[翻译]High Performance JavaScript(011)
Repaints and Reflows 重绘和重排版
Once the browser has downloaded all the components of a page—HTML markup, JavaScript, CSS, images—it parses through the files and creates two internal data structures:
当浏览器下载完所有页面HTML标记，JavaScript，CSS，图片之后，它解析文件并创建两个内部数据结构：
A DOM tree
A representation of the page structure
一棵DOM树
表示页面结构
A render tree
A representation of how the DOM nodes will be displayed
一棵渲染树
表示DOM节点如何显示
The render tree has at least one node for every node of the DOM tree that needs to be displayed (hidden DOM elements don't have a corresponding node in the render tree). Nodes in the render tree are called frames or boxes in accordance with the CSS model that treats page elements as boxes with padding, margins, borders, and position. Once the DOM and the render trees are constructed, the browser can display ("paint") the elements on the page.
渲染树中为每个需要显示的DOM树节点存放至少一个节点（隐藏DOM元素在渲染树中没有对应节点）。渲染树上的节点称为“框”或者“盒”，符合CSS模型的定义，将页面元素看作一个具有填充、边距、边框和位置的盒。一旦DOM树和渲染树构造完毕，浏览器就可以显示（绘制）页面上的元素了。
When a DOM change affects the geometry of an element (width and height)—such as a change in the thickness of the border or adding more text to a paragraph, resulting in an additional line—the browser needs to recalculate the geometry of the element as well as the geometry and position of other elements that could have been affected by the change. The browser invalidates the part of the render tree that was affected by the change and reconstructs the render tree. This process is known as a reflow. Once the reflow is complete, the browser redraws the affected parts of the screen in a
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Cloning Nodes 节点克隆
Another way of updating page contents using DOM methods is to clone existing DOM elements instead of creating new ones—in other words, using element.cloneNode() (where element is an existing node) instead of document.createElement().
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