Use spread operator. Start to slice from beginning until index, then add a slice starting from index + 1 until end.

Redux: Remove an Item from an Array

const immutableReducer = (state = [0,1,2,3,4,5], action) => {
  switch(action.type) {
    case 'REMOVE_ITEM':
      // don't mutate state here or the tests will fail
      return [
        ...state.slice(0, action.index),
        ...state.slice(action.index + 1, state.length)
      ]
    default:
      return state;
  }
};

const removeItem = (index) => {
  return {
    type: 'REMOVE_ITEM',
    index
  }
}

const store = Redux.createStore(immutableReducer);