TailwindMerge

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

Supports Tailwind v3.0+.

Installation

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

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-3 styles 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 px-5")
# border rounded p-5

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

Features

Optimized for speed

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

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'

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'

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'

Watch out when mixing arbitrary properties which could be expressed as Tailwind classes. tailwind_merge does not resolve conflicts between arbitrary properties and their matching Tailwind classes.

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'

Similarly to arbitrary properties, tailwind_merge does not resolve conflicts between arbitrary variants and their matching predefined modifiers.

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'

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 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 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.

Configuration

The tailwind_merge config is an object with several keys:

tailwindMergeConfig = {
    # ↓ Set how many values should be stored in cache.
    cache_size: 500,
    # ↓ 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
    },
}

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.

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. 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
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.

Validators

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), a arbitrary length ([3%], [4px], [length:var(--my-var)]), 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) and arbitrary integer values ([3]).
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_URL 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.
IS_ARBITRARY_WEIGHT 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.
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.

Contributing

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

License

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

marcoroth/tailwind_merge