message: adding default arg
diff: import { BaseToneOptions } from "../Tone";
  -import { isDefined, isObject } from "./TypeCheck";
  +import { isDefined, isObject, isUndef } from "./TypeCheck";
  /**
  * Recursively merge an object
  @@ -55,11 +55,13 @@ export function getDefaultsFromInstance<T>(instance: T): BaseToneOptions {
  }
  /**
  + * Returns the fallback if the given object is undefined.
  * Take an array of arguments and return a formatted options object.
  - * @param args the arguments passed into the function
  - * @param keys an array of keys
  - * @param defaults the class's defaults
  */
  -// export function defaultArg<T>(given: T, fallback): T {
  -
  -// }
  +export function defaultArg<T>(given: T, fallback: T): T {
  + if (isUndef(given)) {
  + return fallback;
  + } else {
  + return given;
  + }
  +}
  