How to get the absolute position of DOM elements in the front-end interface

This time I will show you how to get the absolute position of the DOM element in the front-end interface, and what are the precautions on how to get the absolute position of the DOM element in the front-end interface. The following is a practical case, let's take a look.

When operating page scrolling and animation, the absolute position of DOM elements is often obtained, such as the floating navigation on the left side of this article. When the page scrolls to it, it will be rendered into the document flow normally. When the page scrolls beyond Its position will always be suspended on the left.

This article will detail various methods of obtaining the absolute position of 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

These are the documents and standards corresponding to the APIs involved in this article, for reference:

API	Usage	Documentation	Standard
offsetTop	The position of the relative positioning container	MDN	CSSOM View Module
clientTop	Top border width	MDN	CSSOM View Module
.getBoundingClientRect()	Element size and position relative to the viewport	MDN	CSSOM View Module
.getClientRects()	The size and position of all child CSS boxes	MDN	CSSOM View Module
.getComputedStyle()	Apply all style sheets and CSS properties after calculation	MDN	DOM Level 2 Style CSSOM

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=getboundingclientrectThe upper left corner of the window under IE may not be 0,0. In IE9, it can be set 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!'

Compatibility and limitations:.getComputedStyle() Compatible with almost all browsers, please refer to https://caniuse.com/#search=getComputedStyle. But the value it returns is a CSS property. When using it to obtain the absolute position, you should pay attention to the type of the value. For example, left may be an absolute value like 13px, or it may be a CSS keyword like auto.

Summary To get the position of the DOM element relative to the document, you can directly use offsetTop; To get the position of the DOM element relative to the viewport, you can use .getBoundingClientRect(); To get the SVG element Or the CSS box of an inline element (for example, when used for text highlighting), you can use .getClientRects(); to get the rendered CSS properties of absolutely positioned elements and pseudo elements, you can use .getComputedStyle ()

I believe you have mastered the method after reading the case in this article. For more exciting information, please pay attention to other related articles on the php Chinese website!

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