What are three reasons that a developer should write Jest tests for Lightning web components?

Jest is a powerful JavaScript testing framework used for Lightning Web Components (LWC) to ensure individual units of code function correctly in isolation. One of the primary reasons to use Jest is to verify the DOM output of a component (D). Since LWC is a UI framework, Jest allows developers to inspect the rendered HTML to ensure that elements, classes, and data are displayed as intended after a state change. Furthermore, Jest is essential for testing basic user interaction (A). Developers can simulate events like button clicks or text input and then assert that the component’s state or UI updates accordingly.

Another critical use case is to verify that events fire when expected (E). Components often communicate with parents via custom events; Jest allows you to "spy" on these events to ensure they are dispatched with the correct detail payload at the right time. Conversely, Jest is not intended for testing how multiple components work together (C)—this is the domain of integration tests or end-to-end tests (like UTAM). Additionally, Jest tests should focus on the component's public API and observable behavior; testing non-public (private) properties (B) is generally discouraged as it leads to brittle tests that break upon internal refactoring. By focusing on DOM output, interactions, and events, Jest provides a fast, reliable way to maintain component quality.

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