CODEWITHSUNDEEP
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Fawkes5
Fawkes5

Reputation: 1021

Labeled while loops in Javascript

My confusion stems from this example labelled statement: 

myLoop : while (expression) {
    continue myLoop;
}

and the syntax of a general labelled statement:

identifier : statement

What exactly is being labelled in the example?

isn't the whole block of code:

while (expression) 
    statement

considered a single statement? Or is while(expression) itself a statement? Or is while a statement by itself?

Why isn't the entire:

while (expression) {
    continue myLoop;
}

labelled under myLoop and not just while(expression). Or is that even happening?

Upvotes: 4

Views: 1906

Answers (2)

Ben Clayton
Ben Clayton

Reputation: 82227

I'd never seen labelled while loops before, but according to this http://james.padolsey.com/javascript/looping-in-javascript/ it is the whole while loop that is being labelled.

The use for it is to break out of a particular loop, handy with loops-in-loops e.g. (example taken from the link)

myOuterLoop : while (condition) {
 myInnerLoop : while (condition) {
    if (whatever) {
        break myOuterLoop;
    }
    if (whatever2) {
        break; // Same as 'break myInnerLoop;'
    }
 }
}

Upvotes: 6

McGarnagle
McGarnagle

Reputation: 102783

What's being labelled isn't a block of code, it's just a particular line. So wherever your label myLoop is, writing continue myLoop is like saying "jump to that spot and continue execution".

But actually, in this example:

myLoop : while (expression) {
    continue myLoop;
}

The use of the label is completely redundant. You would write it as follows and the effect would be identical:

while (expression) {
    continue;
}

That's because continue by default means, go to the beginning of the loop's next iteration.

Upvotes: 2

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