Types in Typescript - Any and Unknown

Sometimes, when you write code in a typed language, you have to work with values that are unknown to you at the time you write the code. When that happen, you can use any and unknown and you can use type guards to maintain control of what types your code can handle.

Any

The any type can represent any JavaScript value and it can be useful in several situations:

• when expecting values from a third party library
• when expecting values from a user input
• when you’re gradually migrating from JavaScript

let value: any = 'hey!'
value = true
value = 1

The aforementioned code, won’t throw any error at compilation time. The reason is that the type any skip the type checking.

Obviously, errors runtime errors will be triggers depending on the datatype and the assumption.

let value: any = true;
value.toUpperCase()
// Uncaught TypeError: value.toUpperCase is not a function

Unknown

As we saw in the previous example, the any type is really flexible, to the extent that it can be abused and can often cause unexpected errors.

The recently added unknown type try to solve such problems. It can be assign to any value but you can’t access properties or call or construct the to the value marked as unknown.

An example of unknown in action, along side the any type:

let value: any = 'hello'
value.toUpperCase()
// 'HELLO

value.map(e => console.log(e));
// value.map is not a function


let secondValue: unknown = 'world'
secondValue.toUpperCase()
// Error: Object is of type unknown

value.map(e => console.log(e));
// Error: Object is of type unknown

It’s not possible to interact, in any way, with a variable of type unknown. It’ll always error at compiling time. On the other hand, you can, always interact with a variable of type any and it won’t error at compiling time. However, it might error at runtime.

Conclusion

The unknown type would have been really useful in one of my previous companies. We were using Angular (framework based on TypeScript) but he had to move really fast and build several product from scratch in a very short period of time. We abused of the type any, to the extent that the only difference with JavaScript, was the possibility to use the latest syntactic sugar additions.

The type unknown would have been really useful for the developers that wanted to move fast, without breaking things. We didn’t have it at that time, but now we do and it’s awesome to break things!

If you have been, thank you for reading, keep on coding!