How to use JS to get the absolute position of DOM elements

This time I will show you how to use JS to get the absolute position of DOM elements. What are the precautions for using JS to get the absolute position of DOM elements? The following is a practical case, let's take a look.

DOM elements and the corresponding compatibility. For information on how to obtain the DOM element height and scroll height, please refer to the article Width, Height and Scroll Height of the Viewport.

Overview

offsetTop/offsetLeft

HTMLElement.offsetTop is used to get the position of the current element (excluding the top border) relative to the positioning container. That is to say,

If all ancestor elements are statically positioned position:static; (this is the default situation), offsetTop represents the height from the top of the document Poor (the top of the document may have rolled out of the viewport, and this height may be greater than the viewport height).

If there is an absolutely positioned ancestor element position:absolute/fixed, offsetTop will be relative to this element. Therefore, in order to obtain the height difference relative to the top of the document, you need to call recursively:

function getOffsetTop(el){
 return el.offsetParent
  ? el.offsetTop + getOffsetTop(el.offsetParent)
  : el.offsetTop
}

el.offsetParent is the positioning container of the current element. If the current element is not absolutely positioned Ancestor node, the value of this attribute is null.

Compatibility and limitations: This attribute is supported by almost all browsers. Its value is 0 if the element is hidden, but has no effect in IE9.

clientTop/clientLeft

Don’t be misled by the name, Element.clientTop refers to the width of the top border of the current element an integer value. Always equal to the rounded value of the border-top-width property returned by getComputedStyle().

why? In DOM terminology, client always refers to the rendering box excluding the border (padding content size). Offset always refers to the rendering box containing the border (border, padding, content size), and clientTop is the difference between the two Tops, that is, the border width. For the concept of the box, please refer to: CSS Display attribute and box model

Compatibility and limitations: Same as offsetTop/offsetLeft

.getBoundingClientRect()

Element.getBoundingClientRect() is used to get the size of the element and its position relative to the viewport, and returns a DOMRect object.

> document.querySelector('span').getBoundingClientRect()
DOMRect {x: 2.890625, y: 218.890625, width: 1264, height: 110, top: 218.890625, …}
bottom: 328.890625
height: 110
left: 2.890625
right: 1266.890625
top: 218.890625
width: 1264
x: 2.890625
y: 218.890625

If you want to obtain the position relative to the upper left corner of the document, you need to add the scroll position to the above top and left. The following code comes from MDN and is compatible with almost all browsers:

// For scrollX
(((t = document.documentElement) || (t = document.body.parentNode))
 && typeof t.scrollLeft == 'number' ? t : document.body).scrollLeft
// For scrollY
(((t = document.documentElement) || (t = document.body.parentNode))
 && typeof t.scrollTop == 'number' ? t : document.body).scrollTop

Compatibility and limitations: It is also a feature of CSSOM View Module, but it is compatible with almost all browsers. Please refer to

https://caniuse.com/#feat=getboundingclientrect IE The upper left corner of the window may not be 0,0. In IE9, you can set it to 0,0 like this:

<meta http-equiv="x-ua-compatible" content="ie=edge"/>

.getClientRects()

Element.getClientRects() is used to obtain a collection of DOMRect corresponding to all CSS box models in the DOM element.

If it is a block-level element, there should be only one element in the returned set, which is the size and position of the block. But if it's an inline element (or an element within an SVG), every CSS box within it will be returned. For example, an ordinary <span> that is wrapped by default:

> document.querySelector('span').getClientRects()
DOMRectList {0: DOMRect, 1: DOMRect, 2: DOMRect, length: 5}
0: DOMRect {x: 2.890625, y: 262.890625, width: 1264, height: 22, top: 262.890625, …}
1: DOMRect {x: 2.890625, y: 284.890625, width: 1264, height: 22, top: 284.890625, …}
2: DOMRect {x: 2.890625, y: 306.890625, width: 768, height: 22, top: 306.890625, …}

This <span> has three lines, and the length of the third line is less than one line. Each line break forms a new CSS box.

Compatibility and limitations: In IE8 and below, IE's unique TextRectangle object (instead of ClientRect) will be returned. This object does not have width and height properties, and properties cannot be set on it. Reference: https://webplatform.github.io/docs/dom/HTMLElement/getClientRects/

.getComputedStyle()

Window. getComputedStyle() can get all calculated CSS properties of an element. For simple absolutely positioned elements, the position of the element can be obtained through the top, left and other attribute values ​​returned by this API. For example:

let btn = document.querySelector('#btn-scroll-up')
let {top, left} = getComputedStyle(btn)
console.log('top:', top, 'left:', left)

.getComputedStyle() There is also a useful usage to get style information such as the size and position of pseudo elements:

// 以下代码来自： https://developer.mozilla.org/en-US/docs/Web/API/Window/getComputedStyle
var h3 = document.querySelector('h3'); 
var result = getComputedStyle(h3, ':after').content;
console.log('the generated content is: ', result); // returns ' rocks!'

兼容性和限制：.getComputedStyle() 几乎兼容所有浏览器，可参考 https://caniuse.com/#search=getComputedStyle。但它返回的值是 CSS 属性，用它获取绝对位置时要注意值的类型。例如 left 可能是 13px 这样的绝对值，也可能是 auto 这样的 CSS 关键字。

总结 获取 DOM 元素相对于文档的位置，可以直接使用 offsetTop； 获取 DOM 元素相对于视口的位置，可以使用 .getBoundingClientRect()； 获取 SVG 元素或行内元素的 CSS 盒子（比如用来做文本高亮时），可以使用 .getClientRects()； 获取绝对定位元素、伪元素的渲染后 CSS 属性，可以使用 .getComputedStyle()

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