What should not be written in the render method?

You should not write:

• Side effects: Changing application state, API calls, direct DOM manipulation.
• Business logic: Calculations, data processing, not directly related to rendering.
• Long or resource-intensive operations: Cyclic computations, complex transformations, blocking the main thread.
• Changing props or component state: Render should not modify input data.
• Direct DOM element creation: React manages the DOM, manual creation breaks its model.

The render method should be a pure function that, based on props and state, returns a UI description.