2.2.1 • Public • Published


    BEM helpers for jQuery

    Why you might need it

    BEM (Block, Element, Modifier) is a popular front-end methodology that introduces unified semantic entities across different frontend technologies such as CSS, HTML, JavaScript and templates.

    BEM is not tied to any particular implementation or framework. This jQuery plugin provides basic support for working with modifiers and setting up callbacks based on modifier change.

    Naming convention

    This plugin supports multiple BEM naming conventions.

    By default, it uses BEM naming convention introduced by Yandex:

    • CSS class names are used to denote BEM entities
    • Blocks with and without prefix are supported
    • Elements: block__element
    • Boolean modifiers: block_mod, block__element_mod
    • Key/value modifiers: block_modname_modval, block__element_modname_modval

    You can set alternative naming conventions by calling the $.BEMsyntax method:

    // to read the existing syntax
    var currentSyntax = $.BEMsyntax();
    // to set a new syntax
    var newSyntax = $.BEMsyntax({
        elem: '__',  // separator between block and element name
        modBefore: '--',  // separator between block/element and modifier name
        modKeyVal: '_'  // separator between modifier key and modifier value

    You can omit some properties if you inherit them from a previously defined scheme or default Yandex scheme. Changes are effective immediately and persist until you change them again.

    newSyntax contains an object describing the new syntax (all fields).


    .setMod sets a block modifier on each element of a jQuery collection, and returns the same collection.

    Set a boolean modifier on a block:

    .setMod(blockName, modName, true|false)

    Sets a boolean modifier modName for each block blockName in a provided jQuery collection. Third argument is the modifier value: true adds a modifier, false removes it.

    According to BEM, the DOM element that receives a modifier also needs the original block or element CSS class to be assigned (so, modifier classes are never used standalone). Currently, the library does not check or enforce this rule, but might start doing that in any future release.

    Example usage:

    $('.b-widget').setMod('b-widget', 'hidden', false);

    Set a boolean modifier on an element:

    .setMod(blockName, elemName, modName, true|false)

    Same as above, but the second argument is an element name.

    Set a key/value modifier on a block:

    Key/value modifiers are different from boolean modifiers in that they have distinct string values. Values follow the same limitations as modifier names do; namely, they cannot contain underscores and should comply with general limitations of CSS class names.

    .setMod(blockName, modName, modVal)

    modVal should not be a boolean value, otherwise it will be treated as a boolean modifier.

    Set a key/value modifier on an element:

    .setMod(blockName, elemName, modName, modVal)

    modVal should not be a boolean value.


    This is a simple API to read modifier values. It only reads the first element in a jQuery collection, ignoring all the rest, and returns a primitive value based on what it finds.

    For blocks:

    .getMod(blockName, modName)

    For elements:

    .getMod(blockName, elemName, modName)

    It returns boolean true for a boolean modifier that exists, a String modifier value for a key/value modifier that exists, and boolean false for a modifier that was not found (as it is not there, there is no way to determine whether we're dealing with a boolean or key/value modifier).

    For an empty jQuery collection, the method would return undefined. This might be an inconsistency; I may change it later.


    .hasMod checks whether the first item in a jQuery collection has a modifier set on a specified block/element.

    Returns a boolean value. For key/value modifiers, return true if any modifier value is present, regardless of the actual value.

    For blocks:

    var hasMod = $('.b-block').hasMod('b-block', 'expanded');

    For elements:

    var hasMod = $('.b-block__elem').hasMod('b-block', 'elem', 'expanded');

    Event handlers

    For each modifier being set, a jQuery setMod event is being triggered that propagates up the DOM tree. It also contains metadata that lets you get additional information. This allows for callbacks being assigned to modifier changes.

    Subscribing to setMod events

    For each modifier being set, a custom jQuery event fires which unique name is formed according to a pattern that is described below. The event also contains metadata you can read from the second argument of an event handler.

    For all modifiers (both boolean and key/value pairs), the custom event is formed like this:

    setMod:block_modName_* // for blocks setMod:block__elem_modName_* // for elements

    If you define a custom BEM syntax with $.BEMsyntax() method, you should adjust your event name patterns accordingly. Events being triggered always use the current syntax.

    An asterisk * at the end of an event name means it's triggered for all modifier values. An object passed as a second argument to an event handler contains additional keys:

    • block — block name
    • elem — element name (undefined if modifier is set on a block level)
    • modName — modifier name
    • modVal — modifier value that is set: a boolean true/false for boolean modifiers, a String value for key/value modifiers

    For key/value modifier only, and additional event is triggered which is specific to a modifier value being set. Its custom name is formed like this:

    setMod:block_modName_modVal // for blocks setMod:block__elem_modName_modVal // for elements


    $(document).on('setMod:widget_init_*', function(e, data) {
        console.log('Block name:', data.block);
        console.log('Element name:', data.elem);  // undefined, as it's a block-level modifier
        console.log('Modifier name:', data.modName);
        console.log('Modifier value:', data.modVal);
    $('div.widget').setMod('widget', 'init', true);
    /* Console output:
    >  Block name: widget
    >  Element name: undefined
    >  Modifier name: init
    >  Modifier value: true


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