---
title: Properties
description: Storing and binding values with properties.
---

All elements have properties.
Built-in elements come with common properties such as color or dimensional properties.
You can assign values or entire expressions to them:

```slint
export component Example inherits Window {
    // Simple expression: ends with a semi colon
    width: 42px;
    // or a code block (no semicolon needed)
    height: { 42px }
}
```

When you assign an expression, it becomes a *binding*:
Slint re-evaluates it automatically whenever one of the properties it depends on changes.
This is what makes the user interface [reactive](../../concepts/reactivity/) —
you declare the relationship once, and the value stays up to date.

A binding can also be a code block with multiple statements.
These two bindings are equivalent:

```slint no-test
background: touch-area.is-pressed ? red : blue;

background: {
    if (touch-area.is-pressed) {
        return red;
    } else {
        return blue;
    }
}
```

## Declaring Your Own Properties

Define extra properties on a component by specifying the type, the name, and optionally a default value:

```slint
export component Example {
    // declare a property of type int with the name `my-property`
    property<int> my-property;

    // declare a property with a default value
    property<int> my-second-property: 42;
}
```

Use a qualifier to declare who may read and write the property.
Think of it as the interface of your component:

```slint
export component Button {
    // This is meant to be set by the user of the component.
    in property <string> text;
    // This property is meant to be read by the user of the component.
    out property <bool> pressed;
    // This property is meant to both be changed by the user and the component itself.
    in-out property <bool> checked;

    // This property is internal to this component.
    private property <bool> has-mouse;
}
```

All properties declared at the top level of a component that aren't `private`
are accessible from the outside when using the component as an element,
and via the language bindings from the business logic.

For the exact rules, see the [Properties](../../../../reference/language/properties/) and
[Bindings](../../../../reference/language/bindings/) chapters of the language reference;
[Property Types](../../../../reference/property-types/) lists every type a property can have.
