Programming TypeScript

Appendix A. Type Operators

TypeScript supports a rich set of type operators for working with types. Use Table A-1 as a handy reference for when you want to learn more about an operator.

Table A-1. Type operators

Type operator	Syntax	Use it on	Learn more
Type query	typeof, instanceof	Any type	“Refinement”, “Classes Declare Both Values and Types”
Keys	keyof	Object types	“The keyof operator”
Property lookup	O[K]	Object types	“The keying-in operator”
Mapped type	[K in O]	Object types	“Mapped Types”
Modifier addition	+	Object types	“Mapped Types”
Modifier subtraction	-	Object types	“Mapped Types”
Read-only modifier	readonly	Object types, array types, tuple types	“Objects”, “Classes and Inheritance”, “Read-only arrays and tuples”
Optional modifier	?	Object types, tuple types, function parameter types	“Objects”, “Tuples”, “Optional and Default Parameters”
Conditional type	?	Generic types, type aliases, function parameter types	“Conditional Types”
Nonnull assertion	!	Nullable types	“Nonnull Assertions”, “Definite Assignment Assertions”
Generic type parameter default	=	Generic types	“Generic Type Defaults”
Type assertion	as, <>	Any type	“Type Assertions”, “The const type”
Type guard	is	Function return types	“User-Defined Type Guards”