tailwindcss/src/util/resolveConfig.js

import mergeWith from 'lodash/mergeWith'
import isFunction from 'lodash/isFunction'
import defaults from 'lodash/defaults'
import map from 'lodash/map'
import get from 'lodash/get'

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

function mergeExtensions({ extend, ...theme }) {
  return mergeWith(theme, extend, (themeValue, extensions) => {
    if (!isFunction(themeValue) && !isFunction(extensions)) {
      return {
        ...themeValue,
        ...extensions,
      }
    }

    return resolveThemePath => ({
      ...value(themeValue, resolveThemePath),
      ...value(extensions, resolveThemePath),
    })
  })
}

function resolveFunctionKeys(object) {
  const resolveObjectPath = (key, defaultValue) => get(object, key, defaultValue)

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

export default function resolveConfig(configs) {
  return defaults(
    {
      theme: resolveFunctionKeys(mergeExtensions(defaults({}, ...map(configs, 'theme')))),
      variants: defaults(...map(configs, 'variants')),
    },
    ...configs
  )
}