Type 'void' is not assignable to type '((event: MouseEvent<HTMLInputElement>) => void) | undefined'

   import * as React from "react";
   import "./App.css";
   import PageTwo from "./components/PageTwo";
   
    export interface IPropsk {
        data?: Array<Items>;
        fetchData?(value: string): void;
    }

    export interface IState {
       isLoaded: boolean;
       hits: Array<Items>;
       value: string;
    }
    class App extends React.Component<IPropsk, IState> {
        constructor(props: IPropsk) {
        super(props);

        this.state = {
        isLoaded: false,
        hits: [],
        value: ""
        this.handleChange = this.handleChange.bind(this);
  }   

  fetchData = val => {
        alert(val);
  };

  handleChange(event) {
       this.setState({ value: event.target.value });
  }


  render() {
   return (
      <div>
        <div>
           <input type="text" value={this.state.value} onChange= {this.handleChange}
           <input type="button" onClick={this.fetchData("dfd")} value="Search" />
      </div>
     </div> 

    );

  }
}

 export default App;

In the above code example I tried to call a method(fetchData ) by clicking button with a paremeter.But I gives a error from following line

 <input type="button" onClick={this.fetchData("dfd")} value="Search" />

The error is

type 'void' is not assignable to type '((event: MouseEvent) => void) | undefined'.

In your code this.fetchData("dfd") you are calling the function. The function returns void. void is not assingable to onClick which expects a function.

Fix

Create a new function that calls fetchData e.g. onClick={() => this.fetchData("dfd")} .

More

This is a very common error prevented by TypeScript 

With Functional Components, we use React.MouseEvent and it clears things up...

const clickHandler = () => {
  return (event: React.MouseEvent) => {
    ...do stuff...
    event.preventDefault();
  }
}

You could also do something like

fetchData = (val: string) => (event: any) => {
  alert(val);
};

Alternatively, you can set a type for your event, such as React.MouseEvent. You can read more about it here.

You can just set type this way and you will get no errors

export interface IPropsk {
    data?: Array<Items>;
    fetchData?(): (value: string) => void;
}

As @basarat said above, when you have something like button.onclick = thisFunction(); you are already calling that function. You may want to just assign that function but not to call it,

so you would write like this button.onclick = thisFunction;.