Error Duplicate Const Declaration in Switch Case Statement

Question

I have the following code and I get the error 'Duplicate Declaration query_url'.

  switch(condition) {
    case 'complex':
      const query_url = `something`;
      break;
    default:
      const query_url = `something`;
      break;
  }

I understand that query_url is getting declared twice which isn't right. But i don't know how to resolve this. Can someone please help on what should be the correct way to make this work?

If you need to use query_url outside the switch statement, you can't use const. — Felix Kling
– Felix Kling, Commented Mar 2, 2016 at 14:47

Bergi · Accepted Answer · 2016-03-02 14:43:39Z

424

Try wrapping the cases in blocks:

switch(condition) {
  case 'complex': {
    const query_url = `something`;
    … // do something
    break;
  }
  default: {
    const query_url = `something`;
    … // do something else
    break;
  }
}

edited Mar 2, 2016 at 14:43

answered Mar 2, 2016 at 11:53

Bergi

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8 Comments

user663031 Over a year ago

Umm, won't query_url be confined to the block where it is defined, which sort of defeats the whole purpose of assigning to it?

Bergi Over a year ago

@torazaburo: Yes, it's only to be used in that block. I'm not sure what the actual purpose of the code is, you shouldn't use a switch only to select a single value - an object (or Map) lookup is better suited for that task.

Andreyco Over a year ago

mind blown. I considered case as block. Thanks for this hint!

Emrys Myrooin Over a year ago

Thank you ! This is the real solution. Because, in this trivial case, clearly you can define a global variable, but in more complex case, the global variable can be a non-sense in most of switch branches.

riv Over a year ago

In C/C++ you can't even declare variables inside a switch statement without using a block. The main inconvenience is that all editors have a different idea of how to indent braces in this case.

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rob2d · Accepted Answer · 2023-09-17 05:47:47Z

I personally prefer (and tend to abuse) the following in these sorts of cases:

const query_url = (()=> {
     switch(condition)
           case 'complex': return 'something';
           default:        return 'something-else';
})();

(this requires ES6 or declaring "use-strict" in Node 4.x though)

Update: Alternatively, much more compact depending on if there is any logic there or if it's a simple assignment:

const query_url = { complex: 'something' }[condition] || 'something-else';

Also, of course, depends on the amount of outside-logic embedded in those switch statements!

eltonkamami · Accepted Answer · 2020-06-29 19:51:39Z

13

if you need to redeclare the same variable in each case see @Bergi 's answer bellow

if query_url can have multiple values depending on the switch branch obviously you need a variable ( declare either with var or let ).

const is set once and stays that way.

example usage with let

let query_url = '';
switch(condition) {
  case 'complex':
    query_url = `something`;
    break;
  default:
    query_url = `something`;
    break;
}

edited Jun 29, 2020 at 19:51

answered Mar 2, 2016 at 11:54

eltonkamami

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4 Comments

Kuf Over a year ago

Simply not true. check @bergi answer.

toddg Over a year ago

@eltonkamami while I don't feel your answer deserves a downvote since it properly addresses the question, I do question why you would declare a variable within a switch case and expect to use it outside of that scope.

oldo.nicho Over a year ago

See @Bergi answer below

Patrick Michaelsen Over a year ago

@eltonkamami the answer is sufficient. The question asks how to avoid duplicate definition errors, and this does that. Consider the case where the value query_url passed to another function. In this case, it doesn't matter that query_url is block-scoped because it is simply an intermediate value. You are being unreasonably harsh.

Pang · Accepted Answer · 2022-02-16 08:33:09Z

9

You can use {} to scope your switch case.

For your case, you need to return the variable as long as the var exists and is accessible between curly braces:

 switch(condition) {
    case 'complex': {
      const query_url = `something`;
      return query_url;
    }
    default: {
      const query_url = `something`;
      return query_url;
    }
  }

If you won't use return, you must declare a let query_url above your switch statement.

edited Feb 16, 2022 at 8:33

Pang

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answered Jun 10, 2020 at 11:00

Disfigure

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Comments

Zakaria · Accepted Answer · 2016-03-02 13:33:03Z

2

Just put your switch in a function with some return statements :

var condition;
function aSwitch(condition){
switch(condition) {
    case 'complex':
      return 'something';
    default:
      return 'something';
  }
}
const query_url = aSwitch(condition);

edited Mar 2, 2016 at 13:33

answered Mar 2, 2016 at 11:54

Zakaria

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3 Comments

eltonkamami Over a year ago

if you dont "use strict"; some weird things will occur with your code. each return statement is creating/assinging to a global variable query url.

Zakaria Over a year ago

@antoniskamamis Edited. Thanks for the tip :)

eltonkamami Over a year ago

you just have to return the value without an assignment that will either create a global or cause an error

sbk201 · Accepted Answer · 2018-02-20 04:38:09Z

1

const query_url={
  complex:'something complex',
  other:'other thing'
}[condition]

The drawback is,you can't have default with object,you need to have addition check of condition.

answered Feb 20, 2018 at 4:38

sbk201

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