BEMMED

JavaScript reusable composer class for BEM (Block Element Modifier) CSS classes.

Installation

npm install bemmed

The problem solved

Using BEM notation usually involves a lot of repetition. The name of the block gets repeated for every element and when adding a modifier, it is not uncommon that the className of an element gets quite long and unwieldy.

The goal of this package is to be able to create a reusable object which can be used to build every possible BEM class we want without any repetition of its parts.

The old way:

Here an example of a simple component using JSX:

function Profile({theme, collapsed, avatar, name}) {
  return (
    <article className={"profile profile--" + theme}>
      <header className="profile__header profile__header--with-avatar profile__header--extra-space">
        <h2
          className={
            "profile__title" +
            (collapsed ? " profile__title--small" : "") +
            (theme === "dark" ? " profile__title--inverted" : "")
          }
        >
          {name}
        </h2>
        <figure
          className={
            "profile__avatar" + (collapsed ? " profile__avatar--collapsed" : "")
          }
        >
          <img src={avatar} alt={name} />
        </figure>
      </header>
      ...
    </article>
  );
}

Results in 521b of code when transpiled with babel and then minified with terser.

This is not an uncommon pattern. Ok, some modifiers are a bit weird, but we've all seen worse right? As you can see, there is quite a lot of repetition. The word profile is used 10 times in className attributes, not to mention the header, title and avatar elements.

Usually I would add newlines inside the className to make the ternary operators more readable or compose the classNames outside the JSX, but we want roughly the same number of lines in both examples.

The new way:

// we can pre-initialize the classes used in this component, we don't have to but we can.
const [profileCls, headerCls, titleCls, avatarCls] = new BEM("profile").withElem(
  "header",
  "title",
  "avatar"
);

function Profile({theme, collapsed, avatar, name}) {
  return (
    <article className={profileCls}>
      <header className={headerCls.withMod("with-avatar", "extra-space")}>
        <h2
          className={titleCls.withMod({small: collapsed, inverted: theme === "dark"})}
        >
          {name}
        </h2>
        <figure className={avatarCls.withMod({collapsed})}>
          <img src={avatar} alt={name} />
        </figure>
      </header>
      ...
    </article>
  );
}

Results in 452b of code when transpiled with babel and then minified with terser.

That's 69 bytes (-14%) and we gained the ability to reuse the classes allowing even greater benefits.

As you can see in the example above, there is no repetition. The className variables can be easily minified and mangled. Each BEM instance stays reusable, so it can be modified later which you can see when we use withMod inside the component based on its props.

Features

• ✅ 1.8 kB minified / 837 B minified+gzipped (see: Bundlephobia for the latest numbers)
• ✅ BEM instances are reusable and can be modified.
• ✅ BEM methods are plain or short english, no letters.
• ✅ Methods to ease the creation of multiple classes without duplication.
• ✅ Arguments can be passed consistently without specific syntax requirements such as $dollar variables.
• ✅ BEM or BEMList instances can be converted to a string by simply concatenating them with a string, using their .s property or just calling .toString() like any other JavaScript object.
• ✅ Adding multiple modifiers, requesting the base with an element or modifier or concatenating it results in a BEMList. This is a subclass of Array and renders as proper CSS classes separated by a space character.
• ✅️ Acts like block__element--modifier by default.
• ✅ The separators (__ and --) can be changed by creating a new class using the setup() function.

Usage

Importing in ES6:

import BEM from "bemmed";

or in CommonJS:

// require the named export:
const {BEM} = require("bemmed");

// or as default:
const BEM = require("bemmed");

Example usage:

const cls = new BEM("block", "element", "modifier");
//=> BEM(b: "block", e: "element", m: "modifier")

// Convert to String:
String(cls);
//=> "block__element--modifier"
cls.toString();
//=> "block__element--modifier"
"" + cls;
//=> "block__element--modifier"
`${cls}`;
//=> "block__element--modifier"
cls.s;
//=> "block__element--modifier"

// Use in JSX:
<div className={cls}>x</div>;
//=> <div class="block__element--modifier">x</div>

// Just a block
String(new BEM("block"));
//=> "block"

// Block with element
String(new BEM("block", "element"));
//=> "block__element"

// Set modifier
const modified = cls.modifier("mod2"); // aliased as .mod()
String(modified);
//=> "block__element--mod2"

// Modifications return a new instance, original is unmodified:
String(cls);
//=> "block__element--modifier"

// Set multiple modifiers
String(cls.mod("mod-a", "mod-b", "mod-c"));
//=> "block__element--mod-a block__element--mod-b block__element--mod-c"

// Modify using an object, only keys of truthy values are applied:
String(
  cls.mod({
    foo: true,
    bar: false,
    "foo-bar": "yes",
  })
);
//=> "block__element--foo block__element--foo-bar"

// New instance with another element
const newElement = cls.element("el2"); // aliased as elem()
String(newElement);
//=> "block__el2--modifier"

// New instance with another element and modifiers
const newElementWithMod = cls.element("el3", "modifier");
String(newElementWithMod);
//=> "block__el3 block__el3--modifier"

// Combine the class with a modified variant
const withMod = new BEM("block", "element").withMod("modifier");
//=> BEMList<[BEM(b: "block", e: "element", m: null), BEM(b: "block", e: "element", m: "modifier")]>
String(withMod);
//=> "block__element block__element--modifier"

// Get several elements (useful to pre-generate reusable classes):
const block = new BEM("block");
const [header, body, footer] = block.elements("header", "body", "footer");
String(header);
//=> "block__header"
String(body);
//=> "block__body"
String(footer);
//=> "block__footer"

// Concatenate with multiple strings, Array's, BEM instances or BEMList's
String(
  new BEM("block").concat(
    "just-a-string", // String
    new BEM("b", "e", "m"), // BEM instance
    new BEM("foo").withMod("bar") // BEMList
  )
);
//=> "block just-a-string b__e--m foo foo--bar"

// Create a custom class with modified separators using the setup function.
import {setup} from "bemmed";

const UnderBEM = setup({
  // or just name it `BEM`.
  elementSeparator: "_",
  modifierSeparator: "__",
});
new UnderBEM("block", "element", "modifier").toString();
//=> "block_element__modifier"

// Export this custom BEM class and import it in your application from here.

API

Creating a new instance

Create a new instance. Usually only with a block

const cls = new BEM(block: string[ element: string[ modifier: string]]): BEM

Setting the element or modifier parts

Using the element() (or elem() alias) method returns a new BEM instance with the provided element part. When adding modifiers, a BEMList is returned as if .withMod() was used after adding the element.

cls.element(element: string, ...modifiers: string): BEM|BEMList
cls.elem(element: string, ...modifiers: string): BEM|BEMList

Using the modifier() (or mod() alias) method returns a new BEM instance with the provided modifier part. When given multiple modifiers, a BEMList is returned.

cls.modifier(...modifiers: string): BEM|BEMList
cls.mod(...modifiers: string): BEM|BEMList

Adding an element

To create a block together with one or more elements

cls.withElem(...elements: string): BEMList

new BEM("block").withElem("foo", "bar").toString();
//=> "block block__foo block__bar"

Very useful when destructuring:

const [tableClass, rowClass, cellClass] = new BEM("table").withElem("row", "cell");

Adding a modifier

Usually you want to output a base class and the modifier class. Returns a new BEMList with BEM instances for each part.

cls.withMod(...modifiers: string|Object): BEMList

new BEM("block")
  .withMod("always-add-this", {
    "and-this": true,
    "but-not-this": false,
  })
  .toString();
//=> "block block--always-add-this block--and-this"

A BEMList is just a subclass of Array with a modified toString() method, so it renders as a proper className with spaces between the classes.

Creating multiple elements

Pre-initializing a set of elements for a block is also a common use case. Returns a new BEMList with BEM instances for the given elements.

cls.elements(...element: string): BEMList

new BEM("block").elements("foo", "bar").toString();
//=> "block__foo block__bar"

Combining or concatenating classes

Same method as Array.concat. Returns a new BEMList with the items appended. Remember: Arrays will be flattened! Also removes any duplicates from the given arguments and works on both BEM and BEMList instances.

cls.concat(...items: any): BEMList

new BEM("b1")
  .concat(new BEM("b2"), "just-a-string", ["array", "of", "items"])
  .toString();
// => "b1 b2 just-a-string array of items"

Converting to a string

Returns a string with the class names separated by spaces.

const cls = new BEM("block", "element", "modifier");
cls.toString();
//=> "block__element--modifier"

// or use the getter:
cls.s;
//=> "block__element--modifier"

The s getter is a shorthand for toString() and is especially useful if using a BEM instance causes inspection errors. Although most projects don't mind a BEM instance as a value for className at all.

const cls = new BEM("block", "element", "modifier");
return (
    <div className={cls.s}>Bemmed + JSX = 😀</div>
);
//=> JSX: <div class="block__element--modifier">Bemmed + JSX = 😀</div>

Customizing separators

Use the setup() function to create a customized BEM class.

The function takes an object literal which can contain the following properties:

Property	Default	Description
elementSeparator	"__"	Separator string between the block and element part.
modifierSeparator	"--"	Separator string between the element and modifier part.

Create a module in your project e.g. utils/bem.js.

import {setup} from "bemmed";
export const BEM = setup({
  elementSeparator: "_",
  modifierSeparator: "__",
});

// This would produce classes like "block_element__modifier"

Then in your project just import BEM from that module:

import {BEM} from "./utils/bem";
// BEM is now your customized version that only uses underscores.

PropTypes

BEM provides several PropTypes to ease the usage with libraries such as React. All propTypes also support an .isRequired property to allow undefined values.

Import the prop types from bemmed/proptypes:

// exported as propTypes:
import {propTypes} from "bemmed/proptypes";
// but also available as default export for easier renaming
import BEMTypes from "bemmed/proptypes";

PropType	Description
propTypes.bem	Valid BEM or BEMList instances.
propTypes.className	Utility PropType for checking any valid value which can be used in a className attribute in JSX (including BEM or BEMList instances).
propTypes.element	Valid value for BEM.element().
propTypes.modifier	Valid value for BEM.modifier().

FAQ

Why bother, doesn't gzip solve this already?

Yes it does help in some cases, but it can never yield the same results and does not give any of the benefits such as reusable objects, readability and ease of development.

Improved readability? I find it harder to read

I can imagine never seeing a fully written className can be harder to read at first, but I got used to it quite fast. The habit of destructuring BEM element classes into separate variables makes it more clear what each className is for.

For example, I find className={titleClass.withMod({inverted: myInvertedState})} much easier to read than className={"header__title" + (myInvertedState ? " header__title--inverted" : ""). And I think we've all seen worse classes with more inline logic than that.

Don't forget about the benefits of keeping your code DRY by not repeating the same base class over and over again. This makes changes in your code much easier to make and modifying the class based on a condition is baked in, so no need to use a ternary operator or helper functions.

Now my IDE can't find the usages of a specific class!

True, but are your classes that scattered throughout your application? Also, if that's the case then reusing a BEM instance for that class could help you by simply looking for the usages for that instance instead of searching for the css class string.

Developing

• Build with npm run build
• Run tests with npm run test.

LICENSE

MIT