When parameter validation is enabled, validate the parameters

schema_with_parameter_decorators.ts

function schema(schema: Schema, validateParams: boolean = false) {
  return function validateArgs(target: any) {
    // save a reference to the original constructor
    var original = target;
    // wrap orginal constructor with validation behaviour
    var f: any = function(...args) {
      // When parameter validation is enabled...
      if (validateParams) {
        validateConstructorParams(target, args); // validate the parameters
      }
      const instance = new original(...args);
      const { error } = schema.validate(instance);
      if (error instanceof Error) {
        throw error;
      }

      return instance;
    };
    // set f's prototype to orginal's prototype so f keeps original's type
    f.prototype = original.prototype;
    return f;
  };
}