Angular what's meaning of three dots in @NGRX : <p>what does mean exactly these three dots and why 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>
