code: function sanitizeProperties(custom_properties) {
    var properties = _extends({}, default_properties, custom_properties);
  
    // convert button keyword to associated number
    if (typeof properties.button === 'string') {
      properties.button = button_keywords[properties.button];
    }
  
    return properties;
  }
docs: Constructs mouse event properties object, translates button keywords
  @param {Object} custom_properties
  @returns {Object}
  @private