The unknown type in typescript is a common type that you will likely to come across. It is a strict version of the
any type and interms of accepting data, it accepts any data type which is similar to nature of any type. However
there is a minor differences between them. Since an unknown type also can have multiple type, it forces to use type assertion before using the types function. To further clear , lets look an example
function readAny(val: any){
return val.trim();
}
In the above example, we are using any type for the value. Since trim is a string function, this works well for string data. However on supplying numbers or booleans , we get a runtine error
The same example using unknown
function readAny(val: unknown){
return val.trim(); // typescript error at this line
}
Typescript would throw a compile time error as we are using string function without type assertion. To fix it, the correct code would look like
function readAny(val: unknown){
if( typeof val === 'string')
return val.trim();
}
Top comments (1)
There are several problems with this. Even if we used 'val: string' we would not get real type safety i.e at runtime. To get runtime type safety we have to use typeof or instanceof which are JavaScript primitives, or use a validation library. So then we have another problem how do you unit test the behaviour of non-string input? In typescript the compiler will stop us from passing in a non-string so we resort to either using 'any' or 'undefined' or a long list of '|' separated types which defeats the purpose of typescript and makes the code less readable and does not get added to the production bundle. A JavaScript equivalent of the example given without the superfluous type declarations would be much simpler create, with the added advantage of it being easily testable.