code: function isKeyable(value) {
          var type = typeof value;
          return type == 'number' || type == 'boolean' || type == 'string' && value != '__proto__' || value == null;
        }
docs: Checks if `value` is suitable for use as unique object key.
  
  @private
  @param {*} value The value to check.
  @returns {boolean} Returns `true` if `value` is suitable, else `false`.