function usernameExists(username, cb) {
    if (typeof(cb) != "function") {
      throw new Error("cb must be a function.");
    }

    let url = this.baseURI + username;
    let request = new RESTRequest(url);
    request.get(this._onUsername.bind(this, cb, request));
  }
/**
   * Determine whether a specified username exists.
   *
   * Callback receives the following arguments:
   *
   *   (Error) Describes error that occurred or null if request was
   *           successful.
   *   (boolean) True if user exists. False if not. null if there was an error.
   */
Determine whether a specified username exists.
Callback receives the following arguments.

(Error) Describes error that occurred or null if request was
successful.
(boolean) True if user exists. False if not. null if there was an error.
Determine whether a specified username exists . Callback receives the following arguments . ( Error ) Describes error that occurred or null if request was successful . ( boolean ) True if user exists . False if not . null if there was an error .
function usernameExists(username, cb) {
    if (typeof(cb) != "function") {
      throw new Error("cb must be a function.");
    }
    let url = this.baseURI + username;
    let request = new RESTRequest(url);
    request.get(this._onUsername.bind(this, cb, request));
  }
function usernameExists ( username , cb ) { if ( typeof ( cb ) != "function" ) { throw new Error ( "cb must be a function." ) ; } let url = this . baseURI + username ; let request = new RESTRequest ( url ) ; request . get ( this . _onUsername . bind ( this , cb , request ) ) ; }
Determine whether a specified username exists.
Determine whether a specified username exists .