If you're encountering an "undefined property" error when testing a Lit TypeScript web component, it likely means that the property you are trying to access in your test is not properly initialized or assigned in your component.

To fix this issue, you need to ensure that your component's properties are properly set up and initialized. Here are some steps to troubleshoot and fix the "undefined property" error:

  1. Check Property Initialization: Make sure that the property you are accessing in your test is correctly initialized in your web component's constructor or firstUpdated lifecycle method. For example, if your component has a property named myProperty, you should initialize it like this:

    import { LitElement, html, property } from 'lit'; class MyComponent extends LitElement { @property() myProperty: string = ''; // Initialize with a default value or leave it empty constructor() { super(); // Additional constructor logic if needed } // Other lifecycle methods and component logic }
  2. Test Data Setup: Ensure that you are providing valid test data and properties in your test case. If your component relies on any external data or state, make sure to mock or set up the necessary data for your test.

  3. Render the Component: If you are not rendering your component in the test, it won't go through its lifecycle methods, and properties may not be initialized. Make sure to render your component using testing utilities like render or fixture provided by Lit's lit/testing package:

    import { html, fixture } from '@open-wc/testing'; import { MyComponent } from './my-component'; describe('MyComponent', () => { it('should render with myProperty set', async () => { const element = await fixture(html`<my-component myProperty="Hello"></my-component>`); expect(element.myProperty).to.equal('Hello'); }); });

    Note that fixture from @open-wc/testing will handle the initialization of the component and its properties.

  4. Verify Import Statements: Double-check that your import statements are correct and you are importing the correct version of Lit. Ensure that you are using the correct version of TypeScript and LitElement.

By following these steps and ensuring that your component's properties are properly initialized, you should be able to resolve the "undefined property" error in your Lit TypeScript web component tests.

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