TypeScript for Java Developers: The 'unknown' Type

TypeScript 3.0 introduced the unknown type, which is described as being: “Like any, but type-safe”. This is an exploration of what that means in the Javascript and TypeScript ecosystems. and how that can be compared to strongly-typed languages like Java.

In Java, since the initial release, there has been a “top” type (at least, for non-primitive values…), called Object. All classes in Java directly or indirectly extend from Object, and therefore, all types may be referred to as an Object (String, List, BankAccount, etc.), and there is some degree of method calls available for those types (such as toString(), hashcode(), equals(), etc.) Similarly, Kotlin has the Any type which also represents anything. Kotlin also makes a point to support primitives as children of Any as well, though that is largely a compiler trick (the runtime for Kotlin still treats primitives and objects separately, since it is still a JVM language, and Value Types are not a thing yet).

But Typescript Already Has any

TypeScript, on the surface, has a similar concept with any. As with Java, any may be anything. However, unlike Java, the any type in Typescript means “a value we don’t know because it comes from a dynamic source”. As a result, with any you can treat the variable like any Javascript value, and, as a result, call arbitrary methods on them and request arbitrary fields.

1let aPerson: any = { age: 15, name: "Bob" }
2// Even though it's an 'any', the field can be accessed:
3let name = aPerson.name

In effect, any reverts to JavaScript weak typing for a specific variable. Consequently, the problem with any is that it is too permissive and defeats the strictness of TypeScript. An any variable is, by its very definition treated as “anything” - you can invoke any functions or access any fields.

The goal with unknown is to have a type in its “least capable form”, meaning the top-level and most useless type, and requiring some degree of clarification or refinement before it can be used.

OK, but What About object

With TypeScript 2.2+, the object variable type was introduced, and defines some value that is not a Javascript primitive (so, very similar to Java’s Object). Namely, not: boolean, number, string, symbol, null, or undefined; in other words, all object fields must be something that could be expressed via {}.

So, this ensures the value is not a primitive value (though JS primitives are different in nature). While this is interesting and useful, it is not the same role that a top-type plays in a proper type system (though admittedly, it is very similar to Object in Java!)

Back to unknown

And so, we come back to TypeScript 3.0’s unknown type. Like any, a variable of this type may be numbers, strings, booleans, null, undefined, arrays, and any other object type. However, unlike any, an unknown variable can’t be acted on until it has been clarified and narrowed into a more specific type. As a result, unknown is much like Object in Java or Any in Kotlin: it is known to be “something”, but without a refinement (aka cast) to a specific or narrowed type, it is not of much use (and, as a result, the program is more type-safe). Revisiting our previous example, but with the unknown type:

1let aPerson: unknown = { age: 15, name: "Bob" }
2// Compiler Error: error TS2571: Object is of type 'unknown'.
3let name = aPerson.name

To resolve this, the compiler must have a preceding constraint check (similar to instanceof in Java or is in Kotlin) that checks the minimal required shape of the object. For example:

1let aPerson: unknown = { age: 15, name: "Bob" }
2if(aPerson is { name: String })
3  let name = aPerson.name
4  // do something with name
5}

Note that, unlike Java or Kotlin, the “type check” is based on the existence of fields and methods on the underlying instance (duck typing), rather than some actual runtime type (remember: in TypeScript types are conventional and not actually encoding into the underlying object).

Summary

Hopefully this describes the benefits of unknown. Ideally, most programs can likely transition use of any to unknown seamlessly; for most TypeScript programs any has been treated as unknown. However, occasionally there have been edges where the benefit of an any “dynamic invocation” have been too beneficial to resist due to the neatness of a programming model it provides.

Thankfully, any as a type still exists and migration can be gradual and targeted.

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