deps.js — a technology to declare dependencies in BEM

The process of building a page in different technologies is based on declaring BEM entities in a BEM tree.

There are two ways to form a page:

  • static — make a full page build and publish the result
  • dynamic — start with building all static files (for example, CSS, JS, templates), then create a dynamic BEM tree in runtime and apply templates to it.

During static page building, its BEM tree contains all BEM entities (except blocks without DOM representation and blocks which are expanded from within templates). If a block is defined in the BEM tree, the build process will build all the required technologies (such as CSS or JS) into common files, from all redefinition levels.

When forming a dynamic page, we have no BEM tree on the project build phase; that's why we have to declare all BEM entities which we might need.

In both cases, deps.js files are used to specify explicit dependencies on other blocks, elements, modifiers and technologies.

Let the b1.bemhtml.js template contain the following code:

block b1, content: [
        block: 'b2'
        block: 'b3'

To include styles and scripts for b2 and b3 blocks into the project, we have to create a b1.deps.js with the following contents:

    mustDeps: { block: 'b2' },
    shouldDeps: { block: 'b3' }

If elements and modifiers are standalone files on a file structure and not mentioned in the incoming BEMJSON file, they must be declared the same way.

deps.js syntax

To present a deps-entity in a deps.js file, use this syntax:

    /* deps-entity */

deps-entities in a file can be represented as an array if there is a dependency on a specific technology:

    /* deps-entity 1 */
    /* deps-entity 2 */

Full declaration of a deps-entity looks like this:

    block : 'bBlock',
    elem  : 'elem',
    mod   : 'modName',
    val   : 'modValue',
    tech  : 'techName',
    mustDeps   : [],
    shouldDeps : [],
    noDeps     : []

All of the deps-entity parameters are optional. The block, elem, mod, val, and tech parameters specify the entity to add a dependency for, while mustDeps, shouldDeps and noDeps define the dependency itself:

  • block (string) — block name
  • elem (string) — element name
  • mod (string) — modifier name
  • val (string) — block modifier value
  • tech (string) — technology you include dependencies for (for example, JS)
  • mustDeps (array or object) — defines dependencies that are guaranteed to be included into the resulting build (before the code of a block that originally defined those dependencies)
  • shouldDeps (array or object) — defines dependencies which can be included in any order
  • noDeps (array or object) — cancels a certain dependency (for example, i-bem__dom_init_auto).

As all the fields for the current entity can be read from its file name, the following two dependency definitions for a b1__e1_m1_v1.deps.js file are equivalent:

    block : 'b1',
    elem : 'e1',
    mod : 'm1',
    val : 'v1',
    mustDeps : { block : 'b2' }
    mustDeps : { block : 'b2' }

mustDeps, shouldDeps and noDeps parameters accept these BEM entities as values: block, elem, mods. Alternatively, you can use an extended syntax where elements and modifiers can accept an array:

  • elems (array) — allows to connect several block elements, as well as the block itself
  • mods (object) — an object with arrays as key values.

To declare dependencies on BEM entities which were missing during a build process, use 'mustDeps' or 'shouldDeps'. Build system will add these declarations to a deps.js bundle flat list which will be used as a basis to build all technologies on all redefinition levels.

'noDeps' cancels dependencies on lower redefinition levels.

Build details

In BEM methodology, deps.js is a technology and, as such, it conforms to unified technology building rules.

By default, dependency description file is located in the block folder; its name corresponds to a block name, with an extra '.deps.js' extension.

deps.js allows for defining dependencies on any block, element or modifier, in any technology.

Assume that we have four redefinition levels in our project: a bem-core library, common blocks, platform blocks and page blocks:


You should only point once to '{ block: button }' in a BEM tree for the build system to crawl all redefinition levels and include all the necessary files:

@import url(../../libs/bem-core/common.blocks/button/button.css);
@import url(../../common.blocks/button/button.css);
@import url(../../desktop.blocks/button/button.css);
@import url(blocks/button/button.css);

Building process is the same for any other required technology (for example, JS, templates, documentation).

Assume that the BEM tree is changed in runtime, and in the user's browser, a desktop.blocks/button block includes an e1 element from common.blocks/button block.

Here is how we could define this dependency in desktop.blocks/button/button.deps.js:

    shouldDeps : { block : 'button', elem : 'e1' }

After performing a build for the CSS technology, the following files will be included into the project:

@import url(../../libs/bem-core/common.blocks/button/button.css);
@import url(../../common.blocks/button/button.css);
@import url(../../desktop.blocks/button/button.css);
@import url(blocks/button/button.css);
@import url(../../common.blocks/button/__e1/button__e1.css);
@import url(../../desktop.blocks/button/__e1/button__e1.css);
@import url(blocks/button/__e1/button__e1.css);

According to specified dependencies, all the declared BEM entities are included from both upper and lower redefinition levels.

Examples of dependency declaration

Include an element only

elem includes only an element, not the block itself.

    block : 'b1',
    elem : 'e1'

Same applies to mod and val.

Include several elements

The elems behavior is somewhat different from elem, as it includes the block itself along with the specified elements.

    block : 'b1',
    elems : ['e1', 'e2']

The elems key may contain not only a name, but a full description of elements being included:

    block : 'b1',
    elems : [
        { elem : 'e1' },
        { elem : 'e2', mods : { m1 : 'v1' } }

Include dependencies for specific technologies

To include b1 block templates with _m1_v1 modifier as a part of a client-side JS build, use the following syntax to specify dependencies in a deps-entity:

    tech: 'js',
    mustDeps: [
            block: 'b1',
            mods: { m1: 'v1' },
            tech: 'bemhtml'
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