next.js/lib/head.js

import React from 'react'
import PropTypes from 'prop-types'
import sideEffect from './side-effect'

class Head extends React.Component {
  static contextTypes = {
    headManager: PropTypes.object
  }

  render () {
    return null
  }
}

export function defaultHead () {
  return [<meta charSet='utf-8' className='next-head' />]
}

function reduceComponents (components) {
  return components
  .map((c) => c.props.children)
  .map((children) => React.Children.toArray(children))
  .reduce((a, b) => a.concat(b), [])
  .reverse()
  .concat(...defaultHead())
  .filter((c) => !!c)
  .filter(unique())
  .reverse()
  .map((c) => {
    const className = (c.className ? c.className + ' ' : '') + 'next-head'
    return React.cloneElement(c, { className })
  })
}

function mapOnServer (head) {
  return head
}

function onStateChange (head) {
  if (this.context && this.context.headManager) {
    this.context.headManager.updateHead(head)
  }
}

const METATYPES = ['name', 'httpEquiv', 'charSet', 'itemProp', 'property']

// returns a function for filtering head child elements
// which shouldn't be duplicated, like <title/>.

function unique () {
  const keys = new Set()
  const tags = new Set()
  const metaTypes = new Set()
  const metaCategories = {}

  return (h) => {
    if (h.key) {
      if (keys.has(h.key)) return false
      keys.add(h.key)
    }
    switch (h.type) {
      case 'title':
      case 'base':
        if (tags.has(h.type)) return false
        tags.add(h.type)
        break
      case 'meta':
        for (let i = 0, len = METATYPES.length; i < len; i++) {
          const metatype = METATYPES[i]
          if (!h.props.hasOwnProperty(metatype)) continue

          if (metatype === 'charSet') {
            if (metaTypes.has(metatype)) return false
            metaTypes.add(metatype)
          } else {
            const category = h.props[metatype]
            const categories = metaCategories[metatype] || new Set()
            if (categories.has(category)) return false
            categories.add(category)
            metaCategories[metatype] = categories
          }
        }
        break
    }
    return true
  }
}

export default sideEffect(reduceComponents, onStateChange, mapOnServer)(Head)