The BEM methodology and JavaScript

In the BEM methodology, a web interface is built from independent blocks, which may have elements. Both blocks and elements can have states or characteristics described by modifiers.

A web interface uses various technologies (HTML, CSS, JS, and others). Its implementation is divided into components by block. A block contains a set of technology files that represent aspects of its implementation:

  • my-block.css — The block appearance.
  • my-block.bemhtml — Templates for generating HTML representations of the block.
  • my-block.js — The block dynamic behavior in the browser.

The i-bem.js framework allows us to break down the client JavaScript into components in BEM terms:

  • Block — The JS component that describes the logic of same-type interface elements. For example, all buttons can be implemented as a button block. In this case, button.css defines how all buttons look, and button.js defines how they work. Each page can have more than one block instance (such as buttons). Each block instance corresponds to a JS object in the browser memory that stores its state. The JS object contains a reference to the DOM node that this block instance is bound to.
  • Elements — DOM nodes nested in the block DOM node, with the class attribute pointing to their role in the BEM subject domain (the name of the block and element). Block elements are accessible via the block instance [JS-API] [dom].
  • Modifiers — Provide information about the state of a block and its elements. The state of modifiers is written in the class attribute on the DOM nodes of a block and elements. Modifiers are controlled using a block instance JS-API.


In the BEM methodology, development is modular — each block is programmed separately. The final source code of web pages is generated from the code of individual blocks using assembly procedures.

In the file system, it is convenient to represent a block as a directory, and the block implementation in each of the technologies as a separate file:



For each web page, the code of the blocks used on it can be put in the same types of files:


There are two tools that support the BEM subject domain for assembling separate block descriptions into the code of resulting web pages:

Both tools automate the creation of HTML markup for binding JS blocks and passing parameters to a block instance.

Why i-bem.js is named this way

According to the BEM methodology, the base JS library of the BEM platform was originally developed as a special service block. This approach allows us to work with base libraries the same way as with regular blocks. In particular, it allows us to structure code in terms of elements and modifiers and flexibly configure the library behavior on various redefinition levels.

Service blocks in BEM were conventionally given names with the i- prefix. Thus, the name i-bem.js is read as an implementation of the i-bem block in the JS technology.

How to use i-bem.js

The i-bem.js framework is a part of the bem-core library.

The implementation of i-bem.js consists of two modules:

  • The i-bem module. Base implementation of the i-bem JS block, which all the blocks in i-bem.js inherit from. The i-bem block is written for use in any of the JS environments: both on the client and on the server (for example, in Node.js).
  • The i-bem__dom module. The base implementation of a block bound to a DOM node. Intended for use on the client, and relies on browsers working with DOM. Depends on jQuery.


  • jQuery (only for the i-bem__dom module). When using bem-core, separate installation of jQuery is not necessary.
  • The ym/modules module system. When using bem-tools with .browser.js technology (and derivatives of it), this dependency is satisfied automatically.

You can use i-bem.js as a part of the full stack of BEM tools. In this case, it is convenient to base your project on the project-stub template repository, where automatic installation of dependent libraries and assembly is set up.

If you aren't planning to use other technologies of the BEM platform, you can just put the bem-core library code in an existing project.

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