title: TypeScript error number is not assignable
body: <p>I am writing a reducer and I want RESET to do <code>state.count = 0</code>.  However, this gives me an error that <code>number is not assignable to type CounterState</code>.</p>
  
  <p>Here is my code:</p>
  
  <pre><code>export interface CounterState {
    count: number;
  };
  
  const initialState: CounterState = {
    count: 0
  };
  
  export default function counter(state = initialState, action: Action): CounterState {
    switch (action.type) {
      case TYPES.INCREMENT:
        return assign({}, state, {
          count: state.count + 1
        });
      case TYPES.DECREMENT:
        return assign({}, state, {
            count : state.count - 1
          });
      case TYPES.RESET:
        //error here
        return state.count=0;
      default:
        return state;
    }
  }
  </code></pre>
  
  <p>I am using lodash's assign function.  Thanks</p>