rsnext/packages/next/client/head-manager.js

const DOMAttributeNames = {
  acceptCharset: 'accept-charset',
  className: 'class',
  htmlFor: 'for',
  httpEquiv: 'http-equiv',
}

export default class HeadManager {
  constructor() {
    this.updatePromise = null
  }

  updateHead = head => {
    const promise = (this.updatePromise = Promise.resolve().then(() => {
      if (promise !== this.updatePromise) return

      this.updatePromise = null
      this.doUpdateHead(head)
    }))
  }

  doUpdateHead(head) {
    const tags = {}
    head.forEach(h => {
      const components = tags[h.type] || []
      components.push(h)
      tags[h.type] = components
    })

    this.updateTitle(tags.title ? tags.title[0] : null)

    const types = ['meta', 'base', 'link', 'style', 'script']
    types.forEach(type => {
      this.updateElements(type, tags[type] || [])
    })
  }

  updateTitle(component) {
    let title = ''
    if (component) {
      const { children } = component.props
      title = typeof children === 'string' ? children : children.join('')
    }
    if (title !== document.title) document.title = title
  }

  updateElements(type, components) {
    const headEl = document.getElementsByTagName('head')[0]
    const headCountEl = headEl.querySelector('meta[name=next-head-count]')
    if (process.env.NODE_ENV !== 'production') {
      if (!headCountEl) {
        console.error(
          'Warning: next-head-count is missing. https://err.sh/next.js/next-head-count-missing'
        )
        return
      }
    }

    const headCount = Number(headCountEl.content)
    const oldTags = []

    for (
      let i = 0, j = headCountEl.previousElementSibling;
      i < headCount;
      i++, j = j.previousElementSibling
    ) {
      if (j.tagName.toLowerCase() === type) {
        oldTags.push(j)
      }
    }
    const newTags = components.map(reactElementToDOM).filter(newTag => {
      for (let k = 0, len = oldTags.length; k < len; k++) {
        const oldTag = oldTags[k]
        if (oldTag.isEqualNode(newTag)) {
          oldTags.splice(k, 1)
          return false
        }
      }
      return true
    })

    oldTags.forEach(t => t.parentNode.removeChild(t))
    newTags.forEach(t => headEl.insertBefore(t, headCountEl))
    headCountEl.content = (
      headCount -
      oldTags.length +
      newTags.length
    ).toString()
  }
}

function reactElementToDOM({ type, props }) {
  const el = document.createElement(type)
  for (const p in props) {
    if (!props.hasOwnProperty(p)) continue
    if (p === 'children' || p === 'dangerouslySetInnerHTML') continue

    // we don't render undefined props to the DOM
    if (props[p] === undefined) continue

    const attr = DOMAttributeNames[p] || p.toLowerCase()
    el.setAttribute(attr, props[p])
  }

  const { children, dangerouslySetInnerHTML } = props
  if (dangerouslySetInnerHTML) {
    el.innerHTML = dangerouslySetInnerHTML.__html || ''
  } else if (children) {
    el.textContent = typeof children === 'string' ? children : children.join('')
  }
  return el
}