TypeScript 中是否可以使用编译时标记数字？

Question

As an example, I would like to create a Port type and a Seconds type, both thin wrappers around the number type, such that neither is assignable to the other.

let httpPort: Port = 80;
let oneMinute: Seconds = 60;

httpPort = oneMinute; // type error
oneMinute = httpPort; // type error

Answer 1

We can get some semblance of type safety with unique symbol . Specifically, a field with declared type unique symbol will never compare compatible with another field of the same name, unless it comes from the same declaration. So we can tag our numbers with fake unique symbols to make them incomparable.

type Port = number & { readonly __tag: unique symbol };
type Seconds = number & { readonly __tag: unique symbol };

// Note that we do have to explicitly cast the numbers, as we're
// technically lying to the type system to get this behavior.
let httpPort = 80 as Port;
let oneMinute = 60 as Seconds;

Now httpPort and oneMinute are mutually incompatible. If we try to compare them, we get something like this.

file.ts:8:1 - error TS2322: Type 'Seconds' is not assignable to type 'Port'.
  Type 'Seconds' is not assignable to type '{ readonly __tag: unique symbol; }'.
    Types of property '__tag' are incompatible.
      Type 'typeof __tag' is not assignable to type 'typeof __tag'. Two different types with this name exist, but they are unrelated.

8 httpPort = oneMinute; // type error

Unfortunately, we can still do nonsense things like add two ports or a port or and a second together because Typescript will happily upcast either of them to number (that is how intersection types work, after all), but at least we can't pass a Port to a function expecting a Seconds anymore, or assign the variables incorrectly.

Note that what you're trying to do is often referred to as the "newtype pattern", after the newtype keyword in Haskell (which exists for this exact purpose). In particular, you can read more about its use in Typescript on this page (which is where I learned this little trick too).