mixin class that extends your component and makes `setState` "safe". See https://codesandbox.io/s/6438ymvk8z

SafeSetState.ts

import * as React from 'react';

// see https://github.com/Microsoft/TypeScript/pull/13743 for what the typing in here's all about
type Constructor<T> = new (...args: any[]) => T;

const isMounted_Symbol = Symbol('isMounted');

/**
 * This is for when you're calling setState in uncancellable async callbacks.
 * NOTE: Doing this can mask memory leaks so be careful
 * TODO: typing might need some work
 */
export function makeSafeSetStateComponent<T extends Constructor<React.Component>>(ComponentType: T) {
  return class SafeSetStateComponent extends ComponentType {
    // use symbol so we don't inadvertently override a property on the parent class
    private [isMounted_Symbol] = false;

    setState(...args) {
      if (this[isMounted_Symbol]) {
        super.setState.apply(this, args);
      }
    }

    componentDidMount() {
      this[isMounted_Symbol] = true;
      super.componentDidMount && super.componentDidMount();
    }

    componentWillUnmount() {
      this[isMounted_Symbol] = false;
      super.componentWillUnmount && super.componentWillUnmount();
    }
  }
}