What is the CSS BEM naming convention? Summary of BEM naming conventions (detailed)

BEM is the abbreviation of block, element and modifier. It is a very useful naming convention. It can make your front-end code easier to understand and stricter. Next, let's take a look at the specific bem naming convention.

1 What is the BEM naming convention

Bem is the abbreviation of block, element, and modifier. It is a term proposed by the Yandex team. A front-end CSS naming methodology.

BEM is a simple and very useful naming convention. Make your front-end code easier to read and understand, easier to collaborate with, easier to control, more robust and unambiguous, and more rigorous.

1.1 BEM naming pattern

The BEM naming convention pattern is:

.block {}
.block__element {}
.block--modifier {}

• block represents a higher level abstraction or component.

• block__element represents the descendants of .block and is used to form a complete .block as a whole.

• block--modifier represents different states or different versions of .block.

Use two hyphens and an underscore instead of one so that your own blocks can be delimited by a single hyphen. Such as:

.sub-block__element {}
.sub-block--modifier {}

1.2 Benefits of BEM nomenclature

The key to BEM is that you can get more descriptions And a clearer structure, the meaning of a certain mark can be known from its name. Therefore, by looking at the class attribute in the HTML code, you can know the relationship between elements.

Conventional nomenclature example:

<p class="article">
    <p class="body">
        <button class="button-primary"></button>
        <button class="button-success"></button>
    </p>
</p>

This way of writing can understand the meaning of each element from the DOM structure and class naming, but Its true hierarchical relationship cannot be clarified. When defining CSS, you must also rely on hierarchical selectors to limit the constraint scope to avoid cross-component style pollution.

Examples using the BEM naming method:

<p class="article">
    <p class="article__body">
        <p class="tag"></p>
        <button class="article__button--primary"></button>
        <button class="article__button--success"></button>
    </p>
</p>

Through the BEM naming method, the module hierarchical relationship is simple and clear, and there is no need to write css Making too many level choices.

2 How to use BEM nomenclature

2.1 When should you use BEM format

The trick to using BEM is that you You need to know when and what should be written in BEM format.

Not every place should use BEM naming. The BEM format should be used when explicit module relationships are required.

For example, if there is just a single public style, there is no point in using the BEM format:

.hide {
    display: none !important;
}

2.2 In CSS preprocessing Using the BEM format in the browser

One disadvantage of BEM is that the naming method is long and ugly, and the writing is not elegant. Compared with the convenience brought by the BEM format, we should look at it objectively.

Moreover, CSS is generally written using preprocessor languages ​​such as LESS/SASS, and it is much simpler to write using its language features.

Take LESS as an example:

.article {
    max-width: 1200px;

    &__body {
        padding: 20px;
    }

    &__button {
        padding: 5px 8px;

        &--primary {background: blue;}
        &--success {background: green;}
    }
}

2.3 Using BEM format in components of popular frameworks

In the currently popular front-end frameworks such as Vue.js / React / Angular, there are compilation implementations of CSS component-level scope. The basic principle is to use the CSS attribute selector feature to generate different components for different components. attribute selector.

When you choose this local scope writing method, in smaller components, the BEM format may not seem so important. However, for public and global module style definitions, it is still recommended to use the BEM format.

In addition, for public components released to the outside world, generally for the sake of style customization, this local scope method is not used to define component styles. This is where using the BEM format will also come in handy.

2.4 Avoid the format of .block__el1__el2

In deeply nested DOM structures, overly long style name definitions should be avoided.

3 Summary

One of the hardest parts of BEM is figuring out where a scope starts and ends, and when to use it or not. As you accumulate experience with continuous use, you will slowly know how to use it, and these problems will no longer be a problem. There is no good or bad technology, only the right one is best.

Related recommendations:

BEM syntax detailed explanation

##[English] Use BEM to modularize your CSS code_html /css_WEB-ITnose

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