Question

While creating JavaScript with ASP.NET MVC I noticed several scope warnings and realized that I am missing something with understanding the variable scope inside the switch / case statement.

Warning: ‘i’ is already defined referring to case b and case c

My code looks similar to this:

switch(element) {
  case 'a':
   for(var i=0; i < count; i++){
    do something
   }
   break;

  case 'b':
   for(var i=0; i < count; i++){
    do something
   }
   break;

  case 'c':
   for(var i=0; i < count; i++){
    do something
   }
   break;
}

I thought scope ended with each break statement but it seems that scope does not end until the end of the switch/case. Is scope for the entire switch/case?

Answer 1

From the MDN documentation: "The case and default clauses don’t create lexical scopes."

Therefore, all variables declared inside a switch statement, live within the same scope. Declaring a variable using let/const with the same name with throw an Error.

switch(someVar) {
    case 1:
        let a;
        break;
    case 2:
        let a; // Uncaught SyntaxError: Identifier 'a' has already been declared
        break;
}

If you want to create a separate (nested) block scope inside a case clause, you can use labels:

switch(someVar) {
    case 1: { // <- creates a new block scope
        let a;
        break;
    }
    case 2: { // <- creates a new block scope
        let a; 
        break;
    }
}

There is no C-like language (that I know of) in which each case statement forms an independent scope.

For example, the following C# code will also not compile:

switch(someVar) {
    case 1:
        int a;
        break;
    case 2:
        int a;        // 'a' is already defined
        break;
}