code: function(params) {
      params = _.extend({
        response_type : "code",
        client_id : this.clientId,
        redirect_uri : this.redirectUri
      }, params || {});
      return this.authzServiceUrl +
        (this.authzServiceUrl.indexOf('?') >= 0 ? "&" : "?") +
        querystring.stringify(params);
    }
docs: Get Salesforce OAuth2 authorization page URL to redirect user agent.
  
  @param {Object} params - Parameters
  @param {String} [params.scope] - Scope values in space-separated string
  @param {String} [params.state] - State parameter
  @param {String} [params.code_challenge] - Code challenge value (RFC 7636 - Proof Key of Code Exchange)
  @returns {String} Authorization page URL