# Class & style binding

Class and style bindings in Aurelia allow you to bind to CSS properties and add one or more classes to your HTML elements inside of your views.

## Binding to the class attribute

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 conditionally display, for example, `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.

{% embed url="<https://stackblitz.com/edit/aurelia-conditional-class?embed=1&file=my-app.html&hideExplorer=1&hideNavigation=1&view=preview>" %}

### Binding to multiple classes

Unlike singular class binding, you cannot use the `.class` binding syntax to conditionally bind multiple CSS classes. However, there is a multitude of different ways in which this can be achieved.

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

## Binding to the style attribute

Dynamically set CSS styles on elements in your view templates.

### 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 color to blue:

{% embed url="<https://stackblitz.com/edit/aurelia-conditional-style?embed=1&file=my-app.html&hideExplorer=1&hideNavigation=1&view=preview>" %}

### 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?

{% code title="my-app.ts" %}

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

{% endcode %}

{% code title="my-app.html" %}

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

```

{% endcode %}

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.

{% code title="my-app.ts" %}

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

{% endcode %}

{% code title="my-app.html" %}

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

```

{% endcode %}

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.


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