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



~> 1.1
 Project Readme


Utility function to efficiently merge Tailwind CSS classes without style conflicts. Essentially, a Ruby port of tailwind-merge.

Supports Tailwind v3.0 up to v3.2.


Install the gem and add it to your application's Gemfile by executing:

$ bundle add tailwind_merge

If bundler is not being used to manage dependencies, install the gem by executing:

$ gem install tailwind_merge

To use it, pass in a single string:

require "tailwind_merge"

TailwindMerge::Merger.new.merge("px-2 py-1 bg-red hover:bg-dark-red p-3 bg-[#B91C1C]")
# → 'hover:bg-dark-red p-3 bg-[#B91C1C]'

Or, an array of strings:

require "tailwind_merge"

TailwindMerge::Merger.new.merge(["px-2 py-1", "bg-red hover:bg-dark-red", "p-3 bg-[#B91C1C]"])
# → 'hover:bg-dark-red p-3 bg-[#B91C1C]'

What's it for?

If you use Tailwind with a component-based UI renderer (like ViewComponent or Ariadne), you're probably familiar with the situation that you want to change some styles of an existing component:

<!-- app/components/confirm_email_component.html.erb -->
<div class="border rounded px-2 py-1">Please confirm your email address.</div>
<%= render(ConfirmEmailComponent.new(class: "p-5")) %>

When the ConfirmEmailComponent is rendered, an input with the className border rounded px-2 py-1 gets created. But because of the way the CSS cascade works, the styles of the p-5 class are ignored. The order of the classes in the class string doesn't matter at all and the only way to apply the p-5 style is to remove both px-2 and py-1.

This is where tailwind_merge comes in:

@merger = TailwindMerge::Merger.new
@merger.merge("border rounded px-2 py-1 p-5")
# → "border rounded p-5"

tailwind-merge overrides conflicting classes and keeps everything else untouched. In the case of the implementation of ConfirmEmailComponent, the input now only renders the classes border rounded p-5.


Merging behavior

tailwind_merge is built to be intuitive. It follows a set of rules to determine which class wins when there are conflicts. Here is a brief overview of its conflict resolution.

Last conflicting class wins

@merger.merge('p-5 p-2 p-4') # → 'p-4'

Supports refinements

@merger.merge('p-3 px-5') # → 'p-3 px-5'
@merger.merge('inset-x-4 right-4') # → 'inset-x-4 right-4'

Resolves non-trivial conflicts

@merger.merge('inset-x-px -inset-1') # → '-inset-1'
@merger.merge('bottom-auto inset-y-6') # → 'inset-y-6'
@merger.merge('inline block') # → 'block'

Supports modifiers and stacked modifiers

@merger.merge('p-2 hover:p-4') # → 'p-2 hover:p-4'
@merger.merge('hover:p-2 hover:p-4') # → 'hover:p-4'
@merger.merge('hover:focus:p-2 focus:hover:p-4') # → 'focus:hover:p-4'

The order of standard modifiers does not matter for tailwind-merge.

Supports arbitrary values

@merger.merge('bg-black bg-[color:var(--mystery-var)]') # → 'bg-[color:var(--mystery-var)]'
@merger.merge('grid-cols-[1fr,auto] grid-cols-2') # → 'grid-cols-2'

Warning Labels necessary in ambiguous cases

When using arbitrary values in ambiguous classes like text-[calc(var(--rebecca)-1rem)] tailwind-merge looks at the arbitrary value for clues to determine what type of class it is. In this case, like in most ambiguous classes, it would try to figure out whether calc(var(--rebecca)-1rem) is a length (making it a font-size class) or a color (making it a text-color class). For lengths it takes clues into account like the presence of the calc() function or a digit followed by a length unit like 1rem.

But it isn't always possible to figure out the type by looking at the arbitrary value. E.g. in the class text-[theme(myCustomScale.rebecca)] tailwind-merge can't know the type of the arbitrary value and will default to a text-color class. To make tailwind-merge understand the correct type of the arbitrary value in those cases, you can use CSS data type labels which are used by Tailwind CSS to disambiguate classes: text-[length:theme(myCustomScale.rebecca)].

Supports arbitrary properties

@merger.merge('[mask-type:luminance] [mask-type:alpha]') # → '[mask-type:alpha]'
@merger.merge('[--scroll-offset:56px] lg:[--scroll-offset:44px]') # → '[--scroll-offset:56px] lg:[--scroll-offset:44px]'

#Don't actually do this!
@merger.merge('[padding:1rem] p-8') # → '[padding:1rem] p-8'

Warning > tailwind_merge does not resolve conflicts between arbitrary properties and their matching Tailwind classes to keep the bundle size small.

Supports arbitrary variants

@merger.merge('[&:nth-child(3)]:py-0 [&:nth-child(3)]:py-4') # → '[&:nth-child(3)]:py-4'
@merger.merge('dark:hover:[&:nth-child(3)]:py-0 hover:dark:[&:nth-child(3)]:py-4') # → 'hover:dark:[&:nth-child(3)]:py-4'

# Don't actually do this!
@merger.merge('[&:focus]:ring focus:ring-4') # → '[&:focus]:ring focus:ring-4'

Warning Similarly to arbitrary properties, tailwind_merge does not resolve conflicts between arbitrary variants and their matching predefined modifiers for bundle size reasons. The order of standard modifiers before and after an arbitrary variant in isolation (all modifiers before are one group, all modifiers after are another group) does not matter for tailwind_merge. However, it does matter whether a standard modifier is before or after an arbitrary variant both for Tailwind CSS and tailwind_merge because the resulting CSS selectors are different.

Supports important modifier

@merger.merge('!p-3 !p-4 p-5') # → '!p-4 p-5'
@merger.merge('!right-2 !-inset-x-1') # → '!-inset-x-1'

Supports postfix modifiers

twMerge("text-sm leading-6 text-lg/7"); // → 'text-lg/7'

Preserves non-Tailwind classes

@merger.merge('p-5 p-2 my-non-tailwind-class p-4') # → 'my-non-tailwind-class p-4'

Supports custom colors out of the box

@merger.merge('text-red text-secret-sauce') # → 'text-secret-sauce'

Basic usage

If you're using Tailwind CSS without any extra configs, you can use it right away:

merger = TailwindMerge::Merger.new

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 TailwindMerge::Merger initializer 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 the merge 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.


The tailwind_merge config is an object with several keys:

tailwind_merge_config = {
    # ↓ *Optional* Define how many values should be stored in cache.
    cache_size: 500,
    # ↓ *Optional* modifier separator from Tailwind config
    separator: ':',
    # ↓ *Optional* prefix from Tailwind config
    prefix: 'tw-',
    theme: {
        # Theme scales are defined here
        # This is not the theme object from your Tailwind config
    class_groups: {
        # Class groups are defined here
    conflicting_class_groups: {
        # Conflicts between class groups are defined here

To use the custom configuration, pass it to the TailwindMerge::Merger initializer:

@merger = TailwindMerge::Merger.new(config: tailwind_merge_config)

Class groups

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

position_class_group = ['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:

@merger.merge('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 (the shape is recursive). In the object, each key is joined with all the elements in the corresponding array with a dash (-) in between.

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

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

Sometimes it isn't possible to enumerate every element 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.

For example, here is the fill class group:

is_arbitrary_value = (class_part: string) => /^\[.+\]$/.test(class_part)
fill_class_group = [{ fill: ['current', IS_ARBITRARY_VALUE] }]

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 provides 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
filter_class_group = [{ filter: ['', 'none'] }]

Each class group is defined under its ID in the class_groups 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 merge 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 merge as px-3 pr-4, it's assumed 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 conflicting_class_groups object in the 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.

conflicting_class_groups = {
    px: ['pr', 'pl'],

If a class group creates a conflict, it means that if it appears in a class list string passed to merge, 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.


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. It only supports theme scales which are used in multiple class groups. At the moment these are:

  • colors
  • spacing
  • blur
  • brightness
  • borderColor
  • borderRadius
  • borderSpacing
  • borderWidth
  • contrast
  • grayscale
  • hueRotate
  • invert
  • gap
  • gradientColorStops
  • gradientColorStopPositions
  • 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.


Here's a brief summary for each validator:

  • IS_LENGTH checks whether a class part is a number (3, 1.5), a fraction (3/4), or one of the strings px, full or screen.
  • IS_ARBITRARY_LENGTH checks for arbitrary length values ([3%], [4px], [length:var(--my-var)]).
  • IS_INTEGER checks for integer values (3).
  • IS_PERCENT checks for percent values (12.5%) which is used for color stop positions.
  • IS_ARBITRARY_VALUE checks whether the class part is enclosed in brackets ([something])
  • IS_TSHIRT_SIZEchecks whether class part is a T-shirt size (sm, xl), optionally with a preceding number (2xl).
  • IS_ARBITRARY_SIZE checks whether class part is an arbitrary value which starts with size: ([size:200px_100px]) which is necessary for background-size classNames.
  • IS_ARBITRARY_POSITION checks whether class part is an arbitrary value which starts with position: ([position:200px_100px]) which is necessary for background-position classNames.
  • IS_ARBITRARY_IMAGE checks whether class part is an arbitrary value which is an iamge, e.g. by starting with image:, url:, linear-gradient( or url( ([url('/path-to-image.png')], image:var(--maybe-an-image-at-runtime)]) which is necessary for background-image classNames.
  • IS_ARBITRARY_NUMBER 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.
  • IS_ARBITRARY_SHADOW 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, optionally prepended by inset.
  • IS_ANY always returns true. Be careful with this validator as it might match unwanted classes. I use it primarily to match colors or when it's certain there are no other class groups in a namespace.


Results are cached

Results are cached by default, so you don't need to worry about wasteful re-renders. The library uses a computationally lightweight LRU cache which stores up to 500 different results by default. The cache is applied after all arguments are joined together to a single string. This means that if you call merge repeatedly with different arguments that result in the same string when joined, the cache will be hit.

The cache size can be modified or opted out of by setting the cache_size config variable.

Data structures are reused between calls

Expensive computations happen upfront so that merge calls without a cache hit stay fast.

Lazy initialization

The initial computations are called lazily on the first call to merge to prevent it from impacting startup performance if it isn't used initially.


Bug reports and pull requests are welcome on GitHub at https://github.com/gjtorikian/tailwind_merge.


The gem is available as open source under the terms of the MIT License.


This gem is pretty much just a port of https://github.com/dcastil/tailwind-merge. Thank them, not me!