Breaking out of a PrototypeJS .each() loop

Your are correct, and Prototype has created an object ($break) that can be thrown from the each-function to enable this functionality. According to the Prototype API docs:

Regular loops can be short-circuited in JavaScript using the break and continue statements. However, when using iterator functions, your code is outside of the loop scope: the looping code happens behind the scene.

In order to provide you with equivalent (albeit less optimal) functionality, Prototype provides two global exception objects, $break and $continue. Throwing these is equivalent to using the corresponding native statement in a vanilla loop. These exceptions are properly caught internally by the each method.

Also, note that the $continue object has been deprecated, and to simulate a continue-statement, use a vanilla return statement instead.

Code example:

var result = [];
$R(1,10).each(function(n) {
  if (0 == n % 2)
    return; // this equals continue
  if (n > 6)
    throw $break;
  result.push(n);
});
// result -> [1, 3, 5]

You can read more about the each-function here: http://www.prototypejs.org/api/enumerable/each

if( val == 'bar' ) {
    throw $break;
}

It's documented at the same page you linked. It's an exception specially handled by the each function. When thrown, it prevents your function from being called on further elements.