Existen tres formas de crear un reducer:

• Usando condicionales if:

const reducer = (state, action)=>{
    if(action.type==="ERROR"){
        return  {
            ...state,
            error: true,
            loading: false
        } 
    }else if(action.type==="CHECK"){
        return{
            ...state,
            loading: true,
        }
    }else{
        return {...state};
    }

• Usando switch case:

const reducer = (state, action)=>{
    switch (action.type){
        case "ERROR": 
        return {
            ...state,
            error: true,
            loading: false
        }
        case "CHECK":
            return{
                ...state,
                loading: true,
            }
        default:
            return {
                ...state,
            }
    }
}

• Usando objetos:

const reducerOBJECT = (state) => ({
    'ERROR': {
        ...state,
        error: true,
        loading: false,
    },
    'CONFIRM': {
        ...state,
        error: false,
        loading: false,
        confirmed: true,
    },
 })
  
 const reducer = (state, action) => {
    if (reducerOBJECT(state)[action.type]) {
        return reducerOBJECT(state)[action.type];
    } else {
        return {
            ...state,
        }
    }
 }