typescript

Make second argument of a function optional or required, based on the first argument type

How to make data arg optional only if cat argument is not 1 ?

export class MyClass<T extends 1 | 2 | 3>{
  constructor(
    public cat: T,
    public data?: T extends 1 ? string[] : undefined,
  ) {
    ...
  }
}
Solution

  • You could go with overloads.

    export class MyClass<T extends 1 | 2 | 3> {
  constructor(cat: 1, data: string[]);
  constructor(cat: Exclude<T, 1>);
  constructor(
    public cat: T,
    public data?: string[],
  ) {
    ...
  }
}

    Playground