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Components in Blits have their own internal state and logic, and as such a Component is self-contained. However, as each component is part of a larger App scope, they may need to display different behaviors or appearances based on the rest of the app.

To achieve this, components can receive props from their parent component.

Defining and passing props

In the Component configuration object, you can specify exactly which props a component accepts. These props are passed to the component via attributes in the parent component's template. Any attributes that are not explicitly defined as a prop will be ignored.

The props key in the Component configuration object should be an Array, where each item corresponds to a prop that the component can accept.

The simplest way to define props is to just list their names within the props array:

  props: ['x', 'color', 'index', 'alpha']

Once specified, you can refer to these props inside the template of your component using the $ sign, similar to how you would reference variables defined within the component's internal state (i.e. <Element color="$color" />).

You can also access a prop inside a component's code using this.color (without a dollar sign!). And similar to component state variables, there is no need to specifically reference the props-key. Blits automatically maps all props directly on the this-scope, for easy access.

Since props are used to pass information from a parent to a child, it's important not to attempt to modify props inside your child component. If changes based on the prop from the parent are needed, you should probably use the prop in a so called computed property.

Advanced usage

For more advanced usage, you can define props using an array with an object for each prop, instead of just a string with the accepted name. Within each prop object, you can:

  • Specify a default value for the prop if it's omitted.
  • Validate the value of the prop based on certain criteria.
  • Mark the prop as required.
  • Apply a cast function to modify the value passed as a prop.

As you can see in the following example, you can mix and match the simple string notation with the advanced object notation within the same props array.

export default Blits.Component('MyComponent', {
  // ...
  props: [
      key: 'alpha',
      default: 0.5,
      required: true,
      validate(v) {
        return v <= 1 && v >= 0;
      cast: Number