React/Gatsby Table of Contents

component.tsx

import React, { createRef } from "react"
import throttle from "lodash/throttle"

// Import styled components (see the Styles section below).
import {
  TocDiv,
  TocLink,
  TocIcon,
  TocTitle,
  TocToggleOpener,
  TocToggleCloser,
  TocListItem,
  TocListBullet,
} from "./styles"
import { HeadingTree, TraverseResult, IHeadingData } from "./heading-tree"
import HeadingNode from "./heading-node"

interface IProps {
  containerSelector: string // Selector for a content container
  levels?: number[] // Needed heading levels, by default [2, 3, 4]
  prebuiltHeadings?: IHeadingData[] // Already extracted page headings to speed up

  title?: string // Title, default is "Contents"
  throttleTimeMs?: number // Scroll handler throttle time, default is 300

  // Note: offsetToBecomeActive must not be zero because at least in my chrome browser
  // element.scrollTo() sets window.scrollY = element.offsetTop - 1
  // and some routers use this function to scroll to window.location.hash.
  // The default value is 30 (px).
  offsetToBecomeActive?: number
}

interface IActiveHeadings {
  [key: number]: boolean
}

interface IState {
  open: boolean
  headingTree?: HeadingTree
  activeParents: IActiveHeadings
  activeNode?: HeadingNode
  container?: HTMLElement
}

export default class Toc extends React.Component<IProps, IState> {
  private wrapperRef = createRef<HTMLDivElement>()
  private clickEventListenerWasAdded = false
  private handleScrollThrottled: () => void
  private domObserver: MutationObserver

  constructor(props: IProps) {
    super(props)
    this.state = {
      open: false,
      headingTree: null,
      activeParents: {},
      activeNode: null,
      container: null,
    }
    this.handleClickOutside = this.handleClickOutside.bind(this)
    this.handleResize = this.handleResize.bind(this)
  }

  componentWillUnmount() {
    this.handleClose(false)
    window.removeEventListener(`scroll`, this.handleScrollThrottled)
    window.removeEventListener(`resize`, this.handleResize)
  }

  componentDidMount() {
    const container = this.parseHeadings()
    this.setupEventListeners(container)
  }

  private setupEventListeners(container: HTMLElement) {
    const startedAt = performance.now()

    let handleScroll: any
    if (typeof window === "undefined" || !window.MutationObserver) {
      console.info(`No window or mutationobserver, falling back to recalculating offsets on scroll`)
      handleScroll = () => {
        this.recalcOffsets()
        this.handleScrollImpl()
      }
      this.domObserver = null
    } else {
      handleScroll = this.handleScrollImpl.bind(this)
      this.domObserver = new MutationObserver(mutations => {
        console.info(
          `Toc: content container "${this.props.containerSelector}" mutation detected, recalculating offsets`,
          mutations
        )
        this.recalcOffsets()
      })
      this.domObserver.observe(container, {
        attributes: true,
        childList: true,
        subtree: true,
        characterData: true,
      })
    }
    this.handleScrollThrottled = throttle(handleScroll, this.props.throttleTimeMs || 300)

    window.addEventListener(`scroll`, this.handleScrollThrottled)
    window.addEventListener(`resize`, this.handleResize)

    console.info(`Set up toc event listeners in ${performance.now() - startedAt}ms`)
  }

  private buildActiveParents(activeNode: HeadingNode): IActiveHeadings {
    let curNode = activeNode
    const activeParents = {}
    if (this.state.headingTree) {
      activeParents[this.state.headingTree.getRoot().key] = true
    }
    while (curNode !== null) {
      activeParents[curNode.key] = true
      curNode = curNode.parent
    }
    return activeParents
  }

  private handleResize() {
    console.info(`Handling resize event`)
    this.recalcOffsets()
  }

  private recalcOffsets() {
    if (this.state.headingTree) {
      this.state.headingTree.markOffsetCacheStale()
    }
  }

  private handleScrollImpl() {
    const startedAt = performance.now()
    const activeNode = this.findActiveNode()
    const elapsedMs = performance.now() - startedAt
    if (elapsedMs >= 5) {
      console.info(`Scroll handler: looking for active heading took ${elapsedMs}ms`)
    }
    if (activeNode !== this.state.activeNode) {
      const activeParents = this.buildActiveParents(activeNode)
      this.setState({ activeNode, activeParents })
    }
  }

  private handleClickOutside(event: MouseEvent) {
    if (this.wrapperRef && this.wrapperRef.current && !this.wrapperRef.current.contains(event.target as Node)) {
      this.setState({ open: false })
    }
  }

  private handleOpen() {
    if (!this.clickEventListenerWasAdded) {
      document.addEventListener("mousedown", this.handleClickOutside)
      this.clickEventListenerWasAdded = true
    }
    this.setState({ open: true })
  }

  private handleClose(canSetState: boolean) {
    if (this.clickEventListenerWasAdded) {
      document.removeEventListener("mousedown", this.handleClickOutside)
      this.clickEventListenerWasAdded = false
    }
    if (canSetState) {
      this.setState({ open: false })
    }
  }

  private handleHeadingClick(ev: any, h: HeadingNode) {
    event.preventDefault()
    const elemTopOffset = h.cachedOffsetTop
    window.history.replaceState({}, "", `#${h.id}`)
    window.scrollTo(0, elemTopOffset)
    this.handleClose(true)
    this.setState({ activeNode: h, activeParents: this.buildActiveParents(h) })
  }

  private parseHeadings() {
    const startedAt = performance.now()
    const container = document.querySelector(this.props.containerSelector) as HTMLElement
    if (!container) {
      throw Error(`failed to find container by selector "${this.props.containerSelector}"`)
    }

    let headings = this.props.prebuiltHeadings
    if (headings) {
      const isSSR = typeof window === "undefined"
      if (isSSR) {
        // Just to validate, in client-side code it will be lazy.
        headings.forEach(h => {
          h.htmlNode = h.htmlNode || document.getElementById(h.id)
          if (!h.htmlNode) {
            throw Error(`no heading with id "${h.id}"`)
          }
        })
      }
    } else {
      const levels = this.props.levels || [2, 3, 4]
      const headingSelector = levels.map(level => `h${level}`).join(`, `)
      const htmlNodes: HTMLElement[] = Array.from(container.querySelectorAll(headingSelector))
      headings = htmlNodes.map((node, i) => ({
        value: node.innerText,
        depth: Number(node.nodeName[1]),
        id: node.id,
        htmlNode: node,
      }))
    }

    const tree = new HeadingTree(headings)
    console.info(
      `Built headings tree in ${performance.now() - startedAt}ms from ${
        this.props.prebuiltHeadings ? "prebuilt headings" : "DOM"
      }`
    )
    this.setState({ headingTree: tree, container })
    return container
  }

  private findActiveNode(): HeadingNode | null {
    if (!this.state.headingTree) {
      return null
    }

    const offsetToBecomeActive = this.props.offsetToBecomeActive || 30
    const curScrollPos = window.scrollY + offsetToBecomeActive

    let activeNode = null
    let lastNode = null
    this.state.headingTree.traverseInPreorder((h: HeadingNode) => {
      if (curScrollPos > h.cachedOffsetTop) {
        lastNode = h
        return TraverseResult.Continue
      }

      activeNode = lastNode
      return TraverseResult.Stop
    })

    if (activeNode === null && lastNode !== null && this.state.container) {
      // Mark last heading active only if we didn't scroll after the end of the container.
      if (window.scrollY <= this.state.container.offsetTop + this.state.container.offsetHeight) {
        return lastNode
      }
    }

    return activeNode
  }

  private renderHeadings() {
    if (!this.state.headingTree) {
      return
    }

    const items = []
    this.state.headingTree.traverseInPreorder(h => {
      const isActive = this.state.activeNode && this.state.activeNode.key === h.key
      items.push(
        <TocListItem depth={h.depth} active={isActive} key={h.key}>
          <TocListBullet depth={h.depth} active={isActive} />
          <TocLink href={`#${h.id}`} active={isActive} depth={h.depth} onClick={ev => this.handleHeadingClick(ev, h)}>
            {h.title}
          </TocLink>
        </TocListItem>
      )

      return this.state.activeParents[h.key] ? TraverseResult.Continue : TraverseResult.NoChildren
    })
    return items
  }

  render() {
    return (
      <>
        <TocToggleOpener open={this.state.open} onClick={this.handleOpen.bind(this)} />
        <TocDiv ref={this.wrapperRef} open={this.state.open}>
          <TocTitle>
            <TocIcon />
            {this.props.title || `Contents`}
            <TocToggleCloser onClick={() => this.handleClose(true)} />
          </TocTitle>
          <nav>
            <ul>{this.renderHeadings()}</ul>
          </nav>
        </TocDiv>
      </>
    )
  }
}

heading-node.ts

// src/components/toc/heading-node.ts
export default class HeadingNode {
  title: string
  children: HeadingNode[]
  parent?: HeadingNode
  private offsetCacheVersion: number | null
  cachedOffsetTop: number | null
  private htmlNode: HTMLElement | null
  depth: number // relative depth, starts from 0
  id: string
  key: number // faster comparison than by id

  constructor(
    htmlNode: HTMLElement | null,
    value: string,
    depth: number,
    id: string,
    key: number,
    offsetCacheVersion: number
  ) {
    this.htmlNode = htmlNode
    this.title = value
    this.parent = null
    this.children = []
    this.depth = depth
    this.id = id
    this.key = key

    this.offsetCacheVersion = offsetCacheVersion - 1
    // Don't call this.refetchOffsetIfNeeded(offsetCacheVersion) to save initial expensive fetch:
    // there are a more reflows during intial page loading, we need to delay offset fetching.
  }

  lazyLoad(curCacheVersion: number) {
    if (curCacheVersion === this.offsetCacheVersion) {
      return
    }

    if (!this.htmlNode) {
      this.htmlNode = document.getElementById(this.id)
      if (!this.htmlNode) {
        throw Error(`no heading with id "${this.id}"`)
      }
    }

    this.cachedOffsetTop = this.htmlNode.offsetTop
    this.offsetCacheVersion = curCacheVersion
  }
}

heading-tree.ts

// src/components/toc/heading-tree.ts
import HeadingNode from "./heading-node"

export enum TraverseResult {
  Continue = 1,
  NoChildren = 2,
  Stop = 3,
}

export interface IHeadingData {
  depth: number // h2 => 2, h3 => 3, etc
  value: string
  id: string
  htmlNode?: HTMLElement
}

export class HeadingTree {
  private root?: HeadingNode
  private offsetCacheVersion: number

  constructor(headings: IHeadingData[]) {
    // Make depths of nodes relative
    const minDepth = Math.min(...headings.map(h => h.depth))
    const offsetCacheVersion = 0

    const headingNodes: HeadingNode[] = headings.map((h, i) => {
      return new HeadingNode(h.htmlNode, h.value, h.depth - minDepth, h.id, i, offsetCacheVersion)
    })

    const nodeStack: HeadingNode[] = [new HeadingNode(null, "", -1, "", -1, offsetCacheVersion)] // init with root node
    headingNodes.forEach(node => {
      while (nodeStack.length && nodeStack[nodeStack.length - 1].depth >= node.depth) {
        nodeStack.pop()
      }

      nodeStack[nodeStack.length - 1].children.push(node)
      node.parent = nodeStack[nodeStack.length - 1]
      nodeStack.push(node)
    })

    this.root = nodeStack[0]
    this.offsetCacheVersion = offsetCacheVersion
  }

  getRoot() {
    return this.root
  }

  markOffsetCacheStale() {
    this.offsetCacheVersion++
  }

  traverseInPreorder(f: (h: HeadingNode) => TraverseResult) {
    const visitChildren = (h: HeadingNode): TraverseResult => {
      for (const child of h.children) {
        if (visit(child) === TraverseResult.Stop) {
          return TraverseResult.Stop
        }
      }
      return TraverseResult.Continue
    }

    const visit = (h: HeadingNode): TraverseResult => {
      h.lazyLoad(this.offsetCacheVersion)
      const res = f(h)
      if (res !== TraverseResult.Continue) {
        return res
      }
      return visitChildren(h)
    }

    visitChildren(this.root)
  }
}

index.ts

// src/components/toc/index.ts
export { default } from "./component"

mediaQuery.js

// src/utils/mediaQuery.js
import startCase from "lodash/startCase"

const min = width => `only screen and (min-width: ${width}px)`
const max = width => `only screen and (max-width: ${width - 1}px)`

const mediaQuery = {
  screens: {
    // values are in pixels
    small: 576,
    medium: 768,
    large: 992,
  },
}

for (const key of Object.keys(mediaQuery.screens)) {
  const Key = startCase(key)
  for (const [func, name] of [
    [min, `min`],
    [max, `max`],
  ]) {
    // css query
    const query = func(mediaQuery.screens[key])
    mediaQuery[name + Key] = `@media ` + query
    // js query (see window.matchMedia)
    mediaQuery[name + Key + `Js`] = query
  }
}

export default mediaQuery

styles.ts

// src/components/toc/styles.ts
import styled, { IProps } from "utils/styled"
import { css } from "@emotion/core"
import { FaList as BookContentIcon } from "react-icons/fa"
import { FaTimes as CrossIcon } from "react-icons/fa"

import mediaQuery from "utils/mediaQuery"
import { ITocListProps, ITocToggleProps } from "./types"

const openTocDiv = (props: IProps) => css`
  background: ${props.theme.colors.background};
  color: ${props.theme.colors.text.primary};
  padding: ${props.theme.space[3]} ${props.theme.space[5]};
  border-radius: ${props.theme.space[2]};
  box-shadow: 0 0 1em rgba(0, 0, 0, 0.5);
  border: 1px solid ${props.theme.colors.ui.border};
`

export const TocDiv = styled.div`
  height: max-content;
  z-index: 3;
  line-height: 2em;
  right: ${props => props.theme.space[4]};
  margin-top: ${props => props.theme.space[3]};

  ${mediaQuery.maxMedium} {
    overscroll-behavior: none;
    nav {
      max-height: 60vh;
      overflow-y: scroll;
    }

    position: fixed;
    bottom: 1em;
    left: 1em;
    visibility: ${(props: any) => (props.open ? `visible` : `hidden`)};
    opacity: ${(props: any) => (props.open ? 1 : 0)};
    transition: 0.3s;
    background: white;
    max-width: 20em;

    ${(props: any) => (props.open ? openTocDiv : `height: 0;`)}
  }
  ${mediaQuery.minMedium} {
    font-size: ${props => props.theme.fontSizes[1]};
    position: sticky;
    top: ${props => props.theme.space[7]};
    padding-left: ${props => props.theme.space[4]};
    max-width: 100%;
  }
`

export const TocTitle = styled.p`
  margin: 0;
  padding-bottom: ${props => props.theme.space[2]};

  display: grid;
  grid-auto-flow: column;
  grid-template-columns: auto auto 1fr;
  align-items: center;

  font-size: ${props => props.theme.fontSizes[3]};
  font-weight: ${props => props.theme.fontWeights[`bold`]};
  font-family: ${props => props.theme.fonts[`heading`]};
  line-height: ${props => props.theme.lineHeights[`dense`]};
`

export const TocLink = styled.a`
  font-weight: ${(props: ITocListProps) => props.active && `bold`};
  display: block;
  box-shadow: none;
`

const listItemShiftWidthEm = 1.5
const bulletRadiusPx = 4

export const TocListBullet = styled.span`
  position: absolute;

  border-color: #f0f0f2;
  border-width: 1px;
  border-style: solid;
  border-radius: ${bulletRadiusPx}px;

  background-color: ${(props: ITocListProps) => (props.active ? `rgba(34, 162, 201, 1);` : `#ffffff`)};
  width: ${bulletRadiusPx * 2}px;
  height: ${bulletRadiusPx * 2}px;
  content: "";
  position: absolute;
  display: block;
  z-index: 999;
  top: calc(50% - ${bulletRadiusPx}px);
  left: calc(${(props: ITocListProps) => `${(props.depth + 1 - 1) * listItemShiftWidthEm}em`} - ${bulletRadiusPx}px);
`

export const TocListItem = styled.li`
  margin: 0;
  list-style: none;

  /* You need to turn on relative positioning so the line is placed relative to the item rather than absolutely on the page */
  position: relative;

  /* Use padding to space things out rather than margins as the line would get broken up otherwise */
  padding-left: ${(props: ITocListProps) => `${(props.depth + 1) * listItemShiftWidthEm}em`};

  &::before {
    background-color: ${props => props.theme.colors.palette.grey[20]};
    width: 1px;
    content: "";
    position: absolute;
    top: 0px;
    bottom: 0px;
    left: ${(props: ITocListProps) => `${(props.depth + 1 - 1) * listItemShiftWidthEm}em`};
  }

  &:hover {
    background-color: ${props => props.theme.colors.primaryShades[20]};
  }
`

export const TocIcon = styled(BookContentIcon)`
  width: 1em;
  margin-right: ${props => props.theme.space[1]};
`

const openerToggleCss = (props: ITocToggleProps & IProps) => css`
  position: fixed;
  bottom: calc(1vh + 4em);
  left: 0;
  margin-left: ${props.theme.space[2]};
  transform: translate(${props.open ? `-100%` : 0});
  padding: ${props.theme.space[1]};
  border-radius: 0 50% 50% 0;
  background: ${props.theme.colors.primary};
  color: ${props.theme.colors.background};
`

const closerToggleCss = () => css`
  margin-left: 1em;
  padding: 2px;
  border-radius: 0 50% 50% 0;
`

const toggleCss = () => css`
  width: 1.6em;
  height: auto;
  z-index: 2;
  transition: 0.3s;
  justify-self: end;
  &:hover {
    transform: scale(1.1);
  }
  ${mediaQuery.minMedium} {
    display: none;
  }
`

export const TocToggleOpener = styled(BookContentIcon)`
  ${toggleCss}
  ${openerToggleCss}
`

export const TocToggleCloser = styled(CrossIcon)`
  ${toggleCss}
  ${closerToggleCss}
`

types.ts

// src/components/toc/types.ts
export interface ITocListProps {
  active: boolean
  depth: number
}

export interface ITocToggleProps {
  open: boolean
}