Variables are assigned in if else block of JavaScript but showing undefine when called

So I was writing some code for react-redux and I encounter this anomalous behaviour in javascript. Below is the code.

case CONSTANTS.ADD_CARD: {
      const { listId, card } = action.payload;

      if(typeof state[listId]==="undefined"){
        let newCard = [...state[listId], card] //defined variable based on condition
      }else{
        let newCard = [card] //defined variable for fail condition
      }
      const newState = {
        ...state
      }
      newState[listId] = newCard //error undefined variable showing hear 
    
      return newState;
    }

The above code is a part of switch statement.

Also, I tried declaring a variable above if/else block and then initiated it in the if/else block, but encountered the same error.

I solved the problem using the ternary operator, but still wondering the reason for the problem. Please let me know the reason behind this.

Thank you