What are indexed access types?

An indexed access type is the type-level equivalent of reading a property. T["id"] is the type of the id property, and T[keyof T] is a union of all property types. This avoids duplicating a shape just to name one of its parts.

type User = {
  id: number;
  profile: { name: string; age: number };
  tags: string[];
};

type Profile = User["profile"];
type Age = User["profile"]["age"];
type Tag = User["tags"][number];

Common patterns

• T[K] with a generic key preserves the exact property type
• T[keyof T] produces a union of all value types
• ArrayType[number] extracts the element type
• Nested paths such as T["a"]["b"] reach into object shapes
• Tuple types can be indexed with a literal position

Indexed access is often paired with keyof in generic helpers. The key is constrained to keyof T, and the return type is T[K], so the compiler knows the result matches the requested property. That pattern is safer than returning a broad union or any.