JavaScript 获取页面总高度：document.documentElement.scrollHeight 或 document.body.scrollHeight

This code returns the height of the entire document, including any content that extends beyond the visible viewport. It first checks if the 'scrollHeight' property of the HTML element is defined and returns its value if so. Otherwise, it returns the 'scrollHeight' property of the 'body' element. The 'scrollHeight' property returns the height of an element's content, including any content that overflows outside the element's dimensions and is only accessible through scrolling.