在switch语句中使用break时出现无法访问的代码错误

Question

I am wondering what I am doing wrong in my code. I am trying to do the following:

switch (action.type) {
    case TYPES.ADD_TO_FAVORITES:
      if (state.faves.length <= 10) {
        return assign({}, state, {
          faves: state.faves.concat(action.payload),
          full: false
        });
      } else {
        return assign({}, state, {
          faves: state.faves,
          full: true
        });
      }
    default:
      return state;
  }

My linter says to add a break before default case, but when I do that, it says unreachable code .

Answer 1

A return will act as a break . There is no need to use both. If you receive the message that you have unreachable code, then there is either no point in it being there, or something you did previously is out of order logically.

Answer 2

The linter rule ie 'no-fallthrough' in eslint, acts as to not allow any accidental fallthrough from case to case.

Meaning without break code execution will continue from matching case to next cases unless a break, return etc is encountered.

Sometimes we do need this but unintentional fallthrough can happen and this rule tries to prevent that.

You can disable the rule or configure it as warning . I would recommend to have a variable assigned for return value and at the end of the function and return it without disabling the rule.

function() {
var returnvalue;
Switch(variableA) {
    Case 1:
        returnvalue = somevalue;
        break;
    case 2:
        returnvalue = some other value;
        break;
     default:
         returnvalue= default value;
    }
return returnvalue; 

}

And for the unreachable part, you are returning from your if else block.

So the break will never get any chance to execute.

Answer 3

switch (action.type) {
case TYPES.ADD_TO_FAVORITES:
  if (state.faves.length <= 10) {
    return ...;
  } else {
    return ...;
  }
  break; // YOU HAVE ADDED BREAK HERE
default:
  return state;
}

Inside case TYPES.ADD_TO_FAVORITES , either if or else will be executed. As in both if and else you have returned some object, the break you have added just before default , will never going to be executed!

That's why it says it says unreachable code .

Answer 4

Return in switch statement make no sense and not allowed . You can only return in a function . If you want to get value back from switch statement. Just assign it to a variable like this

var result;
switch (action.type) {
    case TYPES.ADD_TO_FAVORITES:
        if (state.faves.length <= 10) {
            result = assign({}, state, {
                faves: state.faves.concat(action.payload),
                full: false
            });
        } else {
            result = assign({}, state, {
                faves: state.faves,
                full: true
            });
        }
        break;
    default:
        result = state;
        break;
}