CSS classes and styling

Learn how to style elements, components and other facets of an Aurelia application using classes and CSS. Strategies for different approaches are discussed in this section.

Aurelia makes it easy to modify an element inline class list and styles. You can work with not only strings but also objects to manipulate elements.

Binding HTML Classes

The class binding allows you to bind one or more classes to an element and its native class attribute.

Binding to a single class

Adding or removing a single class value from an element can be done using the .class binding. By prefixing the .class binding with the name of the class you want to display conditionally selected.class="myBool" you can add a selected class to an element. The value you pass into this binding is a boolean value (either true or false), if it is true the class will be added; otherwise, it will be removed.

<p selected.class="isSelected">I am selected (I think)</p>

Inside of your view model, you would specify isSelected as a property and depending on the value, the class would be added or removed.

Here is a working example of a boolean value being toggled using .class bindings.

Binding to multiple classes

In addition to binding single classes conditionally, you can also bind multiple classes based on a single boolean expression using a comma-separated list in the .class binding syntax. This allows you to toggle a set of related classes together.

<div alert,alert-danger,fade-in,bold-text.class="hasError">Something went wrong!</div>

In this example, if the hasError property in your view model is truthy, all four classes (alert, alert-danger, fade-in, and bold-text) will be added to the div element. If hasError is falsy, all four classes will be removed.Important Note: When using the comma-separated syntax for multiple classes, ensure there are no spaces around the commas. The parser expects a direct list of class names separated only by commas (e.g., class1,class2,class3).

Besides the .class syntax, there are other ways to achieve dynamic class binding, especially when dealing with complex logic or generating class strings:

Syntax	Input Type	Example
class.bind="someString"	string	'col-md-4 bg-${bgColor}'
class="${someString}"	string	col-md-4 ${someString}

Once you have your CSS imported and ready to use in your components, there might be instances where you want to dynamically bind to the style attribute on an element (think setting dynamic widths or backgrounds).

Binding Inline Styles

Binding to a single style

You can dynamically add a CSS style value to an element using the .style binding in Aurelia.

<p background.style="bg">My background is blue</p>

Inside of your view model, you would specify bg as a string value on your class.

Here is a working example of a style binding setting the background colour to blue:

Binding to multiple styles

To bind to one or more CSS style properties you can either use a string containing your style values (including dynamic values) or an object containing styles.

Style binding using strings

This is what a style string looks like, notice the interpolation here? It almost resembles just a plain native style attribute, with exception of the interpolation for certain values. Notice how you can also mix normal styles with interpolation as well?

my-app.ts

export class MyApp {
  private backgroundColor = 'black';
  private textColor = '#FFF';
}

my-app.html

<p style="color: ${textColor}; font-weight: bold; background: ${backgroundColor};">Hello there</p>

You can also bind a string from your view model to the style property instead of inline string assignment by using style.bind="myString" where myString is a string of styles inside of your view model.

Style binding using objects

Styles can be passed into an element by binding to the styles property and using .bind to pass in an object of style properties. We can rewrite the above example to use style objects.

my-app.ts

export class MyApp {
  private styleObject = {
    background: 'black',
    color: '#FFF'
  };
}

my-app.html

<p style.bind="styleObject">Hello there</p>

From a styling perspective, both examples above do the same thing. However, we are passing in an object and binding it to the style property instead of a string.