Intro to Generics 9 exercises

TypeScript Generics

Section Resources
- Generics section of the TypeScript docs
  The TypeScript docs for generics have info and examples on creating and using generic types.
- The unknown Type from the TypeScript Docs
  Learn more about the unknown type in the TypeScript docs
- Generic Constraints from the TypeScript Docs
  The "Generic Constraints" page from the TypeScript documentation provides detailed information on how to define constraints for generic types
- Classes section of the TypeScript docs
  The Classes section of the TypeScript docs provides detailed information on how to use generics with classes, including examples and best practices.
- gesture.ts from the Excalidraw GitHub Repo
  See a real-world example of how generics are used in a software project.
- Total TypeScript - The Extends Keyword in action
  A refresher from the Type Transformations workshop lesson, "Support Function Type Constraints with Variable Arguments", showing extends in action.

Transcript

0:00 Just like in the previous solution, the right way to think about this is as a ReturnWhatIPassIn type output, where we have a T, and it's a T over here. Now we can say type Example = ReturnWhatIPassIn.

0:15 Let's say we pass in a number, but we don't want to allow numbers to be passed in. Well, the syntax for this in the type world is to say T extends string is to add a constraint on here.

0:27 This means now that we get an error when this is passed in. "Type number does not satisfy the constraint." This is the constraint string. We can do the exact same thing here. We can say T extends string, pass it on there, and now we get all the errors keyed in properly.

0:42 You notice that this slot here is just like something that you would see in the type world. You can do anything that you want to here, and we're going to still keep on hammering this home over the next few exercises.

solution

Add Constraints to a Generic

Before we get to the solution, let's refresh our memory on how to specify constraints.

In the type world, the way to add constraints is with the extends keyword.

So for a test, we could create a ReturnWhatIPassIn type where T extends string. If we then create an additional Example type a

Transcript

0:15 Let's say we pass in a number, but we don't want to allow numbers to be passed in. Well, the syntax for this in the type world is to say T extends string is to add a constraint on here.