How to dynamically change css

Dynamic changes in CSS

With the development of Web technology, web pages are becoming more and more complex. We hope that the elements on the page can dynamically change to meet the different needs of users. Among them, dynamically changing CSS is a common technical method that can achieve dynamic effects on the page. This article will introduce how to use JavaScript to dynamically change CSS.

1. Introduction to CSS

CSS (Cascading Style Sheets) is a language used to describe the style of web pages. It allows us to have fine control over the style of HTML elements. For example, you can set the background color, font, height, width, etc. of the element.

CSS styles can be defined in the following two ways:

1. Internal style sheet: defined by using the

2. External style sheet: defined by linking a tag in the HTML document to the CSS file.

CSS style rules consist of selectors and declarations, as shown below:

选择器 {
    声明1;
    声明2;
    声明3;
    ...
}

Among them, selectors are used to locate elements on the page, and declarations are used to set the style of elements. For example, the following code will set the font color of all p elements to red:

p {
    color: red;
}

2. JavaScript to change CSS

Through JavaScript, we can dynamically change the CSS style after the page is loaded to achieve some Quite an interesting effect. For example, let the navigation bar change to a fixed position when the page scrolls to a certain position. The following is an example:

window.addEventListener('scroll', function() {
    var nav = document.querySelector('nav');
    if (window.pageYOffset > 100) {
        nav.style.position = 'fixed';
        nav.style.top = '0';
    } else {
        nav.style.position = 'relative';
        nav.style.top = '';
    }
});

This code implements the style change of the navigation bar when the scroll position of the document exceeds 100 pixels. Specifically, it sets the navigation bar's position property to fixed to keep it fixed at the top of the screen, and sets its top property to 0 to keep it 0 away from the top of the page. When the scroll position of the document is less than 100 pixels, the position property of the navigation bar will be set to relative and restored to the default state.

Of course, this is just a simple example. In practical applications, we can combine other JavaScript APIs and CSS3 styles to achieve more complex effects.

3. JavaScript operation CSS properties

In JavaScript, we can get and set the CSS style of an element through the style attribute. For example, the following code will set the background color and font size of the element:

var elem = document.querySelector('#foo');
elem.style.backgroundColor = 'red';
elem.style.fontSize = '16px';

Here, we first get the element with ID foo through the document.querySelector() method, and then use the elem.style object to set the style.

If you want to get the style of an element, you can use the getComputedStyle() method. For example, the following code will get the font size of an element:

var elem = document.querySelector('#foo');
var style = window.getComputedStyle(elem);
var fontSize = style.getPropertyValue('font-size');

Here, we first get the element with ID foo, and then use the window.getComputedStyle() method to get the style of the element. Note that the style here is the calculated style, which is the final style of the element. Finally, we use the getPropertyValue() method to get the calculated font size.

In short, JavaScript can operate CSS styles very conveniently, and developers can use JavaScript to achieve more flexible and changeable style effects.

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