Docs: function parameter bivariance · Issue #14973 · microsoft/TypeScript (original) (raw)
I have found these two super useful, but rather unrelated, explanations of why function parameters are bivariant in TypeScript. Would it be helpful to merge them into one?
The first one is that the programmer may know that an object of type T[] will be only read from or only written to, but has no way of communicating that knowledge to TypeScript. Since the methods that write to an array are contravariant in T (T is their argument type) and the methods that read from the array are covariant in T (T is their return type), TypeScript permits either direction even though it cannot statically verify correctness.
The second one is that the programmer may know that the correct type of one argument (which is a function) may depend on the type of another argument; and more specifically, in a common JS pattern, it is the function argument type that varies. Again, there is no way to express it to TypeScript. Rather than giving up completely, TypeScript offers at least partial type safety by asking the programmer to indicate the broadest argument type that the function might accept, which allows it to use bivariance in its type check.