tailwind-merge
Merge Tailwind CSS classes without style conflicts
Last updated 2 years ago by dcas .
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tailwind-merge

tailwind-merge

Utility function to efficiently merge Tailwind CSS classes in JS without style conflicts.

import { twMerge } from 'tailwind-merge'

twMerge('px-2 py-1 bg-red hover:bg-dark-red', 'p-3 bg-[#B91C1C]')
// → 'hover:bg-dark-red p-3 bg-[#B91C1C]'

What is it for

If you use Tailwind with a component-based UI renderer like React or Vue, you're probably familiar with the situation that you want to change some styles of a component, but only in one place.

import React from 'react'

function MyGenericInput(props) {
    const className = `border rounded px-2 py-1 ${props.className || ''}`
    return <input {...props} className={className} />
}

function MySlightlyModifiedInput(props) {
    return (
        <MyGenericInput
            {...props}
            className="p-3" // ← Only want to change some padding
        />
    )
}

When the MySlightlyModifiedInput is rendered, an input with the className border rounded px-2 py-1 p-3 gets created. But because of the way the CSS cascade works, the styles of the p-3 class are ignored. The order of the classes in the className string doesn't matter at all and the only way to apply the p-3 styles is to remove both px-2 and py-1.

This is where tailwind-merge comes in.

function MyGenericInput(props) {
    // ↓ Now `props.className` can override conflicting classes
    const className = twMerge('border rounded px-2 py-1', props.className)
    return <input {...props} className={className} />
}

tailwind-merge overrides conflicting classes and keeps everything else untouched. In the case of the MySlightlyModifiedInput, the input now only renders the classes border rounded p-3.

Features

Optimized for speed

  • Results get cached by default, so you don't need to worry about wasteful re-renders. The library uses a LRU cache which stores up to 500 different results. The cache size can be modified or opt-out of by using extendTailwindMerge.
  • Expensive computations happen upfront so that twMerge calls without a cache hit stay fast.
  • These computations are called lazily on the first call to twMerge to prevent it from impacting app startup performance if it isn't used initially.

Last conflicting class wins

twMerge('p-5 p-2 p-4') // → 'p-4'

Allows refinements

twMerge('p-3 px-5') // → 'p-3 px-5'
twMerge('inset-x-4 right-4') // → 'inset-x-4 right-4'

Resolves non-trivial conflicts

twMerge('inset-x-px -inset-1') // → '-inset-1'
twMerge('bottom-auto inset-y-6') // → 'inset-y-6'
twMerge('inline block') // → 'block'

Supports modifiers and stacked modifiers

twMerge('p-2 hover:p-4') // → 'p-2 hover:p-4'
twMerge('hover:p-2 hover:p-4') // → 'hover:p-4'
twMerge('hover:focus:p-2 focus:hover:p-4') // → 'focus:hover:p-4'

Supports arbitrary values

twMerge('bg-black bg-[color:var(--mystery-var)]') // → 'bg-[color:var(--mystery-var)]'
twMerge('grid-cols-[1fr,auto] grid-cols-2') // → 'grid-cols-2'

Supports arbitrary properties

twMerge('[mask-type:luminance] [mask-type:alpha]') // → '[mask-type:alpha]'
twMerge('[--scroll-offset:56px] lg:[--scroll-offset:44px]')
// → '[--scroll-offset:56px] lg:[--scroll-offset:44px]'

// Don't do this!
twMerge('[padding:1rem] p-8') // → '[padding:1rem] p-8'

Watch out for mixing arbitrary properties which could be expressed as Tailwind classes. tailwind-merge does not resolve conflicts between arbitrary properties and their matching Tailwind classes to keep the bundle size small.

Supports arbitrary variants

twMerge('[&:nth-child(3)]:py-0 [&:nth-child(3)]:py-4') // → '[&:nth-child(3)]:py-4'
twMerge('dark:hover:[&:nth-child(3)]:py-0 hover:dark:[&:nth-child(3)]:py-4')
// → 'hover:dark:[&:nth-child(3)]:py-4'

// Don't do this!
twMerge('[&:focus]:ring focus:ring-4') // → '[&:focus]:ring focus:ring-4'

Similarly to arbitrary properties, tailwind-merge does not resolve conflicts between arbitrary variants and their matching predefined modifiers for bundle size reasons.

Supports important modifier

twMerge('!p-3 !p-4 p-5') // → '!p-4 p-5'
twMerge('!right-2 !-inset-x-1') // → '!-inset-x-1'

Preserves non-Tailwind classes

twMerge('p-5 p-2 my-non-tailwind-class p-4') // → 'my-non-tailwind-class p-4'

Supports custom colors out of the box

twMerge('text-red text-secret-sauce') // → 'text-secret-sauce'

Ignores undefined, null and false values

twMerge('some-class', undefined, null, false) // → 'some-class'

Basic usage

If you're using Tailwind CSS without any extra config, you can use twMerge right away. You can safely stop reading the documentation here.

Usage with custom Tailwind config

If you're using a custom Tailwind config, you may need to configure tailwind-merge as well to merge classes properly.

The default twMerge function is configured in a way that you can still use it if all the following points apply to your Tailwind config:

  • Only using color names which don't clash with other Tailwind class names
  • Only deviating by number values from number-based Tailwind classes
  • Only using font-family classes which don't clash with default font-weight classes
  • Sticking to default Tailwind config for everything else

If some of these points don't apply to you, you can test whether twMerge still works as intended with your custom classes. Otherwise, you need create your own custom merge function by either extending the default tailwind-merge config or using a completely custom one.

The tailwind-merge config is different from the Tailwind config because it's expected to be shipped and run in the browser as opposed to the Tailwind config which is meant to run at build-time. Be careful in case you're using your Tailwind config directly to configure tailwind-merge in your client-side code because that could result in an unnecessarily large bundle size.

Shape of tailwind-merge config

The tailwind-merge config is an object with a few keys.

const tailwindMergeConfig = {
    // ↓ Set how many values should be stored in cache.
    cacheSize: 500,
    // ↓ Optional prefix from TaiLwind config
    prefix: 'tw-',
    theme: {
        // Theme scales are defined here
        // This is not the theme object from your Tailwind config
    },
    classGroups: {
        // Class groups are defined here
    },
    conflictingClassGroups: {
        // Conflicts between class groups are defined here
    },
}

Class groups

The library uses a concept of class groups which is an array of Tailwind classes which all modify the same CSS property. E.g. here is the position class group.

const positionClassGroup = ['static', 'fixed', 'absolute', 'relative', 'sticky']

tailwind-merge resolves conflicts between classes in a class group and only keeps the last one passed to the merge function call.

twMerge('static sticky relative') // → 'relative'

Tailwind classes often share the beginning of the class name, so elements in a class group can also be an object with values of the same shape as a class group (yes, the shape is recursive). In the object each key is joined with all the elements in the corresponding array with a dash (-) in between.

E.g. here is the overflow class group which results in the classes overflow-auto, overflow-hidden, overflow-visible and overflow-scroll.

const overflowClassGroup = [{ overflow: ['auto', 'hidden', 'visible', 'scroll'] }]

Sometimes it isn't possible to enumerate all elements in a class group. Think of a Tailwind class which allows arbitrary values. In this scenario you can use a validator function which takes a class part and returns a boolean indicating whether a class is part of a class group.

E.g. here is the fill class group.

const isArbitraryValue = (classPart: string) => /^\[.+\]$/.test(classPart)
const fillClassGroup = [{ fill: ['current', isArbitraryValue] }]

Because the function is under the fill key, it will only get called for classes which start with fill-. Also, the function only gets passed the part of the class name which comes after fill-, this way you can use the same function in multiple class groups. tailwind-merge exports its own validators, so you don't need to recreate them.

You can use an empty string ('') as a class part if you want to indicate that the preceding part was the end. This is useful for defining elements which are marked as DEFAULT in the Tailwind config.

// ↓ Resolves to filter and filter-none
const filterClassGroup = [{ filter: ['', 'none'] }]

Each class group is defined under its ID in the classGroups object in the config. This ID is only used internally, and the only thing that matters is that it is unique among all class groups.

Conflicting class groups

Sometimes there are conflicts across Tailwind classes which are more complex than "remove all those other classes when a class from this group is present in the class list string".

One example is the combination of the classes px-3 (setting padding-left and padding-right) and pr-4 (setting padding-right).

If they are passed to twMerge as pr-4 px-3, I think you most likely intend to apply padding-left and padding-right from the px-3 class and want pr-4 to be removed, indicating that both these classes should belong to a single class group.

But if they are passed to twMerge as px-3 pr-4, I assume you want to set the padding-right from pr-4 but still want to apply the padding-left from px-3, so px-3 shouldn't be removed when inserting the classes in this order, indicating they shouldn't be in the same class group.

To summarize, px-3 should stand in conflict with pr-4, but pr-4 should not stand in conflict with px-3. To achieve this, we need to define asymmetric conflicts across class groups.

This is what the conflictingClassGroups object in the tailwind-merge config is for. You define a key in it which is the ID of a class group which creates a conflict and the value is an array of IDs of class group which receive a conflict.

const conflictingClassGroups = {
    px: ['pr', 'pl'],
}

If a class group creates a conflict, it means that if it appears in a class list string passed to twMerge, all preceding class groups in the string which receive the conflict will be removed.

When we think of our example, the px class group creates a conflict which is received by the class groups pr and pl. This way px-3 removes a preceding pr-4, but not the other way around.

Theme

In the Tailwind config you can modify theme scales. tailwind-merge follows the same keys for the theme scales, but doesn't support all of them. tailwind-merge only supports theme scales which are used in multiple class groups to save bundle size (more info to that in PR 55). At the moment these are:

  • colors
  • spacing
  • blur
  • brightness
  • borderColor
  • borderRadius
  • borderSpacing
  • borderWidth
  • contrast
  • grayscale
  • hueRotate
  • invert
  • gap
  • gradientColorStops
  • inset
  • margin
  • opacity
  • padding
  • saturate
  • scale
  • sepia
  • skew
  • space
  • translate

If you modified one of these theme scales in your Tailwind config, you can add all your keys right here and tailwind-merge will take care of the rest. If you modified other theme scales, you need to figure out the class group to modify in the default config.

Extending the tailwind-merge config

If you only need to extend the default tailwind-merge config, extendTailwindMerge is the easiest way to extend the config. You provide it a configExtension object which gets merged with the default config. Therefore, all keys here are optional.

import { extendTailwindMerge } from 'tailwind-merge'

const customTwMerge = extendTailwindMerge({
    // ↓ Add values to existing theme scale or create a new one
    theme: {
        spacing: ['sm', 'md', 'lg'],
    },
    // ↓ Add values to existing class groups or define new ones
    classGroups: {
        foo: ['foo', 'foo-2', { 'bar-baz': ['', '1', '2'] }],
        bar: [{ qux: ['auto', (value) => Number(value) >= 1000] }],
    },
    // ↓ Here you can define additional conflicts across class groups
    conflictingClassGroups: {
        foo: ['bar'],
    },
})

Using completely custom tailwind-merge config

If you need to modify the tailwind-merge config and need more control than extendTailwindMerge gives you or don't want to use the default config (and tree-shake it out of your bundle), you can use createTailwindMerge.

The function takes a callback which returns the config you want to use and returns a custom twMerge function.

import { createTailwindMerge } from 'tailwind-merge'

const customTwMerge = createTailwindMerge(() => ({
    cacheSize: 500,
    theme: {},
    classGroups: {
        foo: ['foo', 'foo-2', { 'bar-baz': ['', '1', '2'] }],
        bar: [{ qux: ['auto', (value) => Number(value) >= 1000] }],
    },
    conflictingClassGroups: {
        foo: ['bar'],
    },
}))

The callback passed to createTailwindMerge will be called when customTwMerge is called the first time, so you don't need to worry about the computations in it affecting app startup performance in case you aren't using tailwind-merge at app startup.

Using third-party tailwind-merge plugins

You can use both extendTailwindMerge and createTailwindMerge with third-party plugins. Just add them as arguments after your config.

import { extendTailwindMerge, createTailwindMerge } from 'tailwind-merge'
import { withMagic } from 'tailwind-merge-magic-plugin'
import { withMoreMagic } from 'tailwind-merge-more-magic-plugin'

// With your own config
const twMerge1 = extendTailwindMerge({ … }, withMagic, withMoreMagic)

// Only using plugin with default config
const twMerge2 = extendTailwindMerge(withMagic, withMoreMagic)

// Using `createTailwindMerge`
const twMerge3 = createTailwindMerge(() => ({  … }), withMagic, withMoreMagic)

API reference

Reference to all exports of tailwind-merge.

twMerge

function twMerge(...classLists: Array<string | undefined | null | false>): string

Default function to use if you're using the default Tailwind config or are close enough to the default config. Check out basic usage for more info.

If twMerge doesn't work for you, you can create your own custom merge function with extendTailwindMerge.

getDefaultConfig

function getDefaultConfig(): Config

Function which returns the default config used by tailwind-merge. The tailwind-merge config is different from the Tailwind config. It is optimized for small bundle size and fast runtime performance because it is expected to run in the browser.

fromTheme

function fromTheme(key: string): ThemeGetter

Function to retrieve values from a theme scale, to be used in class groups.

fromTheme doesn't return the values from the theme scale, but rather another function which is used by tailwind-merge internally to retrieve the theme values. tailwind-merge can differentiate the theme getter function from a validator because it has a isThemeGetter property set to true.

It can be used like this:

extendTailwindMerge({
    theme: {
        'my-scale': ['foo', 'bar']
    },
    classGroups: {
        'my-group': [{ 'my-group': [fromTheme('my-scale'), fromTheme('spacing')] }]
        'my-group-x': [{ 'my-group-x': [fromTheme('my-scale')] }]
    }
})

extendTailwindMerge

function extendTailwindMerge(
    configExtension: Partial<Config>,
    ...createConfig: Array<(config: Config) => Config>
): TailwindMerge
function extendTailwindMerge(...createConfig: Array<(config: Config) => Config>): TailwindMerge

Function to create merge function with custom config which extends the default config. Use this if you use the default Tailwind config and just extend it in some places.

You provide it a configExtension object which gets merged with the default config.

const customTwMerge = extendTailwindMerge({
    cacheSize: 0, // ← Disabling cache
    // ↓ Add values to existing theme scale or create a new one
    //   Not all theme keys form the Tailwind config are supported by default.
    theme: {
        spacing: ['sm', 'md', 'lg'],
    },
    // ↓ Here you define class groups
    classGroups: {
        // ↓ The `foo` key here is the class group ID
        //   ↓ Creates group of classes which have conflicting styles
        //     Classes here: foo, foo-2, bar-baz, bar-baz-1, bar-baz-2
        foo: ['foo', 'foo-2', { 'bar-baz': ['', '1', '2'] }],
        //   ↓ Functions can also be used to match classes.
        //     Classes here: qux-auto, qux-1000, qux-1001, …
        bar: [{ qux: ['auto', (value) => Number(value) >= 1000] }],
    },
    // ↓ Here you can define additional conflicts across different groups
    conflictingClassGroups: {
        // ↓ ID of class group which creates a conflict with …
        //     ↓ … classes from groups with these IDs
        foo: ['bar'],
    },
})

Additionally, you can pass multiple createConfig functions (more to that in createTailwindMerge) which is convenient if you want to combine your config with third-party plugins.

const customTwMerge = extendTailwindMerge({ … }, withSomePlugin)

If you only use plugins, you can omit the configExtension object as well.

const customTwMerge = extendTailwindMerge(withSomePlugin)

createTailwindMerge

function createTailwindMerge(
    ...createConfig: [() => Config, ...Array<(config: Config) => Config>]
): TailwindMerge

Function to create merge function with custom config. Use this function instead of extendTailwindMerge if you don't need the default config or want more control over the config.

You need to provide a function which resolves to the config tailwind-merge should use for the new merge function. You can either extend from the default config or create a new one from scratch.

// ↓ Callback passed to `createTailwindMerge` is called when
//   `customTwMerge` gets called the first time.
const customTwMerge = createTailwindMerge(() => {
    const defaultConfig = getDefaultConfig()

    return {
        cacheSize: 0,
        classGroups: {
            ...defaultConfig.classGroups,
            foo: ['foo', 'foo-2', { 'bar-baz': ['', '1', '2'] }],
            bar: [{ qux: ['auto', (value) => Number(value) >= 1000] }],
        },
        conflictingClassGroups: {
            ...defaultConfig.conflictingClassGroups,
            foo: ['bar'],
        },
    }
})

Same as in extendTailwindMerge you can use multiple createConfig functions which is convenient if you want to combine your config with third-party plugins. Just keep in mind that the first createConfig function does not get passed any arguments, whereas the subsequent functions get each passed the config from the previous function.

const customTwMerge = createTailwindMerge(getDefaultConfig, withSomePlugin, (config) => ({
    // ↓ Config returned by `withSomePlugin`
    ...config,
    classGroups: {
        ...config.classGroups,
        mySpecialClassGroup: [{ special: ['1', '2'] }],
    },
}))

But don't merge configs like that. Use mergeConfigs instead.

mergeConfigs

function mergeConfigs(baseConfig: Config, configExtension: Partial<Config>): Config

Helper function to merge multiple config objects. Objects are merged, arrays are concatenated, scalar values are overridden and undefined does nothing. The function assumes that both parameters are tailwind-merge config objects and shouldn't be used as a generic merge function.

const customTwMerge = createTailwindMerge(getDefaultConfig, (config) =>
    mergeConfigs(config, {
        classGroups: {
            // ↓ Adding new class group
            mySpecialClassGroup: [{ special: ['1', '2'] }],
            // ↓ Adding value to existing class group
            animate: ['animate-magic'],
        },
    })
)

validators

interface Validators {
    isLength(classPart: string): boolean
    isArbitraryLength(classPart: string): boolean
    isInteger(classPart: string): boolean
    isArbitraryValue(classPart: string): boolean
    isTshirtSize(classPart: string): boolean
    isArbitrarySize(classPart: string): boolean
    isArbitraryPosition(classPart: string): boolean
    isArbitraryUrl(classPart: string): boolean
    isArbitraryWeight(classPart: string): boolean
    isArbitraryShadow(classPart: string): boolean
    isAny(classPart: string): boolean
}

An object containing all the validators used in tailwind-merge. They are useful if you want to use a custom config with extendTailwindMerge or createTailwindMerge. E.g. the classGroup for padding is defined as

const paddingClassGroup = [{ p: [validators.isLength] }]

A brief summary for each validator:

  • isLength checks whether a class part is a number (3, 1.5), a fraction (3/4), a arbitrary length ([3%], [4px], [length:var(--my-var)]), or one of the strings px, full or screen.
  • isArbitraryLength checks for arbitrary length values ([3%], [4px], [length:var(--my-var)]).
  • isInteger checks for integer values (3) and arbitrary integer values ([3]).
  • isArbitraryValue checks whether the class part is enclosed in brackets ([something])
  • isTshirtSizechecks whether class part is a T-shirt size (sm, xl), optionally with a preceding number (2xl).
  • isArbitrarySize checks whether class part is an arbitrary value which starts with size: ([size:200px_100px]) which is necessary for background-size classNames.
  • isArbitraryPosition checks whether class part is an arbitrary value which starts with position: ([position:200px_100px]) which is necessary for background-position classNames.
  • isArbitraryUrl checks whether class part is an arbitrary value which starts with url: or url( ([url('/path-to-image.png')], url:var(--maybe-a-url-at-runtime)]) which is necessary for background-image classNames.
  • isArbitraryWeight checks whether class part is an arbitrary value which starts with number: or is a number ([number:var(--value)], [450]) which is necessary for font-weight classNames.
  • isArbitraryShadow checks whether class part is an arbitrary value which starts with the same pattern as a shadow value ([0_35px_60px_-15px_rgba(0,0,0,0.3)]), namely with two lengths separated by a underscore.
  • isAny always returns true. Be careful with this validator as it might match unwanted classes. I use it primarily to match colors or when I'm certain there are no other class groups in a namespace.

Config

interface Config { … }

TypeScript type for config object. Useful if you want to build a createConfig function but don't want to define it inline in extendTailwindMerge or createTailwindMerge.

Writing plugins

This library supports classes of the core Tailwind library out of the box, but not classes of any plugins. But it's possible and hopefully easy to write third-party plugins for tailwind-merge. In case you want to write a plugin, I invite you to follow these steps:

  • Create a package called tailwind-merge-magic-plugin with tailwind-merge as peer dependency which exports a function withMagic and replace "magic" with your plugin name.
  • This function would be ideally a createConfig function which takes a config object as argument and returns the modified config object.
  • If you create new class groups, prepend them with magic. (your plugin name with a dot at the end) so they don't collide with class group names from other plugins or even future class groups in tailwind-merge itself.
  • Use the validators and mergeConfigs from tailwind-merge to extend the config with magic.

Here is an example of how a plugin could look like:

import { mergeConfigs, validators, Config } from 'tailwind-merge'

export function withMagic(config: Config): Config {
    return mergeConfigs(config, {
        classGroups: {
            'magic.my-group': [{ magic: [validators.isLength, 'wow'] }],
        },
    })
}

This plugin can then be used like this:

import { extendTailwindMerge } from 'tailwind-merge'
import { withMagic } from 'tailwind-merge-magic-plugin'

const twMerge = extendTailwindMerge(withMagic)

Also, feel free to check out tailwind-merge-rtl-plugin as a real example of a tailwind-merge plugin.

Versioning

This package follows the SemVer versioning rules. More specifically:

  • Patch version gets incremented when unintended behavior is fixed, which doesn't break any existing API. Note that bug fixes can still alter which styles are applied. E.g. a bug gets fixed in which the conflicting classes inline and block weren't merged correctly so that both would end up in the result.

  • Minor version gets incremented when additional features are added which don't break any existing API. However, a minor version update might still alter which styles are applied if you use Tailwind features not yet supported by tailwind-merge. E.g. a new Tailwind prefix magic gets added to this package which changes the result of twMerge('magic:px-1 magic:p-3') from magic:px-1 magic:p-3 to magic:p-3.

  • Major version gets incremented when breaking changes are introduced to the package API. E.g. the return type of twMerge changes.

  • alpha releases might introduce breaking changes on any update. Whereas beta releases only introduce new features or bug fixes.

  • Releases with major version 0 might introduce breaking changes on a minor version update.

  • A non-production-ready version of every commit pushed to the main branch is released under the dev tag for testing purposes. It has the format 0.0.0-dev.<git SHA>.

  • A changelog is documented in GitHub Releases.

Contributing

Please see CONTRIBUTING for details.

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