Narrowing & Guards

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1. 1 How does control-flow narrowing work with `typeof`, `instanceof`, and truthiness? Medium
2. 2 What is a user-defined type guard (`x is Foo`), and when do you need one? Medium
3. 3 What are assertion functions (`asserts x is Foo`), and how do they differ from type guards? Medium
4. 4 Why does narrowing get lost after an `await` or a callback, and how do you preserve it? Hard
5. 5 What is the `satisfies` operator, and how does it differ from a type annotation or `as`? Hard