Const assertion or "as const" has been supported in TypeScript since late 2019, yet many developers still don’t know about it. What are the benefits it brings, and can they improve any project?
Ondrej PolesnyPublished on Apr 3, 2023
In this article, I’ll explain const assertion in TypeScript, show how to use it, and explain the benefits it brings. I will also share two examples of how we use const assertion on this website and touch on the support for strongly typed content resolution using the Kontent.ai model generator.
What is const assertion?
Let’s say we have a variable:
We can always change the value of the variable:
And the type of the
text variable will be a
string. Now, when we use const assertion:
We effectively create a type
string. And while you can still change the value, you can only change it to
"Hello" and not anything else.
The same can be applied to objects and arrays:
You can now change the person object’s properties.
But if you use the const assertion:
The object’s properties’ values are now read-only. Const assertion looks at the used values, infers the most specific type possible, and applies it.
Now, why is this beneficial, and when can you use it? It mainly helps with:
- Type safety
You can ensure that a variable or property has a specific value and nothing else and catch potential errors at compile time.
You can make objects and their properties used in multiple places throughout your project read-only, which helps avoid errors and unintentional mutations at a compile time. You will know you’re making a mistake while you’re writing the code.
You no longer need to explicitly define types everywhere in your code when they can be inferred from the values.
Using const assertion in a real project
Let me show you two examples of how we use const assertion in our project:
This code defines search categories – their titles, tags CSS classes, and priority.
Now, the const assertion helps here narrow down the type of the search categories’ titles which we then use in an interface that unifies data for a search engine:
The extracted type
SearchCategoryTitle is essentially this:
As opposed to a simple
string if we didn’t use
This is great as we get the type safety on the level of the search categories titles. It prevents any developer to assign a custom value that would break the front-end part of the search functionality.
Safe rich text resolution with const assertion
Another example is the marketing landing pages on this site. Editors can compose them using predefined components, and we resolve those components in code when we’re pre-building the site. During that process, we need to:
- Make sure each used component in the CMS is allowed for the UMLP page; that is, we have a React component for it
- Find the React component corresponding to the used content type and render it using the respective content item data
We do this using a mapper from content type to React component:
So where is the const assertion? It’s actually in the model generator. Note that we’re not using string literals to define content types’ codenames. Instead, we’re taking them from the generated
contentTypes file, which indeed marks them with
as const keyword.
In this case, the type
UmlpCodenames will be this:
If there was no const assertion in the generated file, the type would look like this:
Which would be useless for any additional type of safety checks. Now, having the type more specific, we can do a number of checks at compile time and ensure that each React component receives the data in the format it expects. We do that using a list of
UmlpBinding types and a type guard:
Now, this code sample is way beyond the const assertion part of this article, so I won’t go into too much detail here. However, const assertion allowed us to:
- Introduce compile-time type checks for resolved components even though we didn’t know which components and data we’ll receive until runtime
- Remove 1000 lines long large switch statement that used to handle these components and their data one by one
In this article, I explained what const assertion or
as const is and how it can help with type safety, readability, and debugging of your code. I also shared two examples of how we use const assertion in our web project. Const assertion is a very powerful tool that has the potential to make your code safer and your development more effective.
If you’re interested in learning more about any of the examples mentioned above or just want to talk to other developers in the Kontent.ai community, get in touch on our Discord server.
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