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 In this exercise, we have our returnWhatIPassIn function from the previous exercise. We have our T, and it's basically returning what if we pass in, which is really nice.

0:09 Now our test is saying it should only allow strings to be passed in. This means that we want to actually constrain the type of T to make sure it's only a string.

0:19 We could of course do this, where we could say, "OK, you can pass in a string here," but actually, you end up with a being a string instead of the exact thing that you're passing in.

0:28 Your job is to find a way and find a method of doing this syntax so that the thing that we pass in must be a string but still gets inferred as its literal type.

Problem

Restricting Generic Argument Types

In this exercise, we start with the returnWhatIPassIn function from the previous exercise. It has the T and it's basically returning whatever we pass in:


const returnWhatIPassIn = <T>(t: T) => {
  return t;
};

But now our test is saying it should only allow strings to be pass

Transcript

0:00 In this exercise, we have our returnWhatIPassIn function from the previous exercise. We have our T, and it's basically returning what if we pass in, which is really nice.

0:09 Now our test is saying it should only allow strings to be passed in. This means that we want to actually constrain the type of T to make sure it's only a string.

0:19 We could of course do this, where we could say, "OK, you can pass in a string here," but actually, you end up with a being a string instead of the exact thing that you're passing in.

0:28 Your job is to find a way and find a method of doing this syntax so that the thing that we pass in must be a string but still gets inferred as its literal type.