I have React component:
<Dropdown
    placeholder={field[propName].label}
    id={propName}
    fluid
    multiple
    selection
    search
    defaultValue={defaultOptions}
    options={options}
/>
So options and defaultOptions is the same structure arrays {text: 'string, value: 'string'}.
In semantic UI source code I found this:
/** Initial value or value array if multiple. */
    defaultValue: PropTypes.oneOfType([
      PropTypes.string,
      PropTypes.number,
      PropTypes.arrayOf(PropTypes.oneOfType([
        PropTypes.string,
        PropTypes.number,
      ])),
    ])
That the reason why my code above gives me error:
`Warning: Failed propType: Invalid prop `defaultValue` supplied to `Dropdown`. Check the render method of `View`.`
So question is how then I should set defaultValue for multi selection type of Dropdown?
defaultValue cannot be an object for semantic-UI-react. It can only be a value. http://react.semantic-ui.com/modules/dropdown. If you look at the props of defaultValue, the docs say that it can be a string, number, or arrayOf.
I usually set mine to value of the dropdown - using immutabilityJS - when it is switched onChange.
<Dropdown
    placeholder={field[propName].label}
    id={propName}
    fluid
    multiple
    selection
    search
    defaultValue={dropdownList.get('forWhat')}
    options={options}
    onChange={(e, {value}) => this.updateDropdownList('forWhat',[value:value, text:"works"])}
/>
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