How to Make Your TypeScript Generic Functions Safer and Easier to Use

Correctly working with types in Typescript might be very confusing (if not maddening and mind-blowing) for someone who comes from the JavaScript world. Our company has started migrating from JavaScript to TypeScript not so long ago, so most of the developers are still learning how to work with it (including myself). For some people this article might be trivial, but it was a real challenge for me. And I believe that sharing experience and knowledge about challenging things is the best we can do because it could help someone with the same challenge.

Problem

So, once, when doing a code review of a merge request from another team, my colleague and I stumbled upon these two peculiar functions:

The functions were supposed to be used something like this:

Anyway here we see two main problems:

1. We have no arguments safety we can pass blah and blahblah as parameters to our mapByKey function and Typescript wont stop us from it.

2. We have to do the Type assertions to calm Typescript down, adding a substantial risk of an error. This is because TS will treat the result variable as a Map of any T[keyof T] to any Tkeyof T, and any type assertion that satisfies those types will be legal.

If you are not sure what this all means, I recommend you to read some Typescript documentation on keyof operator and generics.

Partial Solution

The first solution that comes to our mind is to use not the string type in our mapByKey function, but keyof T:

The Real Solution

After some digging and brainstorming (and long moments of despair) we finally found the solution we need to add more generic parameters and use type parameters with constraints. This way Typescript will be able to know the exact types of the function arguments. Our functions will now look like this:

Conclusion

Generics and other concepts might be scary, but try to make Typescript your friend, and not your enemy. Explore the Typescript documentation and other resources extensively, dont be afraid to try out stuff, and dont get demotivated when something doesnt work. In the end it will absolutely work out. And when it does it will do wonders for you.

This is my first article, and I'm planning to write more, so any feedback is very much appreciated!

Photo by Mohammad Rahmani on Unsplash