Discuss how to implement addClass() in JavaScript

JavaScript is a widely used programming language. In web application development, it is often necessary to operate the class attribute of HTML elements to control style and behavior. One of the commonly used methods is addClass, which is used to add one or more class names to the specified element.

In this article, we will explore how to implement addClass in JavaScript. First, we need to understand some basic knowledge about the class attribute of elements.

Introduction to element class attribute

In HTML, each element can have one or more class names (class names), which are used to specify the specific style and behavior of the element. The class name is a string value that can contain characters such as letters, numbers, dashes, and underscores.

The following is an example that shows how to specify a class name for an element:

<div class="my-class">This is a div element with class "my-class".</div>

In the above code, the class attribute value of the <div> element is "my-class".

An element can have multiple class names, separated by spaces. For example:

<div class="my-class1 my-class2">This is a div element with class "my-class1" and "my-class2".</div>

addClass Method implementation principle

In most JavaScript frameworks and libraries, the addClass method is provided for convenient operation The element's class attribute. However, it is not difficult to actually implement addClass. Below we will introduce a common implementation method.

First, we need to get the target element to be operated, you can use querySelector or getElementById and other methods:

var elem = document.querySelector('.my-element');

Next, we need to The added class name is converted into an array form, so that multiple class names can be handled. For example:

var classesToAdd = ['class1', 'class2', 'class3'];

Then, we need to get the current class attribute value of the target element and convert it into an array form:

var currentClasses = elem.className.split(' ');

Next, traverse the array of class names to be added. If the current class If the name does not exist in the class attribute of the target element, add it to the array:

for (var i = 0; i < classesToAdd.length; i++) {
  var className = classesToAdd[i];
  if (currentClasses.indexOf(className) === -1) {
    currentClasses.push(className);
  }
}

Finally, reassign the spliced ​​class name array to the class attribute of the target element:

elem.className = currentClasses.join(' ');

Complete addClass The function implementation is as follows:

function addClass(elem, classesToAdd) {
  // 将要添加的类名转换为数组形式
  if (!Array.isArray(classesToAdd)) {
    classesToAdd = [classesToAdd];
  }

  // 获取目标元素当前的 class 属性值，并将其转换为数组形式
  var currentClasses = elem.className.split(' ');

  // 遍历要添加的类名数组，如果当前类名在目标元素的 class 属性中不存在，则将其添加到数组中
  for (var i = 0; i < classesToAdd.length; i++) {
    var className = classesToAdd[i];
    if (currentClasses.indexOf(className) === -1) {
      currentClasses.push(className);
    }
  }

  // 将拼接好的 class 名称数组重新赋值给目标元素的 class 属性
  elem.className = currentClasses.join(&#39; &#39;);
}

Use addClass method

Use addClass method is very simple, just need Just pass the element to be operated on and the class name to be added as parameters. For example, adding a class name active when a button is clicked:

<button onclick="addClass(document.querySelector(&#39;.my-element&#39;), &#39;active&#39;)">Add class "active"</button>

<div class="my-element">This is a div element.</div>

Summary

In this article, we introduced a basic addClass method This implementation provides an easy way to add element class names to any JavaScript-developed application. It should be noted that when using this method, the class name in the form of an array should be passed to the function, and the class attribute value of the target element should be as expected.

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