If you're using Tailwind CSS without any extra config, you can use twMerge right away. You can safely stop reading the documentation here.
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.
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-',
// ↓ Optional separator from TaiLwind config
separator: '_',
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
},
conflictingClassGroupModifiers: {
// Conflicts between postfox modifier of a class group and another class group are defined here
},
}
}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.
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, 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, 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.
Tailwind CSS allows postfix modifiers for some classes. E.g. you can set font-size and line-height together with text-lg/7 with /7 being the postfix modifier. This means that any line-height classes preceding a font-size class with a modifier should be removed.
For this tailwind-merge has the conflictingClassGroupModifiers object in its config with the same shape as conflictingClassGroups explained in the section above. This time the key is the ID of a class group whose modifier creates a conflict and the value is an array of IDs of class groups which receive the conflict.
const conflictingClassGroupModifiers = {
'font-size': ['leading'],
}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:
colorsspacingblurbrightnessborderColorborderRadiusborderSpacingborderWidthcontrastgrayscalehueRotateinvertgapgradientColorStopsgradientColorStopPositionsinsetmarginopacitypaddingsaturatescalesepiaskewspacetranslate
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.
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] }],
baz: ['baz-sm', 'baz-md', 'baz-lg'],
},
// ↓ Here you can define additional conflicts across class groups
conflictingClassGroups: {
foo: ['bar'],
},
// ↓ Define conflicts between postfix modifiers and class groups
conflictingClassGroupModifiers: {
baz: ['bar'],
},
})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] }],
baz: ['baz-sm', 'baz-md', 'baz-lg'],
},
conflictingClassGroups: {
foo: ['bar'],
},
conflictingClassGroupModifiers: {
baz: ['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.
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)Next: Recipes
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