TypeScript is a language that is moving forward really fast and sometimes is hard to keep up with it, there are so many features being released constantly, that we may be missing some really important gems. I think that the unknown type is one of those.
What is it?
The unknown type is a basic TypeScript type that was introduced in version 3.0 of the language.
And by basic, I mean that you can use it to define a simple variable or parameter, as you can see in the following example.
let movie: unknown = "";
function printMovie(movie: unknown) {
console.log(movie);
}
Why is it there?
The quick explanation is: to represent all types that at the moment of the declaration, the developer is not sure (or does not know) how that value will be used, or more specifically, how its members (if any) will be accessed.
It is a complement to any but it acts in a more type-safe way. Meaning that until the moment that you are really using the value of type unknown it won't type check it, but as soon as your code starts to make assumptions about it, the compiler will begin to type-check its use.
How so? Let's see an example to make it more clear.
Let's take that printMovie function from above. That function works fine because, console.log expects a value of type any for its first argument, and unknown can be assigned to another unknown value or any.
Let's explore a little more on the example:
function formatMovie(movie: string) {
return `The movie title is ${movie}`;
}
function printMovie(movie: unknown) {
console.log(formatMovie(movie));
}
The above snippet will not compile 💣 because we are now making an assumption about the movie argument, we're trying to pass it down to another function formatMovie that is expecting an argument of type string. When we try to call that function and pass an unknown value where a string was expected, then the compiler will yield an error stating exactly that.
Now you have a choice, either type the first movie argument as a string or something else that you can turn in to a string.
Here's another example:
function printMovie(movie: unknown) {
console.log(`The movie title is ${movie.title} and its rating is ${movie.rating}`);
}
This is another example that will fail 💣 at compile-time because we are assuming that movie is an object with the properties title and rating. Again, at this point, we are able to type it accordingly, at least with an inline type.
function printMovie(movie: { title: string; rating: number; })
And what about any?
Well, I am a strong advocate against the abuse of the any type in TS 🙅. The main idea of any is to represent types that can actually be anything. We cannot know what those types would be at compile-time, my experience is that people reach for any too soon instead of taking the time and write the appropriate types.
If it's possible to write the types, even if those types would be of different structures or values, then we should write them. There are a few ways to do so, using generics or union types for instance, but the point is that they can be typed. And I don't want to over-extend myself in this article explaining how to type them.
The point is, any is for when you know that the type in question can't be specified and unknown is for when you don't know yet what it is.
When should you use it?
I'd say there are a few use cases for the unknown type. I certainly reach for it in these scenarios:
1) When migrating from a JavaScript file
When migrating from a JavaScript codebase, one will probably (maybe) do that on a file per-file basis. Let's say you open a given file, rename it to .ts and start seeing some errors because of missing types. One thing you could do is something like this:
type Movie = unknown;
You can create type aliases for the types in that file and then apply the given type whenever it's needed such as in functions function printMovie(movie: Movie).
After you have that completed you can come back to the Movie type and complete it with the needed properties.
The strategy applied here will be the same for the other scenarios as well. 🔁
2) When using a third party lib that does not have type definitions.
You can start creating a definition file (.d.ts) and begin filling it with unknown types for the things you are unsure of and not using at the moment, and type the things that you are using, and maybe make a pull request for that later on. 😀
3) When consuming an API that is not implemented.
If you are consuming an API to get some data and use that data, chances are you know the types already so make it safe 🔒and type it already. If you're not sure of its use or you don't know how data should be sent, then that's a good use of unknown.
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