切换案例的Javascript范围变量？

Question

In C you can scope a variable to a switch case, like this .

With javascript I get unexpected token using the following:

 const i = 1 switch (i) { // variables scoped to switch var s var x = 2342 case 0: s = 1 + x break case 1: s = 'b' break } 

Is there another way to do this or should I just declare my variables outside the switch?

EDIT:

This is a workaround I considered but it didn't end up working. The reason being that each case has its own scope.

 const i = 1 switch (i) { case i: // variables scoped to switch var s var x = 2342 case 0: s = 1 + x break case 1: s = 'b' break } 

Answer 1

some alternative:

/* curly braces inside the case */

const i = 1

switch (i) {
  case 0: {
    let x = 2342;
    let s = 1 + x;
    console.log(x+' & '+s+' from inside');
  } break;
  case 1: {
    let x = 2342;
    let s = 'b';
    console.log(x+' & '+s+' from inside'); /* 2342 & b from inside */
  } break;
}

console.log(x+' & '+s+' from outside'); /* Uncaught ReferenceError */

or

/* curly braces outside the switch */

const i = 1

{
  let x = 2342;
  let s;
  switch (i) {
    case 0:
      s = 1 + x;
      break;
    case 1:
      s = 'b';
      break;
  }
  console.log(x+' & '+s+' from inside'); /* 2342 & b from inside */
}

console.log(x+' & '+s+' from outside'); /* Uncaught ReferenceError */

Answer 2

Since var creates variables at function scope anyway, using it is pretty pointless. For this to work at a granularity below function scopes you'll have to use let and a browser/compiler which supports it, and then introduce a new block which you can scope things to (within switch it's simply invalid syntax):

if (true) {
    let s;

    switch (i) {
        ...
    }
}

This scopes s to the if block, which for all intents and purposes is identical to the " switch scope" here.

If you cannot support let , you'll need to use an IIFE:

(function () {
    var s;

    switch (...) ...
})();

Answer 3

No, this is invalid syntax. A case or default statement is expected within a switch . You can check the specification here: http://www.ecma-international.org/ecma-262/5.1/#sec-12.11

You can also try entering your code in a JSLinter and see that this an error: http://jslint.com/

Expected 'case' and instead saw 'var'.

The workaround that you're considering would be the same thing as putting them outside the switch statement. Remember, var has function -level scope, not block-level scope. That means they are bound to the entire function containing the switch. You should declare them outside of the switch because that is where they are accessible.

const i = 1;
var s;
var x = 2342;

switch (i) {
  case 0:
    s = 1 + x;
    break;
  case 1:
    s = 'b';
    break;
  default:
    break;
}

console.log("s is " + s);

Answer 4

It should be declared outside the switch. The below might help:

var i = 1, x = 2342;
var s;
switch (i)
 {        
    case 0:
      s = 1 + x;
      break;
    case 1:
      s = 'b';
      break;
}
console.log("s is " + s);

Answer 5

JavaScript defines 3 levels of scope:

• Global - Anything not delcared in a function
• Function - Anything declared in a function using the var keyword
• Block - Anything declared in a block container ( {} ) using let

So, to creae a scope an entire construct, you have two choices: Function or Block

In order to get the the behavior you are looking for with a function:

 const i = 1 function doSwitch(data){ // variables are not scoped to switch, but are // scoped to the function, which only contains // the switch. var s; var x = 2342; switch (data) { case 0: s = 1 + x; break; case 1: s = 'b'; break; default: s = "other"; } console.log("s is " + s) } doSwitch(18); 

Or, in order to get the functionality with a block using let

 const i = 1; // Wrapping the switch in its own block allows for let to work: { // variables are not scoped to switch, but are // scoped to the function, which only contains // the switch. let s; let x = 2342; switch (i) { case 0: s = 1 + x; break; case 1: s = 'b'; break; default: s = "other"; } console.log("s is " + s) } // Test: console.log(s, x); // ReferenceError