Generics in Typescript.

February 17, 20234 min read



Learning Typescript has been a major boost to my career. The basics were not too difficult to grasp, and after going through a Vscode Livestream and a couple of exercises from Matt Pocock, I believed I was ready. Until I met generics.

Generics were a strange feature I couldn't wrap my head around. They weren't simple type annotations I was used to and when combined, confused me further. I gave up on it...for some time.

One thing about me though is that I always get back up. This was a subject I didn't understand yet I was going to learn it. If one resource doesn't help, I will try another.

With that, I went to the docs and read that while waiting to see the doctor. I got a bit out of it but it still wasn't 100% clear. The text that did it for me was this blog post by Shu Uesugi. Shu is an amazing teacher.

What I do wanna do today though, is break down an example he illustrated in my own words. In a bid to explain it, I believe I will understand it better.

Breaking down a generic Func

Here is a sample generic function:

function MakeState<F extends number | string,
 S extends string | F
  let pair: { first: F, second: S}

  function getPair(){
    return pair

  function setPair(x: F, y: S){
    pair = {
      first: x,
      second: y

return { getPair, setPair }


On to the generics stuff.

The function MakeState is a generic function. In the body of the function, we have two functions. One called getPair and another setPair which are used for getting and setting the pair variable with a set of object keys. Then we return an object with those two functions.

Though that's not the interesting part.

The function MakeState is generic because it takes in
type parameters and with that determines what type the values of the object should be.

The first type F extends number | string means that, our first parameter is constrained to be a subtype of the type number or string and no other type. This means that, when we call the function, we can only apply types that are a subset of number or string. This is what I mean:

We can pass in the type number

  let newState = MakeState<number, number>()

or we could also use a literal type

  let newState = MakeState<2,2>()

Why does a literal type work?

Well think of types as a set. Thank you Lere for reminding me of this. In the a string set, string values are a subset of that set. Therefore, for type string we can use the string type or a literal value.

If thinking of types as set is a bit confusing, check out this resource

The second type S extends string | F means that, our second parameter is constrained to the type string, and the type of F we initially typed in.

Here's an example:

  let newState = MakeState<string, string>()

We could have it like this:

  let newState = MakeState<number, number>()

or with literal types:

  let newState = MakeState<"kehinde","kehinde">()

Like I have earlier mentioned, the types we pass to the generic function, constrain the type of argument we can pass in.

Let's experiment with this in the TS playground. You can check it out by clicking on this link

If we call the generic fn and assign it the string types. We can't pass any other types of arguments to the fn:

Typescript playground

The typescript compiler returns the error:

Typescript compiler error

This helps us handle catch that error before runtime.


That sums up my short dive into the world of generics. This might just be a simple example but I recently learned this and wanted to break it down into a manner I understood. It is mostly for myself to know it better but for any other person who might come across this.

I think TS adds a layer of safety to code that I really appreciate. I will be using it in all my projects moving forward.

Thank you for reading and see you next time.


  1. fn means function