Mutating State without setState()

• Causes state changes without making component re-render.
• Whenever setState gets called in future, the mutated state gets applied.

Bad

class SampleComponent extends Component {
  constructor(props) {
    super(props);

    this.state = {
      items: ['foo', 'bar']
    };

    this.handleClick = this.handleClick.bind(this);
  }

  handleClick() {
    // BAD: We mutate state here
    this.state.items.push('lorem');

    this.setState({
      items: this.state.items
    });
  }

  render() {
    return (
      <div>
        {this.state.items.length}
        <button onClick={this.handleClick}>+</button>
      </div>
    )
  }
}

Good

class SampleComponent extends Component {
  constructor(props) {
    super(props);

    this.state = {
      items: ['foo', 'bar']
    };

    this.handleClick = this.handleClick.bind(this);
  }

  handleClick() {
    // We update using setState() - concat return new array after appending new item.
    this.setState(prevState => ({
      items: prevState.items.concat('lorem')
    }));
  }

  render() {
    return (
      <div>
        {this.state.items.length}
        <button onClick={this.handleClick}>+</button>
      </div>
    )
  }
}

Related links:

React Design Patterns and best practices by Michele Bertoli.