In computer science, type safety is the extent to which a programming language discourages or prevents type errors. Type-safe languages are sometimes also called strongly or strictly typed. The behaviors classified as type errors by a given programming language are usually those that result from attempts to perform operations on values that are not of the appropriate data type, e.g. trying to add a string to an integer.

"Type safe" redirects here. For Typesafe Inc, see Akka.io.

Type enforcement can be static (catching potential errors at compile time), dynamic (associating type information with values at run-time and consulting them as needed to detect imminent errors), or a combination of both.¹ Dynamic type enforcement can often run programs that would be invalid under static enforcement, but at the cost of introducing errors at runtime.

In the context of static (compile-time) type systems, type safety usually involves (among other things) a guarantee that the eventual value of any expression will be a legitimate member of that expression's static type. The precise requirement is more subtle than this — see, for example, subtyping and for complications.