Error Duplicate Const Declaration in Switch Case Statement

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;
  }
}

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;
}

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.

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!