TypeIsT

TypeIsT is a type-level utility used in statically typed languages to test whether two types are exactly the same. It is not a runtime function but a compile-time construct that can influence generic constraints, type narrowing, or branching in type-dependent code. The concept is particularly common in TypeScript, where conditional types enable precise type comparisons without executing any code.

In TypeScript, a typical implementation of TypeIsT takes two type parameters and resolves to a boolean literal. For example:

type TypeIsT<T, U> = T extends U ? (U extends T ? true : false) : false;

This pattern treats T and U as identical when both directions of extension hold. It can be used to enforce or infer type relationships in generic utilities, to gate features behind specific type predicates, or to refine overloads and mapped types.

Limitations and considerations include the fact that type equality checks can be sensitive to how types are constructed (aliases, conditional types, or unions can affect the result). The construct operates only at compile time and has no runtime footprint. In practice, TypeIsT is a tool in a developer’s toolbox for type-safe generic programming, rather than a fundamental language feature.

Examples of outcomes: TypeIsT<string, string> yields true, TypeIsT<string, number> yields false, and TypeIsT<string | number, string | number> depends on how the types are composed. TypeIsT is commonly discussed in type utility libraries and documentation as a technique for expressing exact type equality in generic contexts. See also: type utilities, conditional types, type equality tricks.