Is it possible to somehow combine the description of the two types of the React component props below.

On isEdit == true:

type DialogProps = {
  isEdit: boolean
  obj: ICustom
}

and on isEdit == false:

type DialogProps = {
  isEdit: boolean
  obj: null
}

>Solution :

Try using discriminated union:

type EditDialogProps = {isEdit: true, obj: ICustom};
type NonEditDialogProps = {isEdit: false}; // just remove obj in this one :)
type DialogProps = EditDialogProps | NonEditDialogProps;