I was unaware of this before inspecting these types.

Type predicate inference with generics is possible.

The type definitions can inform how these are expected to be used and uncover unique use cases like using type guards directly.

When trying to improve type-safety it's important to inspect and understand third-party types to know what they do and don't support.

While these examples are specific to a few array methods, there are some takeaways.

The find method then finds the exact square based on the size and returns the same type it received, which is Square. The filter method first filters to only squares and properly narrows the type to Square since the type guard was passed directly. The simplest 'fix' to this problem is to cast the result of find.Ĭonst firstSquare = shapes. What if we could inform TypeScript so it could properly narrow the type to only the element we're finding? Casts There is no way for TypeScript to know what the predicate function filters on so from it's perspective any element in the array could be returned. However, the return type includes the generic T which is the type that represents all elements in the array, or in this case: Square | Rectangle | Circle. From the example above, we expected firstSquare to be of type Square.

The return type of find is T | undefined since undefined is returned when no element in the array matches the predicate. This generic T is then also used for the value passed to the predicate testing function since each element in the array can be passed as a value. In this example, T would be the inferred type of the elements in shapes covered earlier: Square | Rectangle | Circle. The Array interface provides a generic T which represents the type of elements in the array. It's type signature defines three arguments, but again for the purposes of this we only care about the first argument: value. The predicate is a testing function that defines the criteria for what to find within the array.

For the purposes of this we only care about the first argument: predicate. The types define find as a method that accepts two arguments.

Array for each typescript