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Validate Property Inputs with the CSS Properties and Values API in JavaScript

Last updated: December 12, 2024

Modern web development often demands a great deal of flexibility and dynamism. One such area of flexibility has been the manipulation of CSS properties via JavaScript, which allows for dynamic styling of web pages. The CSS Properties and Values API plays a crucial role in providing developers with the ability to define and use custom, typed CSS properties, offering opportunities for validating property inputs right from within the CSS styling code. In this article, we will explore how to validate property inputs using this API in JavaScript and how it integrates seamlessly with CSS.

What is the CSS Properties and Values API?

The CSS Properties and Values API is part of the Houdini set of APIs, which allows developers to hook into the styling and layout processes of web browsers. This particular API enables developers to register custom properties (also known as CSS variables) with specific syntaxes and default values. These properties are then used within CSS, but their values can be constrained and validated in a more predictable way.

Registering Custom Properties

To utilize this API, you need to register custom properties using JavaScript within your web application. The CSS.registerProperty() method is employed for this purpose, where the developer can define the name, syntax, initial value, and whether the property should inherit.

CSS.registerProperty({
  name: '--main-color',
  syntax: '<color>',
  inherits: true,
  initialValue: 'black'
});

In the example above, we are registering a new CSS property called --main-color, specifying that its value should conform to standard CSS color syntax, it should inherit values, and it has an initial value of black.

Validating Inputs with Typed Values

The key advantage of using the CSS Properties and Values API is the ability to enforce type checking at the CSS level. This potentially reduces errors by limiting the range of acceptable values, thus adding a layer of validation.

CSS.registerProperty({
  name: '--header-height',
  syntax: '<length>',
  inherits: false,
  initialValue: '50px'
});

Here, we define a custom property --header-height with the <length> syntax, which means values should be specified in a valid CSS length unit, such as px, em, or %, and these will be enforced automatically by the browser.

Applying Custom Properties

Once registered, these properties can be used in your CSS files just like standard CSS variables:

:root {
  --main-color: navy;
  --header-height: 60px;
}

.header {
  color: var(--main-color);
  height: var(--header-height);
}

Reacting to Invalid Values

While CSS inherently won’t throw errors in the console when encountering invalid values, it will simply fall back to the initial values or disregard invalid ones. However, using JavaScript, you can add more robust error reporting or validation mechanisms if needed.

document.styleSheets[0].cssRules[0].style.setProperty('--main-color', 'invalid-color');
console.assert(getComputedStyle(document.documentElement).getPropertyValue('--main-color') === 'black');

In this JavaScript code snippet, we purposefully assign an invalid property value, check the computed style to ensure it fell back to the initial value specified in registration, and assert the fallback behavior, confirming proper validation handling.

Conclusion

The CSS Properties and Values API offers a powerful way to enhance the integrity of web applications by ensuring that CSS properties adhere to expected types and constraints. By registering properties and specifying syntaxes, developers can create safer, more reliable styling capabilities. This API, while still evolving, provides an exciting addition to the web's capabilities, reducing refactoring risks and enhancing the maintainability of complex styling systems. By leveraging the examples discussed, you can begin exploring the power and flexibility of dynamic CSS properties.

Next Article: Refine Inheritance Using the CSS Properties and Values API in JavaScript

Previous Article: Themeable Styles via the CSS Properties and Values API in JavaScript

Series: Web APIs – JavaScript Tutorials

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