Question

If I create a trivial labeled statement such as:

foo: do { alert('hi') } while (false)

Is there any way to clone foo?

Answer 1

No, apparently this is not possible. Labels cannot be used in an assignment statement and the scope of a label is the conditional block of the loop in which it is defined.

pastureLoop:
for (i = 0; i < pastureLen; i++)
{
   var pasture = pastures[i];
   var cows = pasture.getCows();

   var j, numCows = cows.length;
   for (j = 0; j < numCows; j++)
   {
      var cow = cows[j];
      if (cow.isSick())
         { break pastureLoop; }

      healthyCows++;
   }
}


pastureLoop:
for (i = 0; i < pastureLen; i++)
{
   var pasture = pastures[i];
   var cows = pasture.getCows();

   var j, numCows = cows.length;
   for (j = 0; j < numCows; j++)
   {
      var cow = cows[j];
      if (cow.isEating())
         { continue pastureLoop; }
   }

   // No cows were eating, so fire the callback for pasture[i]
   pasture.executeCallback();    // or whatever
}

The closest alternative is a labelled function declaration in non-strict mode:

B.3.2 Labelled Function Declarations

Prior to ECMAScript 2015, the specification of LabelledStatement did not allow for the association of a statement label with a FunctionDeclaration. However, a labelled FunctionDeclaration was an allowable extension for non-strict code and most browser-hosted ECMAScript implementations supported that extension. In ECMAScript 2015, the grammar productions for LabelledStatement permits use of FunctionDeclaration as a LabelledItem but 13.13.1 includes an Early Error rule that produces a Syntax Error if that occurs. For web browser compatibility, that rule is modified with the addition of the underlined text:

LabelledItem : FunctionDeclaration

It is a Syntax Error if any strict mode source code matches this rule.

This can be used to create a declaration which is inaccessible:

function show_alert()
  {
  label:
    {
    break label;
    var foo = 1;
    }
  console.log(foo);
  }

but can be made accessible through string manipulation and the function constructor.

References

• ECMAScript-6 Annex B, Section 3.2: Labelled Function Declarations

• Loop unrolling and completion-values in es6

• The JavaScript Goto

• Speed up your JavaScript, Part 1