title: Angular what's the meaning of these three dots in @NGRX
body: <p>What do these three dots mean exactly, and why do I need them?</p>
  <pre><code>export function leadReducer(state: Lead[]= [], action: Action {
      switch(action.type){
          case ADD_LEAD:
              return [...state, action.payload];
          case REMOVE_LEAD:
              return state.filter(lead =&gt; lead.id !== action.payload.id )
  }
  }
  </code></pre>