>
<li class=”menu__item menu__item_promo”>
<a href=”/more”>Read More</a>
</li>
<li class=”menu__item”>
<a href=”/buy”>Buy Online</a>
</li>
<li class=”menu__item”>
<a href=”/buy”>Contact</a>
</li>
</ul>

31. so structure
much semantics
wow
much semantics
very code
such frontend

32. BEM CSS: best practices
1. Map the whole document to BEM blocks
2. No CSS outside of blocks
3. Independent blocks → no global CSS
resets

33. Benefits!
Drop tag names and IDs
● Faster selectors
● Re-use same semantics on any tag:
— <DIV class=”block”>
— <SPAN class=”block”>
— <TABLE class=”block”>

34. Benefits!
CSS specificity magic solved
Priority of CSS rules:
by specificity first, then by rule order
td.data { background-color: gray }
td.summary { background-color: white }
.total-summary { background-color: yellow }
<TD class="summary total-summary">
<!-- Still gray, baby :-( -->
</TD>

35. Benefits!
CSS specificity magic solved
Priority of CSS rules:
by specificity first, then by rule order
td.data { background-color: gray }
td.summary { background-color: white }
td.total-summary { background-color: yellow }
<TD class="summary total-summary">
<!-- This works, I'm yellow now -->
</TD>

36. Benefits!
Bye-bye CSS cascade?!
Only one CSS class needed to:
● style a block container
● style any element within a block
● add extras/overrides with a modifier
Doesn't it cover 90% of your styling
needs?

37. Benefits!
Bye-bye CSS cascade?!
...well, not exactly.
Example of an element affected by a block
modifier:
/* theme menu items for a dark theme */
.menu_theme_dark .menu__item
{
color: white;
background-color: darkgray;
}

38. HOWTO:
Block dependencies

39. LoginLoginpassword
Main
username
Download Help Contact

40. LoginLoginpassword
Main
username
Download Help Contact
headerheader
text inputtext input text inputtext input buttonbutton
menumenu

41. LoginLoginpassword
Main
username
Download Help Contact
_size_small _size_small _primary

42. LoginLoginpassword
Main
username
Download Help Contact
.header .input { font-size: 0.85em }
.header .button { background: navy }

43. LoginLoginpassword
Main
username
Download Help Contact
.header .input { font-size: 0.85em }
.header .button { background: navy } !

44. HOWTO: JavaScript

45. JavaScript
Components → Blocks
Work with BEM tree, not DOM tree

46. JavaScript deals with BEM
blockObj
blockObj.setMod('active');
// <div class=”block block_active”>
blockObj.delMod('active);
// <div class=”block”>

47. JavaScript deals with BEM
BlockObj.do({
'active': function() {
// do smth when active
},
'disabled': function() {
// do something when disabled
}
});

48. JavaScript
i-bem.js framework by Yandex +
tutorial
http://bit.ly/bem-js-tutorial/
● First English docs (expect more!)
● 100% BEM-based declarative API
● Part of a larger bem-core library

49. HTML is no longer semantic.
JavaScript is.

50. HOWTO: Design / UX

51. BEM is the universal language
for developers and designers,
the bridge across technology
gaps.

52. Build your block library.
The code itself is the styleguide.

53. UX + Frontend
● Live style guide
● Always up-to-date
● Prototyping mapped to code from
day one
● Designers and devs speak the
same language
● Good for making early estimates

54. HOWTO: File
structure

55. File and folder structure
Flat block structure with a folder for each block.
Simple structure for BEM beginners:
/block
block.css
block.js
block.tpl
...whatever you need

56. File and folder structure
Advanced structure to expose semantics
/block
/__elem1
block__elem1.css
block__elem1.tpl
/_mod
block_mod.css
block.css
block.js
block.tpl

57. Bundles for browsers
page.css:
@import url(“header/header.css”);
@import url(“button/button.css”);
@import url(“button/button_promo.css”);
page.js:
/* include: button.js */
/* include: login.js */

58. Build process and deployment
Use a build tool!
Borschik:
an open-source build tool by Yandex
Code:
https://github.com/bem/borschik
English docs:
http://bem.info/articles/borschik

59. http://bem.info

60. Thank you Booking!
@varya_en
http://varya.me