classnames

What is classnames?

The classnames package is a simple utility for conditionally joining class names together. It is commonly used in React projects but can be used anywhere that requires dynamic class name computation.

What are classnames's main functionalities?

String arguments

Combine multiple string arguments into one class name.

classnames('foo', 'bar'); // 'foo bar'

Object arguments

Use an object to include class names conditionally based on the truthiness of the object's values.

classnames({ 'foo': true, 'bar': false }); // 'foo'

Array arguments

Pass an array of class names which will be joined together.

classnames(['foo', 'bar']); // 'foo bar'

Mixed arguments

Combine string, object, and array arguments to form a class name string.

classnames('foo', { 'bar': true, 'duck': false }, ['baz', { 'quux': true }]); // 'foo bar baz quux'

Other packages similar to classnames

clsx

clsx is a tiny utility for constructing className strings conditionally. It is similar to classnames but is smaller and faster, making it a popular alternative.

emotion

Emotion is a performant and flexible CSS-in-JS library. While it is more than a class name utility, it provides similar functionalities in terms of composing and applying class names conditionally.

README

Classnames

A simple JavaScript utility for conditionally joining classNames together.

Install from the npm registry with your package manager:

npm install classnames

Use with Node.js, Browserify, or webpack:

const classNames = require('classnames');
classNames('foo', 'bar'); // => 'foo bar'

Alternatively, you can simply include index.js on your page with a standalone <script> tag and it will export a global classNames method, or define the module if you are using RequireJS.

Project philosophy

We take the stability and performance of this package seriously, because it is run millions of times a day in browsers all around the world. Updates are thoroughly reviewed for performance implications before being released, and we have a comprehensive test suite.

Classnames follows the SemVer standard for versioning.

There is also a Changelog.

Usage

The classNames function takes any number of arguments which can be a string or object. The argument 'foo' is short for { foo: true }. If the value associated with a given key is falsy, that key won't be included in the output.

classNames('foo', 'bar'); // => 'foo bar'
classNames('foo', { bar: true }); // => 'foo bar'
classNames({ 'foo-bar': true }); // => 'foo-bar'
classNames({ 'foo-bar': false }); // => ''
classNames({ foo: true }, { bar: true }); // => 'foo bar'
classNames({ foo: true, bar: true }); // => 'foo bar'

// lots of arguments of various types
classNames('foo', { bar: true, duck: false }, 'baz', { quux: true }); // => 'foo bar baz quux'

// other falsy values are just ignored
classNames(null, false, 'bar', undefined, 0, 1, { baz: null }, ''); // => 'bar 1'

Arrays will be recursively flattened as per the rules above:

const arr = ['b', { c: true, d: false }];
classNames('a', arr); // => 'a b c'

Dynamic class names with ES2015

If you're in an environment that supports computed keys (available in ES2015 and Babel) you can use dynamic class names:

let buttonType = 'primary';
classNames({ [`btn-${buttonType}`]: true });

Usage with React.js

This package is the official replacement for classSet, which was originally shipped in the React.js Addons bundle.

One of its primary use cases is to make dynamic and conditional className props simpler to work with (especially more so than conditional string manipulation). So where you may have the following code to generate a className prop for a <button> in React:

import React, { useState } from 'react';

export default function Button (props) {
	const [isPressed, setIsPressed] = useState(false);
	const [isHovered, setIsHovered] = useState(false);

	let btnClass = 'btn';
	if (isPressed) btnClass += ' btn-pressed';
	else if (isHovered) btnClass += ' btn-over';

	return (
		<button
			className={btnClass}
			onMouseDown={() => setIsPressed(true)}
			onMouseUp={() => setIsPressed(false)}
			onMouseEnter={() => setIsHovered(true)}
			onMouseLeave={() => setIsHovered(false)}
		>
			{props.label}
		</button>
	);
}

You can express the conditional classes more simply as an object:

import React, { useState } from 'react';
import classNames from 'classnames';

export default function Button (props) {
	const [isPressed, setIsPressed] = useState(false);
	const [isHovered, setIsHovered] = useState(false);

	const btnClass = classNames({
		btn: true,
		'btn-pressed': isPressed,
		'btn-over': !isPressed && isHovered,
	});

	return (
		<button
			className={btnClass}
			onMouseDown={() => setIsPressed(true)}
			onMouseUp={() => setIsPressed(false)}
			onMouseEnter={() => setIsHovered(true)}
			onMouseLeave={() => setIsHovered(false)}
		>
			{props.label}
		</button>
	);
}

Because you can mix together object, array and string arguments, supporting optional className props is also simpler as only truthy arguments get included in the result:

const btnClass = classNames('btn', this.props.className, {
	'btn-pressed': isPressed,
	'btn-over': !isPressed && isHovered,
});

Alternate dedupe version

There is an alternate version of classNames available which correctly dedupes classes and ensures that falsy classes specified in later arguments are excluded from the result set.

This version is slower (about 5x) so it is offered as an opt-in.

To use the dedupe version with Node.js, Browserify, or webpack:

const classNames = require('classnames/dedupe');

classNames('foo', 'foo', 'bar'); // => 'foo bar'
classNames('foo', { foo: false, bar: true }); // => 'bar'

For standalone (global / AMD) use, include dedupe.js in a <script> tag on your page.

Alternate bind version (for css-modules)

If you are using css-modules, or a similar approach to abstract class 'names' and the real className values that are actually output to the DOM, you may want to use the bind variant.

Note that in ES2015 environments, it may be better to use the "dynamic class names" approach documented above.

const classNames = require('classnames/bind');

const styles = {
	foo: 'abc',
	bar: 'def',
	baz: 'xyz',
};

const cx = classNames.bind(styles);

const className = cx('foo', ['bar'], { baz: true }); // => 'abc def xyz'

Real-world example:

/* components/submit-button.js */
import { useState } from 'react';
import classNames from 'classnames/bind';
import styles from './submit-button.css';

const cx = classNames.bind(styles);

export default function SubmitButton ({ store, form }) {
  const [submissionInProgress, setSubmissionInProgress] = useState(store.submissionInProgress);
  const [errorOccurred, setErrorOccurred] = useState(store.errorOccurred);
  const [valid, setValid] = useState(form.valid);

  const text = submissionInProgress ? 'Processing...' : 'Submit';
  const className = cx({
    base: true,
    inProgress: submissionInProgress,
    error: errorOccurred,
    disabled: valid,
  });

  return <button className={className}>{text}</button>;
}

Polyfills needed to support older browsers

classNames >=2.0.0

Array.isArray: see MDN for details about unsupported older browsers (e.g. <= IE8) and a simple polyfill.

LICENSE MIT

Copyright (c) 2018 Jed Watson. Copyright of the Typescript bindings are respective of each contributor listed in the definition file.