1 // React change state
2this.setState({state to change:value});

1import React, {Component} from 'react';
2
3class ButtonCounter extends Component {
4  constructor() {
5    super()
6    // initial state has count set at 0
7    this.state = {
8      count: 0
9    }
10  }
11
12  handleClick = () => {
13    // when handleClick is called, newCount is set to whatever this.state.count is plus 1 PRIOR to calling this.setState
14    let newCount = this.state.count + 1
15    this.setState({
16      count: newCount
17    })
18  }
19
20  render() {
21    return (
22      <div>
23        <h1>{this.state.count}</h1>
24        <button onClick={this.handleClick}>Click Me</button>
25      </div>
26    )
27  }
28}
29
30export default ButtonCounter
31

1// Correct
2this.setState(function(state, props) {
3  return {
4    counter: state.counter + props.increment
5  };
6});

1  const [a, b] = React.useState(['hi','world']);
2  const dup = [...a]; //won't work without spread operator
3  b(dup);