Cast object to interface in TypeScript

Question

I m trying to make a cast in my code from the body of a request in express  using body-parser middleware  to an interface  but it s not enforcing type safety   This is my interface   export interface IToDoDto     description  string    status  boolean       This is the code where I m trying to do the cast    Post   addToDo  Response   res   Request   req      const toDo  IToDoDto    lt IToDoDto gt  req body      lt  lt  lt  cast here   this toDoService addToDo toDo     return res status HttpStatus CREATED  end        And finally  the service method that s being called   public addToDo toDo  IToDoDto   void     toDo id   this idCounter    this todos push toDo     this idCounter        I can pass whatever arguments  even ones that don t come close to matching the interface definition  and this code will work fine  I would expect  if the cast from response body to interface is not possible  that an exception would be thrown at runtime like Java or C    I have read that in TypeScript casting doesn t exist  only Type Assertion  so it will only tell the compiler that an object is of type x  so    Am I wrong  What s the right way to enforce and ensure type safety

User · Answer

If it helps anyone  I was having an issue where I wanted to treat an object as another type with a similar interface  I attempted the following   Didn t pass linting  const x   new Obj a as b     The linter was complaining that a was missing properties that existed on b  In other words  a had some properties and methods of b  but not all  To work around this  I followed VS Code s suggestion   Passed linting and testing  const x   new Obj a as unknown as b     Note that if your code attempts to call one of the properties that exists on type b that is not implemented on type a  you should realize a runtime fault

User · Answer

Here s another way to force a type-cast even between incompatible types and interfaces where TS compiler normally complains   export function forceCast lt T gt  input  any   T             do runtime checks here        ts-ignore  lt -- forces TS compiler to compile this as-is   return input      Then you can use it to force cast objects to a certain type   import   forceCast   from    forceCast    const randomObject  any       const typedObject   forceCast lt IToDoDto gt  randomObject     Note that I left out the part you are supposed to do runtime checks before casting for the sake of reducing complexity  What I do in my project is compiling all my  d ts interface files into JSON schemas and using ajv to validate in runtime

User · Answer

There s no casting in javascript  so you cannot throw if  casting fails   Typescript supports casting but that s only for compilation time  and you can do it like this   const toDo    lt IToDoDto gt  req body     or const toDo   req body as IToDoDto    You can check at runtime if the value is valid and if not throw an error  i e    function isToDoDto obj  any   obj is IToDoDto       return typeof obj description      string   amp  amp  typeof obj status      boolean       Post   addToDo  Response   res   Request   req        if   isToDoDto req body             throw new Error  invalid request               const toDo   req body as IToDoDto      this toDoService addToDo toDo       return res status HttpStatus CREATED  end          Edit  As  huyz pointed out  there s no need for the type assertion because isToDoDto is a type guard  so this should be enough   if   isToDoDto req body         throw new Error  invalid request       this toDoService addToDo req body

[object] Cast object to interface in TypeScript

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