Generics exist in more languages than Typescript (notably C# and Java). They allow for creating reusable components that work across a variety of types, rather than a single one. This allows users to consume these components and provide their own types.
function identity<T>(arg: T): T {
return arg;
}
// Both work
let output = identity<string>("myString");
let output = identity("myString");
The generic type variable (T) can be named anything - it's helpful to give it a more meaningful name in more complicated functions.
Generics can be used in classes, types, interfaces, etc. This is where a lot of power comes from, as they can be passed through a type, to a function, to an interface, and then finally to a React prop for type safety.
They can also be constrained: e.g. T extends Lengthwise so now arg.length is valid, and you won't be able to pass in a number as it doesn't have a .length