Question

I’m looking at somebody’s javascript code that is minimized and I see a syntax that doesn’t make any sense.

firstObject.init = function() {
  void 0 === secondObject.properties && thirsObject.reportError("Something is wrong");
  firstObject.doSomething();
}

My guess is that packer is checking for an undefined property, breaking out of the execution context and returning null in that case.

if (secondObject.properties === undefined) {
  thirdObject.reportError("Something is wrong");
  return NULL;
}

What’s going on here?

Answer 1

If you look at the result of void 0 it quickly becomes clear what’s happening:

> void 0
undefined

Therefore, writing

void 0 === secondObject.properties

is simply a different way of writing

typeof secondObject.properties === "undefined"

---

Does this end the execution of the function or simply continues after executing the code to the right of the && operator?

Condition evaluation is usually lazy “short-circuit”, that is, if it is already certain that a condition will evaluate to true or false, further sub-conditions are not evaluated anymore. For example:

false && alert("foo"); // Will never alert foo
true && alert("foo"); // Will always alert foo

If you have any two conditions combined by a && operator, if the first (left) condition is false, the entire condition will be false no matter the it’s value. Therefore, evaluation of the second condition is not necessary, and thus discarded.

More information:

• Lazy evaluation on Wikipedia
• Short-circuit evaluation