Rule 4 of 9 · Chapter II — Build, Test, Debug
The test harness is your first environment.
Why this rule exists
npm start watch gives you a render loop in seconds. An environment round-trip costs minutes per attempt and pollutes a shared tenant with half-working versions.
In practice
Prove rendering, property changes, and updateView behavior in the harness before the component ever touches Dataverse. Keep a browser tab on the harness for the whole dev loop.
When it doesn't apply
Anything involving the WebAPI, real record context, or model-driven form events genuinely needs an environment — mock what you can, then test those paths on a dev tenant.