Interfaces, Unions & Objects

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1. 1 What are union and intersection types, and when does an intersection produce `never`? Medium
2. 2 What is a discriminated union, and why is it the safest way to model variants? Medium
3. 3 What's the difference between `readonly`, `const` assertions, and `Readonly<T>`? Medium
4. 4 How do index signatures work, and what are their downsides vs `Record`? Medium
5. 5 What is declaration merging, and when is it useful vs dangerous? Medium
6. 6 How do optional properties (`?`) differ from `| undefined` under `exactOptionalPropertyTypes`? Hard