tailwindcss/src/util/resolveConfig.js

import some from 'lodash/some'
import mergeWith from 'lodash/mergeWith'
import isFunction from 'lodash/isFunction'
import isUndefined from 'lodash/isUndefined'
import defaults from 'lodash/defaults'
import identity from 'lodash/identity'
import get from 'lodash/get'
import map from 'lodash/map'
import get from 'lodash/get'
import toPath from 'lodash/toPath'
import negateValue from './negateValue'

const configUtils = {
  negative(scale) {
    return Object.keys(scale)
      .filter(key => scale[key] !== '0')
      .reduce(
        (negativeScale, key) => ({
          ...negativeScale,
          [`-${key}`]: negateValue(scale[key]),
        }),
        {}
      )
  },
}

function applyPluginConfigModifications(config, plugins) {
  return plugins.reduce((modified, plugin) => {
    return get(plugin, 'modifyConfig', identity)(modified)
  }, config)
}

function value(valueToResolve, ...args) {
  return isFunction(valueToResolve) ? valueToResolve(...args) : valueToResolve
}

function mergeThemes(themes) {
  const theme = (({ extend: _, ...t }) => t)(
    themes.reduce((merged, t) => {
      return defaults(merged, t)
    }, {})
  )

  return {
    ...theme,

    // In order to resolve n config objects, we combine all of their `extend` properties
    // into arrays instead of objects so they aren't overridden.
    extend: themes.reduce((merged, { extend }) => {
      return mergeWith(merged, extend, (mergedValue, extendValue) => {
        if (isUndefined(mergedValue)) {
          return [extendValue]
        }

        if (Array.isArray(mergedValue)) {
          return [...mergedValue, extendValue]
        }

        return [mergedValue, extendValue]
      })
    }, {}),
  }
}

function mergeExtensions({ extend, ...theme }) {
  return mergeWith(theme, extend, (themeValue, extensions) => {
    // The `extend` property is an array, so we need to check if it contains any functions
    if (!isFunction(themeValue) && !some(extensions, isFunction)) {
      return {
        ...themeValue,
        ...Object.assign({}, ...extensions),
      }
    }

    return (resolveThemePath, utils) => ({
      ...value(themeValue, resolveThemePath, utils),
      ...Object.assign({}, ...extensions.map(e => value(e, resolveThemePath, utils))),
    })
  })
}

function resolveFunctionKeys(object) {
  const resolveThemePath = (key, defaultValue) => {
    const path = toPath(key)

    let index = 0
    let val = object

    while (val !== undefined && val !== null && index < path.length) {
      val = val[path[index++]]
      val = isFunction(val) ? val(resolveThemePath) : val
    }

    return val === undefined ? defaultValue : val
  }

  return Object.keys(object).reduce((resolved, key) => {
    return {
      ...resolved,
      [key]: isFunction(object[key]) ? object[key](resolveThemePath, configUtils) : object[key],
    }
  }, {})
}

export default function resolveConfig([userConfig, defaultConfig]) {
  const modifiedDefaultConfig = applyPluginConfigModifications(
    defaultConfig,
    get(userConfig, 'plugins', [])
  )
  const configs = [userConfig, modifiedDefaultConfig]

  return defaults(
    {
      // Need to get a default empty object if the config has no theme
      theme: resolveFunctionKeys(
        mergeExtensions(mergeThemes(map(configs, t => get(t, 'theme', {}))))
      ),
      variants: (firstVariants => {
        return Array.isArray(firstVariants)
          ? firstVariants
          : defaults({}, ...map(configs, 'variants'))
      })(defaults({}, ...map(configs)).variants),
    },
    ...configs
  )
}