🔸.boundingBox

browser ⟩ DOM ⟩ type ⟩ Element ⟩ boxes ⟩ Element+boxes ⟩ .boundingBox

a Rect relative to viewport.

• size: (may be fractional)

  • almost the same as .borderBox size (integer-valued).

  • "box-sizing: border-box" size = (width, height) -> padding/border included.

  • standard box model： size = content(width, height) + padding + border.

• position: (.x, .y) or (.left, .top) relative to viewport.

👉 compare： .borderBox (a Rect relative to .offsetParent)

🗺️ 圖解

💾 Element+boxes, 👉 coordinates

"elem.boundingBox" is a Rect relative to viewport

🧨 雷區

the elements must be in the document to read offsetHeight and other properties, a hidden (display:none) or out of the document element has no size.

coordinates right/bottom are different from CSS right/bottom properties, in CSS,

• right: the distance from the right edge.

• bottom: the distance from the bottom edge.

⭐️ 重點

inline elements (such as <span>, <code>, <b>)

• may span multiple lines and consist of multiple rectangles.

• the bounding rect returned by .getBoundingClientRect() will include the entire width/height of these lines.

• call .getClientRects() to get individual rectangles of inline elements.

👥 相關

• coordinates of document / viewport / container

• elem.position() - document coordinates of Element

• will change while window scrolling

• CSS positioning

• .borderBox

⬇️ 應用

• elem.position() - document coordinates of Element.

💈範例

• elem.showNote() - show note beside element.

💾 replit: elem.getBoundingClientRect()

const { log } = console;

// ⭐️ $all(): select all element
function $all(selector, parent = document) {
    return parent.querySelectorAll(selector);
}

// 🔸 elem.showMessageUnder()
Element.prototype.showMessageUnder = function(html, {
    timeout = 5,
    showRectInfo = true,
}={}) {

    // ⭐️ bounding rect 
    let rect = this.getBoundingClientRect();    
    log(rect);

    // create <div> element
    let div = document.createElement('div');
    
    // better to use a css class for the style here (⭐️ don't forget "px"!)
    div.style.cssText = `position: fixed; color: red; background: lightyellow; left: ${rect.left}px; top: ${rect.bottom}px; padding: 4px; border: solid black 1px; opacity: 0.7`;

    if (showRectInfo) {
        html += `<br><code>{x: ${rect.x.toFixed(0)}, y: ${rect.y.toFixed(0)}, width: ${rect.width.toFixed(0)}, height: ${rect.height.toFixed(0)}}</code>`;
    }

    div.innerHTML = html;
    document.body.append(div);        // add to the DOM tree
    
    if (timeout > 0) {
        setTimeout(() => div.remove(), timeout * 1000);    // remove from DOM
    }

    return div;
};


// click handler
document.onclick = (event) => {

    const tag = event.target.tagName;

    if (tag !== 'BUTTON') {
        log(`clicked on a <${tag}>`); return;
    }

    $all('span').forEach(span => {
        span.showMessageUnder(`bounding rect:`);    // 🔸 custom method
    });
};

📗 參考

• JavaScript: The Definitive Guide (15.5 Document Geometry & Scrolling)

📘 手冊

• Element ⟩

  • .getBoundingClientRect()

  • .getClientRects() - individual rectangles of inline elements.

• DOMRectReadOnly ⟩

  • DOMRect (NOT read-only) ⟩ (.x, .y)

    • .right, .bottom

• box-sizing