title: How do you set the value of a TextField from Dropdown Selection?
body: <p>In the following code, I was curious as to how you would set the value of the following TextField. 
  For this example, how do I set the TextField to the selected item in the Dropdown?
  If the user selects "TOP LEVEL" in the Dropdown, then I want to populate the TextField to be "TOP LEVEL". The Dropdown is called <strong>ChildComponent</strong></p>
  
  <pre><code>import * as React from "react";
  import ChildComponent from './Operations/ChildComponent';
  import { DropdownMenuItemType, IDropdownOption } from 'office-ui-fabric-react/lib/Dropdown';
  import { TextField} from 'office-ui-fabric-react/lib/TextField';
  export interface ParentComponentState {
      selectedItem?: { key: string | number | undefined };
      value: {key: string};
    }
  
    export default class ParentComponent extends React.Component&lt;{}, ParentComponentState&gt; {
      constructor(props, context) {
          super(props, context);
  
        }
      public state: ParentComponentState = {
        selectedItem: undefined,
        value: undefined,
      };
  
    render(){
      const { selectedItem } = this.state;
      const options: IDropdownOption[] = [
          { key: 'blank', text: '' },
          { key: 'topLevelMake', text: 'Parents', itemType: DropdownMenuItemType.Header },
          { key: 'topLevel', text: 'TOP LEVEL' },
          { key: 'make', text: 'MAKE ITEM' },
          { key: 'divider_1', text: '-', itemType: DropdownMenuItemType.Divider },
          { key: 'Purchased', text: 'Purchases', itemType: DropdownMenuItemType.Header },
          { key: 'rawMaterial', text: 'RAW MATERIAL' },
          { key: 'buyItem', text: 'BUY ITEM' },
        ];
        return(
          &lt;div&gt;
              &lt;ChildComponent 
                  options={options} 
                  selectedKey={selectedItem ? selectedItem.key : undefined}
                  onChange={this._onChange}
              /&gt;
              &lt;TextField
                  name="textTest"
                  label={"Test"}
              /&gt;
          &lt;/div&gt;
            );
     }
     private _onChange = (event: React.FormEvent&lt;HTMLDivElement&gt;, item: IDropdownOption): void =&gt; {
      this.setState({ selectedItem: item });
      let opValue = item.text;
      console.log(event);
      console.log(opValue);
    };
  }
  </code></pre>
  
  <p>After inserting Muhammad's logic, here is the error I am getting. Do I need to add an onChange event for the TextField? and then put "this.state.selectedItem" in the handleChange event? Do I need to make a new child component and have the TextField rollup to ParentComponent?</p>
  
  <p><a href="https://i.stack.imgur.com/yQ1GU.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/yQ1GU.png" alt="enter image description here"></a></p>