3 awesome features of the TypeScript type system

(Note: This originally appeared on Codeburst)

As of 2017 TypeScript has emerged as one of the most popular languages which can be “compiled down” to JavaScript for both browser and nodejs development. Compared to alternatives like Facebook’s Flow or CoffeeScript one of TypeScript’s most unique features is its expressive type system. Although TypeScript’s type system is technically dynamic, if you fully embrace it you’ll be able to leverage several features of the TypeScript compiler.

So what are a couple of these features? We’ll be looking at code from Setfive’s CloudWatch Autowatch an AWS cloud monitoring tool written in TypeScript. Since TypeScript is evolving so quickly it’s worth noting that these examples were run on version 2.4.2. Anyway, enough talk lets code!

Non-nullable types

If you have any experience with Java you’ve probably encountered the dreaded NullPointerException when you tried to deference a variable holding a “null” value. It’s certainly a pain and has been derided as a “billion dollar mistake” by its creator. To tackle NullPointerException bugs TypeScript 2.0 introduced the concept of non-nullable types. It’s “opt-in” via the “strictNullChecks” compiler flag which you can set your tsconfig.json

Consider this simple sample:

By default, the TypeScript compiler will compile that code since “null” is assignable to string[] but you’ll get an error about half the time you run it. Now, if we set “strictNullChecks: true” and run the compiler we’ll get an error:

partyguests.ts(5,9): error TS2322: Type ‘null’ is not assignable to type ‘string[]’.

Since the compiler can infer that at least one code path in the function produces a null which is now incompatible with an array. An example of this in the Autowatch code are the checks to ensure that PutMetricAlarmInput instances aren’t created with null dimensions. At line 462 for example.

Exhaustive type matching

Most programming languages with a Hindley–Milner type system have some functionality to perform a “pattern match” over a type. In practice, that allows a developer to make decisions about what to do with a set of objects based on the concrete type vs. their abstract signatures. For example, in Scala:

At face value it appears as if you can replicate this ES2015 JavaScript with something like:

That misses an important piece from the Scala example, the compiler guarantees an exhaustive match. If you added a new subclass of “Notification” like BulkSMS the Scala compiler would error because showNotification is missing that case. The JavaScript example wouldn’t be able to provide this and you’d end up getting a runtime error.

However, with TypeScript’s “never” type it’s possible to have the compiler guarantee an exhaustive match for us. We could do something like the following:

The important part is the call to “assertNever” which the compiler will error on if it detects is a reachable code path. We can confirm this if we add “BulkMessage” to the “MyNotification” type but not the if:

If you run the TypeScript compiler against that you’ll get an error highlighting that your if isn’t exhaustive since it’s hitting the “never”:

match2.ts(19,24): error TS2345: Argument of type ‘BulkMessage’ is not assignable to parameter of type ‘never’.

It’s certainly not as elegant as the Scala example but it does the job. You can see a real example in Autowatch starting at line 168 where we used it to guarantee exhaustive matching on the available AWS services.

Read-only class properties

Marking a class property “readonly” signals to the compiler that code shouldn’t be able to modify the value after initialization. Although “readonly” may sound similar to marking a property as “private” it actually enhances the type system in important ways. First, “readonly” properties make it possible to more faithfully represent immutable data. For example, a HTTP Request has a “url” which will never change after the request has started. And by marking properties as “readonly” as opposed to private you’re still able to return literal objects with matching properties.

Let’s look at an example:

If you run that through the TypeScript compiler you’ll get an error advising that the property is readonly:

readonly.ts(6,5): error TS2540: Cannot assign to ‘url’ because it is a constant or a read-only property.

Now, if you try and mark the url property as private and create a literal HttpRequest you’ll notice you’ll get an error:

But if you switch it back to “readonly” it’ll work as expected.

You can see real world usage of this in Autowatch, where we marked properties in our Config class as readonly since they should never change once the object has been constructed.

That’s a wrap

Well that’s three pretty cool features of the TypeScript type system that should help you be more productive and write better code. If you found these interesting, there’s several other interesting type related features that landed in 2.3+ versions of TypeScript that are worth checking out.

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