What is the reason for Range.getBoundingClientRect() returning 0 for a range that is inside a textarea

When working with a <textarea> element and creating a range inside it, the following steps are taken:

  • A new range is created using document.createRange()
  • The only child node of the <textarea> is retrieved using textarea.childNodes[0]
  • Range start and range end are set using range.setStart and range.setEnd

After setting up the range as described above, range.getBoundingClientRect() is called:

let textarea = document.querySelector('textarea');
let node = textarea.childNodes[0];

let range = document.createRange();
range.setStart(node, 3);
range.setEnd(node, 5);
console.log(range.getBoundingClientRect());
<textarea>aa bb cc</textarea>

However, the received ClientRect object shows all zero fields - left = top = width = height = 0. The question arises: Why are these fields returning zeros?


An important note is that everything works properly if we substitute the <textarea> with a regular div:

let textarea = document.querySelector('div');
let node = textarea.childNodes[0];

let range = document.createRange();
range.setStart(node, 3);
range.setEnd(node, 5);
console.log(range.getBoundingClientRect());
<div>aa bb cc</div>

Answer №1

<textarea> is considered a substitute element

Objects that are added using the CSS content property serve as anonymous replaced elements. They are referred to as "anonymous" because they do not appear in the actual HTML code.

Answer №2

If you're wondering how to accomplish this task, here's a helpful example:

The process involves creating a div, transferring all the styles from the input element to that div, copying the text up to the selection into the div, inserting a span within the div to mark the selection, and then determining the position of this span in relation to the div before ultimately removing the div.

Answer №3

If you're looking for a robust "caret position listener" that is functional for text areas, look no further than this solution inspired by the codepen found here. The technique involves using a "shadow" element that mirrors the styles of the textarea and gracefully handles scenarios where the textarea is resized or removed from the DOM. Be sure to review the code for insights into handling small issues like 'phantomNewline'.

function keepTrackOfCaretPosition(textArea, action) {
  const shadow = createShadowElement(textArea);
  shadow.style.visibility = 'hidden'; // Conceal the shadow element

  function adjustShadowSizeAndPosition() {
    let boundingRect = textArea.getBoundingClientRect();
    let scrollLeft = window.pageXOffset || document.documentElement.scrollLeft;
    let scrollTop = window.pageYOffset || document.documentElement.scrollTop;
    shadow.style.left = `${boundingRect.left + scrollLeft}px`;
    shadow.style.top = `${boundingRect.top + scrollTop}px`;
    shadow.style.width = `${textArea.offsetWidth}px`;
    shadow.style.height = `${textArea.offsetHeight}px`;
    shadow.scrollTop = textArea.scrollTop;
  }

  function updateCaret() {
    adjustShadowSizeAndPosition();
    const caretPos = getCurrentCaretPosition(textArea, shadow);
    action(caretPos);
  }

  textArea.addEventListener('input', updateCaret);
  textArea.addEventListener('click', updateCaret);
  textArea.addEventListener('scroll', updateCaret);
  textArea.addEventListener('keydown', function(e) {
    if(["ArrowRight", "ArrowLeft", "ArrowUp", "ArrowDown"].includes(e.key)) {
      updateCaret();
    }
  });

  // Resize shadow if textarea is resized:
  const resizeObserver = new ResizeObserver(() => adjustShadowSizeAndPosition());
  resizeObserver.observe(textArea);

  // Remove shadow if textarea is removed:
  const mutationObserver = new MutationObserver(mutations => {
    for(const mutation of mutations) {
      if(Array.from(mutation.removedNodes).includes(textArea)) {
        shadow.remove();
        mutationObserver.disconnect();
        resizeObserver.disconnect();
      }
    }
  });
  mutationObserver.observe(textArea.parentNode, { childList: true });

  function createShadowElement(textArea) {
    const shadow = document.createElement('div');
    const style = getComputedStyle(textArea);

    const propertiesToCopy = ['overflow-x', 'overflow-y', 'display', 'font-family', 'font-size', 'font-weight', 'word-wrap', 'white-space', 'padding-left', 'padding-right', 'padding-top', 'padding-bottom', 'border-left-width', 'border-top-width', 'border-right-width', 'border-bottom-width', 'border-style', 'text-align'];
    propertiesToCopy.forEach(key => shadow.style[key] = style[key]);

    Object.assign(shadow.style, {
      position: 'absolute',
      left: `${textArea.offsetLeft}px`,
      top: `${textArea.offsetTop}px`,
    });

    document.body.appendChild(shadow);
    return shadow;
  }

  function getCurrentCaretPosition(textArea, shadow) {
    const { selectionStart, selectionEnd } = textArea;
    const value = textArea.value;
    let phantomNewline = false;

    // Add a 'phantom' character for a final newline due to HTML vs. textarea disparity
    if(selectionStart === selectionEnd && value[selectionStart - 1] === '\n') {
      phantomNewline = true;
    }

    shadow.textContent = phantomNewline ? value.substring(0, selectionStart) + ' ' + value.substring(selectionStart) : value;

    if(!shadow.firstChild) {
      const style = getComputedStyle(textArea);
      return {
        x: textArea.offsetLeft + parseFloat(style.paddingLeft),
        y: textArea.offsetTop + parseFloat(style.paddingTop),
      };
    }

    const range = document.createRange();
    range.setStart(shadow.firstChild, phantomNewline ? selectionStart + 1 : selectionStart);
    range.setEnd(shadow.firstChild, phantomNewline ? selectionEnd + 1 : selectionEnd);

    const rect = range.getBoundingClientRect();
    const scrollLeft = window.pageXOffset || document.documentElement.scrollLeft;
    const scrollTop = window.pageYOffset || document.documentElement.scrollTop;

    return {
      x: rect.left + scrollLeft,
      y: rect.top + scrollTop,
    };
  }

}

Ready to implement? Here's how you can use it:

keepTrackOfCaretPosition(textareaEl, position => {
  console.log(`Caret location: x=${position.x}, y=${position.y}`);
});

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