Some utilities for detecting the caret position inside a contenteditable element

contenteditable-caret-position.js

/**
 * Get the number of characters in an element
 *
 * @param {Element} element
 * @return {number}
 */
function getTextLength(element) {
  let range = element.ownerDocument.createRange()
  range.selectNodeContents(element)

  return range.toString().length
}

/**
 * Get the character offset the caret is currently at
 *
 * @param {Element} element
 * @return {number}
 */
function getCaretOffset(element) {
  let sel = element.ownerDocument.defaultView.getSelection()
  if (sel.rangeCount === 0) return 0

  let range = element.ownerDocument.defaultView.getSelection().getRangeAt(0)
  let preCaretRange = range.cloneRange()
  preCaretRange.selectNodeContents(element)
  preCaretRange.setEnd(range.endContainer, range.endOffset)
  return preCaretRange.toString().length
}

/**
 * Check if the caret is at the start of an element
 * Returns `false` when the caret is part of a selection
 *
 * @param {Element} element
 * @return {boolean}
 */
function isCaretAtStart(element) {
  if (element.ownerDocument.activeElement !== element) return false
  if (
    element.ownerDocument.defaultView.getSelection().getRangeAt(0).toString()
      .length > 0
  )
    return false

  return getCaretOffset(element) === 0
}

/**
 * Check if the caret is at the end of an element
 * Returns `false` when the caret is part of a selection
 *
 * @param {Element} element
 * @return {boolean}
 */
function isCaretAtEnd(element) {
  if (element.ownerDocument.activeElement !== element) return false
  if (
    element.ownerDocument.defaultView.getSelection().getRangeAt(0).toString()
      .length > 0
  )
    return false

  return getCaretOffset(element) === getTextLength(element)
}

/**
 * Check if the caret is on the first line of an element
 * Returns `false` when the caret is part of a selection
 *
 * @param {Element} element
 * @return {boolean}
 */
function isCaretOnFirstLine(element) {
  if (element.ownerDocument.activeElement !== element) return false

  // Get the client rect of the current selection
  let window = element.ownerDocument.defaultView
  let selection = window.getSelection()
  if (selection.rangeCount === 0) return false

  let originalCaretRange = selection.getRangeAt(0)

  // Bail if there is text selected
  if (originalCaretRange.toString().length > 0) return false

  let originalCaretRect = originalCaretRange.getBoundingClientRect()

  // Create a range at the end of the last text node
  let startOfElementRange = element.ownerDocument.createRange()
  startOfElementRange.selectNodeContents(element)

  // The endContainer might not be an actual text node,
  // try to find the last text node inside
  let startContainer = startOfElementRange.endContainer
  let startOffset = 0
  while (startContainer.hasChildNodes() && !(startContainer instanceof Text)) {
    startContainer = startContainer.firstChild
  }

  startOfElementRange.setStart(startContainer, startOffset)
  startOfElementRange.setEnd(startContainer, startOffset)
  let endOfElementRect = startOfElementRange.getBoundingClientRect()

  return originalCaretRect.top === endOfElementRect.top
}

/**
 * Check if the caret is on the last line of an element
 * Returns `false` when the caret is part of a selection
 *
 * @param {Element} element
 * @return {boolean}
 */
function isCaretOnLastLine(element) {
  if (element.ownerDocument.activeElement !== element) return false

  // Get the client rect of the current selection
  let window = element.ownerDocument.defaultView
  let selection = window.getSelection()
  if (selection.rangeCount === 0) return false

  let originalCaretRange = selection.getRangeAt(0)

  // Bail if there is a selection
  if (originalCaretRange.toString().length > 0) return false

  let originalCaretRect = originalCaretRange.getBoundingClientRect()

  // Create a range at the end of the last text node
  let endOfElementRange = document.createRange()
  endOfElementRange.selectNodeContents(element)

  // The endContainer might not be an actual text node,
  // try to find the last text node inside
  let endContainer = endOfElementRange.endContainer
  let endOffset = 0
  while (endContainer.hasChildNodes() && !(endContainer instanceof Text)) {
    endContainer = endContainer.lastChild
    endOffset = endContainer.length ?? 0
  }

  endOfElementRange.setEnd(endContainer, endOffset)
  endOfElementRange.setStart(endContainer, endOffset)
  let endOfElementRect = endOfElementRange.getBoundingClientRect()

  return originalCaretRect.bottom === endOfElementRect.bottom
}

Alternative

function getCaretPositionRelativeToElement(element: HTMLElement): DOMRect | null {
  const selection = window.getSelection()
  if (!selection || selection.rangeCount === 0) {
    return null
  }

  const range = selection.getRangeAt(0).cloneRange()

  if (!range.collapsed) {
    return getRelativeRect(range.getBoundingClientRect(), element)
  }

  let caretRect = range.getBoundingClientRect()

  if (caretRect.width === 0 && caretRect.height === 0) {
    range.setEnd(range.endContainer, range.endOffset + 1)
    caretRect = range.getBoundingClientRect()
    range.collapse(true) 
  }

  return getRelativeRect(caretRect, element)
}

function getRelativeRect(absoluteRect: DOMRect, relativeElement: HTMLElement): DOMRect {
  const elementRect = relativeElement.getBoundingClientRect()
  return new DOMRect(
    absoluteRect.left - elementRect.left,
    absoluteRect.top - elementRect.top,
    absoluteRect.width,
    absoluteRect.height,
  )
}