What are union, intersection, keyof and typeof?

Definition:

A union allows a value to match one of several types, while an intersection requires it to satisfy all included types. keyof extracts property names from a type, and TypeScript's type-position typeof derives a type from a value declaration. Together they help express safe property access and reusable contracts.

Key points:

• A | B: either type. A & B: must satisfy both types.

• keyof User gives the property names of the User type.

• type User = typeof user gets a type from a value; runtime typeof user returns a string.

• For a union, narrow the value before using members that are not present on every possibility.

• An intersection of incompatible property requirements can produce an impossible type such as never.

Interview point:

• Runtime typeof returns a string; type-position typeof is a compile-time type query. They solve different problems.