BEM helper functions for resolving and joining blocks to elements, blocks to modifiers and elements to modifiers.
Last updated 4 years ago by jedmao .
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BEM helper functions for resolving and joining blocks to elements, blocks to modifiers and elements to modifiers.


What is BEM? See MindBEMding – getting your head ’round BEM syntax for a primer.

These BEM helper functions help you to join block names to elements and any number of modifiers. Modifers are defined as objects where the keys are the modifier names and the values determine whether the modifier name is activated or not.


$ npm install bem-helpers

`BEMBlock( blockName [, options] )

Returns a function that can be used to construct BEM class names. For example, in React, you might have a component that supports a dark theme as well as an expandable content area. This could be achieved like so:

const b = BEMBlock('section')
export const Section = ({ heading, isDark, isExpanded, children }) => (
  <section className={b({ dark: isDark })}>
    <h1 className={b('heading')}>
    <div className={b('body', { expanded: isExpanded })}>

If isDark and isExpanded props were both truthy, the result would be the following HTML:

<section class="section section--dark">
  <h1 class="section__heading">
  <div class="section__body section__body--expanded">

Of course, if isDark and isExpanded were false, no --dark or --expanded modifiers would be constructed for them.


If you want more control over the separators that are used to construct BEM selectors, the following options are available to you:


A string that stands between the block and element names (i.e., the __ in foo__bar). The default value is __.


A string that separates a block or element from a modifier (i.e., the -- in foo--bar or foo__bar--baz). The default value is --.

joinBEMElement( block, element [, separator] )

Joins a BEM block to an element.

joinBEMElement('foo', 'bar');
// "foo__bar"

joinBEMElement('foo', 'bar', '__custom__');
// "foo__custom__bar"

joinBEMModifiers( blockOrElement, modifiers [, separator] )

Joins a BEM block or element to any number of modifiers.

joinBEMModifiers('foo__bar', { baz: true, qux: true });
// "foo__bar foo__bar--baz foo__bar--qux"

joinBEMModifiers('foo', { bar: true }, '--custom--');
// foo foo--custom--bar

See the tests for more examples.


Run the following command:

$ npm test

This will build scripts, run tests and generate a code coverage report. Anything less than 100% coverage will throw an error.


For much faster development cycles, run the following commands in 2 separate processes:

$ npm run build:watch

Compiles TypeScript source into the ./dist folder and watches for changes.

$ npm run watch

Runs the tests in the ./dist folder and watches for changes.

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