Similar in some sense to this question, but rather than waiting for any change in a particular variable, this would be by value: breaking on a particular primitive value (probably only particularly useful for numbers and strings) being assigned.
e.g. When watching for assignment of value 'asdf' (in some browser's dev tools or something like Firebug):
function example(){
var x = 'as';
x += 'df'; /*break here*/
return x;
}
y= example(); /*break here, as well*/
I suspect that the answer is that this doesn't exist (searching has turned up nothing), but just in case, I thought I'd ask.
