eslint-plugin-react/docs/rules/no-direct-mutation-state.md

Prevent direct mutation of this.state (react/no-direct-mutation-state)

NEVER mutate this.state directly, as calling setState() afterwards may replace the mutation you made. Treat this.state as if it were immutable.

The only place that's acceptable to assign this.state is in a ES6 class component constructor.

Rule Details

This rule is aimed to forbid the use of mutating this.state directly.

The following patterns are considered warnings:

var Hello = createReactClass({
  componentDidMount: function() {
    this.state.name = this.props.name.toUpperCase();
  },
  render: function() {
    return <div>Hello {this.state.name}</div>;
  }
});

class Hello extends React.Component {
  constructor(props) {
    super(props)

    // Assign at instance creation time, not on a callback
    doSomethingAsync(() => {
      this.state = 'bad';
    });
  }
}

The following patterns are not considered warnings:

var Hello = createReactClass({
  componentDidMount: function() {
    this.setState({
      name: this.props.name.toUpperCase();
    });
  },
  render: function() {
    return <div>Hello {this.state.name}</div>;
  }
});

class Hello extends React.Component {
  constructor(props) {
    super(props)

    this.state = {
      foo: 'bar',
    }
  }
}