Fluent UI | React | Controlled multi-select Dropdown - Selected Keys

import { Dropdown, IDropdownOption } from 'office-ui-fabric-react'; // office-ui-fabric-react@7.204.0

export interface State {
	itemList: IDropdownOption[];
	selectedItems: string[];
}

export default class DropDownExample extends React.Component<any, State> {
	constructor(context) {
		super(context);
		this.state = {
			itemList: [{ key: 'None', text: 'None' }],
			selectedItems: []
		}
		this.GetItems();
		this.LoadPreselection();
	}
		
	private async GetItems(){
		// ... load items into itemList
	}

	private async LoadPreselection(){
		// ... load pre-selected items into selectedItems
	}

	public onItemChange = (event: React.FormEvent<HTMLDivElement>, item: IDropdownOption): void => {

		if (item) {
      		let tempItemsArray = selectedKeys;
      		const index = tempItemsArray.indexOf(item.key, 0); // Obtain item index if exists - returns -1 if it does not

      		if (index > -1) {
      		   tempItemsArray.splice(index, 1); // If the item exists, remove the item from the existing array through the splice function
			} else {
      		  tempItemsArray.push(item.key); // If the item does not exist, add the item to the existing array
      		}

      		setSelectedKeys(tempItemsArray); // Update State
    	}

	}

	render() {
		return (

			...

			<Dropdown
				placeholder="[Select Item]"
				disabled={this.state.itemList.length === 0}
				label='Items'
				id='items'
				responsiveMode={1}
				multiSelect
				options={this.state.itemList}
				selectedKeys={this.state.selectedItems}
				onChange={this.onItemChange.bind(this)}
			/>

			...

		);
	}
}

...this.state.selectedItems, item.key as string

this.state.selectedItems.filter(key => key !== item.key)

public onItemChange = (event: React.FormEvent<HTMLDivElement>, item: IDropdownOption): void => {

		if (item) {
			this.setState({
				selectedItems:
					item.selected
						? [...this.state.selectedItems, item.key as string]
						: this.state.selectedItems.filter(key => key !== item.key),
			});
		}

	}