Swift language interview questions

Optionals, value semantics, protocols, generics, closures, error handling and the modern Swift features interviews test.

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let vs var
let binds a constant and var a variable. Prefer let; the compiler warns when var is never mutated.
Subscripts
subscript(index:) -> T adds [] access to your types, with optional setters and multiple parameters.
@discardableResult
Silences the unused-result warning. Use it only when ignoring the result is normal (like removeFirst), not to hide errors.
Nominal typing
Swift types are identified by name, not shape: two structs with the same fields are different types. Tuples and functions are structural.
Never
A function returning Never doesn't return (fatalError, preconditionFailure). The compiler knows code after it is unreachable.
@dynamicMemberLookup
Lets obj.anything compile by routing to a subscript(dynamicMember:), often with key paths for type safety.
Any vs AnyObject
Any holds any value, including structs and functions; AnyObject holds only class instances.
class vs static methods
Both belong to the type. static can't be overridden; class (classes only) can be overridden by subclasses.