Id is used to uniquely identify elements, such as navigation bar, footer; class is used to reusable styles, such as buttons, and status switching. 1. The id must be unique, suitable for anchor positioning and JavaScript precise operation; 2. Class can be used for multiple elements, supports multiple selection and combination, and is suitable for component design; 3. The priority of id in CSS is higher than class; 4. Use id for layout, and class for interactive logic; 5. The naming should be clear and semantic to avoid abuse or vague description.

In web development, class and id are two of the most commonly used attributes in HTML, used to add identifiers to elements, making it convenient for CSS style control and JavaScript operations. They look a lot like it, but have different uses and rules. Simply put: an id represents a unique identifier, and a class can be reused .

When to use id?

The biggest feature of id is "uniqueness" - there cannot be two identical ids in the page. This makes it ideal for identifying structures or components unique to the page.

Common usage scenarios include:

• Global elements such as page main title, navigation bar, footer, etc.
• Positions that need to be positioned through anchor jump (for example <a href="#section1"></a> )
• A specific element in JavaScript that needs to be operated accurately

For example:

 <div id="main-header">This is the main title of the website</div>

A few points to note:

• The id name must be unique
• Naming suggestions are clear and semantic, such as #sidebar and #user-profile
• Don't abuse id to write styles because reusability is poor

When to use class?

The advantage of class lies in its "reusability". The same class can be applied to multiple elements, which is the key to style reuse and component design.

Suitable occasions include:

• Multiple elements share the same style (such as buttons, cards)
• Elements that need to switch styles according to state (such as .active , .hidden )
• In componentized front-end frameworks (such as React, Vue)

For example:

 <button class="btn">Submit</button>
<button class="btn">Cancel</button>

Some tips:

• class can have multiple, separated by spaces, for example <div class="card featured"> <li> Class names are recommended to be concise and clear, avoiding excessively long or vague naming</li> <li> Using naming specifications such as BEM can help maintain large projects</li> <hr> <h3 id="The-difference-between-id-and-class-in-CSS-and-JS"> The difference between id and class in CSS and JS</h3> <p> Functionally, both can be accessed by CSS and JavaScript, but there are differences in usage and priority.</p> <p> CSS Level:</p> <ul> <li> The id selector has a higher priority than class (which means that the id style is more likely to take effect if it conflicts)</li> <li> class supports multiple selection and can be used in combination, while id can only have one</li> </ul> <p> JavaScript Level:</p> <ul> <li> Get the id element with <code>document.getElementById('xxx')
• Get the class element with document.getElementsByClassName() or document.querySelectorAll('.xxx')
• id is more suitable for direct positioning of a single element, class is more suitable for batch processing

Recommendations for use in actual development

How to choose id and class when actually writing code? Here are a few practical suggestions:

• Use id for layout structure, and use class for style components
  For example, the page container can be used with #container , but the buttons are uniformly used with .btn

• Avoid overuse of id writing styles
  Because the id has high priority, it can easily lead to style coverage problems and debugging is troublesome

• Dynamic interaction logic prioritizes class
  For example, toggle menu expand and close, use classList.toggle() to operate class more flexible

• Keep semantics clear and naming meaningful
  Do not use class names that describe styles such as .red or .big , but should use semantic names such as .error-message or .highlight

Basically that's it. class and id have their own bits. Only by understanding their differences can you write clearer and easier to maintain HTML structure and style logic.