What is BEM (Block, Element, Modifier) methodology?
Jun 24, 2025 am 12:40 AM
BEM is a naming specification for writing maintainable HTML and CSS. 1. Block is an independent and reusable component, such as .menu or .button, which should be named semantically; 2. Element is an integral part of Block, named such as .menu__item, which should not exist alone and should not be nested too deeply; 3. Modifier represents state or variant, such as .button--primary, which should only define different styles and avoid complex combinations; BEM improves naming clarity, reduces conflicts, facilitates maintenance and unify team structure.
BEM (Block, Element, Modifier) ??is a naming specification for writing maintainable, well-structured HTML and CSS. It is especially suitable for large projects or teamwork, and can help developers understand the relationship between components more clearly.
What is Block?
Block is a separate, reusable section, such as a button, navigation bar, or search box. It can exist alone without relying on other components.
- Naming method:
block-name - Example:
.menu,.button
Important: Block should be semantic, describing what it is rather than how it looks. For example, .search-form is more suitable than .blue-box .
What is Element?
Element is part of the Block and cannot exist alone from the Block. For example, a menu item is part of the menu, and the icons in the button are also elements of the button.
- Naming method:
block-name__element-name - Example:
.menu__item,.button__icon
Common misunderstandings:
- Don't nest too many levels, such as
.block__elem1__elem2is not recommended. - Element belongs to only one block, do not use across blocks.
The function of Modifier
Modifier is used to represent the state or variant of Block or Element, such as "disable", "large", "red" and other style changes.
- Naming method:
block-name--modifierorblock-name__element-name--modifier - Example:
.button--primary,.menu__item--active
Recommended usage:
- Modifier should only define styles that are different from the default state, avoiding duplicate code.
- Multiple Modifiers can be combined, but pay attention to naming conflicts and complexity controls.
Why is BEM useful?
In projects that are collaborative or long-term maintenance, BEM offers the following benefits:
- Clear naming: You can tell which module this class belongs to by looking at the name.
- Reduce style conflicts: class names are more unique and are not prone to global pollution.
- Easy to maintain: it is easier to locate the scope of influence when modifying styles.
- Consistent structure: Team members can write in a unified way to reduce communication costs.
Although the class name of BEM looks a bit long, the readability and stability it brings are far more important than short.
Basically that's it. It may feel a bit long-winded at first, but once you get used to it, you will find that it makes front-end development more orderly and more controllable.
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