Button Click -- https://reactjs.org/docs/react-component.html#setstate

increment-once.js

onClick=>{ 
  () => this.setState({pageNumber:this.state.pageNumber+1})
}

increment-per-click.js

onClick={
  () => this.setState(({pageNumber}) => {pageNumber:pageNumber+1})
}

NOTE

The way increment-per-click is implemented, each time the click the button, they will increment the page number. So if you click 3 times, it'll advance the page from page x to x+3.

For increment-once, this would mean that until the page gets re-rendered, clicking the button 3 times would still just move from x to x+1 -- and it will always be x+1 until it's re-rendered with an updated pageNumber.