Checking for null properties in Typescript objectsorHow to verify if a

What is a simple way to determine if the properties of an object in TypeScript are nullable? For example

export default interface UserDto{
     ID?:int;

      USER_NAME?:string;
  
      FIRST_NAME?:string;
  
      LAST_NAME?:string;
  
      USER_ROLE?:string;
  
       TEAM?:string;
     IS_ACTIVE?:Boolean;
    CREATE_DATI?:DateTime;
     UPDATE_DATI?:DateTime;
       PASSWORD?:string;
}

The USER_ROLE property mentioned above is nullable. How can I check if it is nullable or not within an if statement?

javascriptreactjstypescriptvue.js

Answer №1

It seems like your goal is to create a conditional statement using an if clause that verifies if the property of an object is optional (meaning it is not necessarily undefined, but specified as optional in the type declaration).

Unfortunately, this cannot be achieved since types are not present during runtime.

Your best bet is to validate the value itself or explicitly list the optional keys and include them in your condition logic.

Answer №2

During runtime, it is not feasible because all types are stripped away when the code is compiled (Refer to "What is type erasure?").

However, you can utilize Conditional Types at compile time to analyze whether a key in the UserDto object is optional or mandatory.

interface UserDto {
  USER_ROLE?: string;
  REQUIRED: string;
}

type IsOptionalKey<T, K extends keyof T> = {} extends Pick<T, K>
  ? true
  : false;

type Result = IsOptionalKey<UserDto, "USER_ROLE">; // true
type Result2 = IsOptionalKey<UserDto, "REQUIRED">; // false

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