bemmed
v1.6.2
Published
BEM class name composer.
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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 className
s 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
$dollar
variables. - ✅
BEM
orBEMList
instances can be converted to a string by simply concatenating them with a string, using their.s
property or just calling.toString()
like any other JavaScript object. - ✅ Adding multiple modifiers, requesting the base with an element or modifier or
concatenating it results in a
BEMList
. This is a subclass ofArray
and renders as proper CSS classes separated by a space character. - ✅️ Acts like
block__element--modifier
by default. - ✅ The separators (
__
and--
) can be changed by creating a new class using thesetup()
function.
Usage
Importing in ES6:
import BEM from "bemmed";
or in CommonJS:
// require the named export:
const { BEM } = require("bemmed");
Example usage:
const cls = new BEM("block", "element", "modifier");
//=> BEM(b: "block", e: "element", m: "modifier")
// Convert to String:
String(cls);
//=> "block__element--modifier"
cls.toString();
//=> "block__element--modifier"
"" + cls;
//=> "block__element--modifier"
`${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