Test Harness

Blits exposes a small testing utility that lets you render a single Component without launching a full Application.

This can be useful for unit testing Components in isolation. You can pass props, update props or state, apply focus and trigger input handlers. The harness runs with a small test platform, so it can be used in a Node test environment without a browser DOM.

Rendering a Component

Import the test helper from the testing namespace:

import { renderComponent } from '@lightningjs/blits/testing'
import Button from './Button.js'

const fixture = renderComponent(Button, {
  props: {
    label: 'Play',
  },
})

The returned fixture contains the mounted Component instance and a few helper methods to interact with it.

Snapshot

Use snapshot() to get a plain JavaScript object representation of the current Component.

const snapshot = fixture.snapshot()

console.log(snapshot.props.label) // Play
console.log(snapshot.tree.children[0].attributes.content)

The snapshot contains:

Key	Description
name	Component name
props	Declared props and their current values
state	Declared state and their current values
hasFocus	Whether the Component has focus
isHovered	Whether the Component is hovered
attributes	Attributes on the Component holder element
tree	Rendered element tree of the Component

Nested Components are included in the tree as Component snapshots as well.

Updating Props and State

Props can be updated with setProps().

fixture.setProps({
  label: 'Pause',
})

const snapshot = fixture.snapshot()

State can be updated with setState().

fixture.setState({
  selected: true,
})

Both methods update the mounted Component, so reactive template bindings are updated before the next snapshot is taken.

Focus and Input

Input handling follows the normal Blits focus behavior. This means input only runs after focus is set.

await fixture.focus()

fixture.input('enter')

You can also pass a custom keyboard event. Use createKeyboardEvent() when running tests in Node.

const event = fixture.createKeyboardEvent('enter', {
  keyCode: 13,
})

fixture.input('enter', event)

Cleanup

Call destroy() after the test to clean up the mounted Component and restore the test platform.

fixture.destroy()