Optional Attribute Type Definition as inline object

def.ts

/**
 * Definition of arg fields for typesafe event handling.
 * 
 * @example
 * {
 *   parameters: 'param1' | 'param2'
 *   arguments: 'arg1' | 'arg2'
 * }
 */
interface ParameterArgDefinition {
  /**
   * Definition of query params keys. Type of value is `boolean`.
   * @example 'param1' | 'param2'
   */
  parameters?: string
  /**
   * Definition of path params. Type of value is `number`.
   * @example 'arg1' | 'arg2'
   */
  arguments?: string
}

/**
 * TheInterface with nicely defineable optional parameters/arguments.
 * 
 * Define `parameters` and `arguments` if required.
 * 
 * @example
 * TheInterface<{
 *   parameters: 'param1' | 'param2'
 *   arguments: 'arg1' | 'arg2'
 * }>
 */
export interface TheInterface<Def extends ParameterArgDefinition = never> {
  otherField: string;
  definedParameters: [Def['parameters']] extends [string] ? { [P in Def['parameters']]: boolean } : unknown;
  definedArguments: [Def['arguments']] extends [string] ? { [P in Def['arguments']]: number } : unknown;
}