CODEWITHSUNDEEP
javascriptbreakcontrol-flow
Peter Olson
Peter Olson

Reputation: 142939

Is the break keyword in Javascript only meant for breaking out of loops?

One of my friends is teaching a programming class with Javascript and one of his assignments was to create a number guessing game. This was his example implementation:

funProgram: for(;;) {
  numberGuesser: {
    var num = (Math.random() * 100) | 0;
    var guess = +prompt("I'm thinking of a number between 0 and 100. Try to guess it.", 0);
    var guesses = 1;
    guess: for(;;) {
      higher: {
        lower: {
          if(guess === num) break guess;
          if(guess > num) break lower;
          guess = +prompt("Too low. Try again.", 0);
          break higher;
        }
        guess = +prompt("Too high. Try again.", 0);
      }
      guesses++;
    }
    alert("You got it in " + guesses + " guesses! The number is " + num);
  }
  var again = prompt("Do you want to guess again (y/n)?", "y") === "y";
  if(!again) break funProgram;
}

He told me that it's a good practice to label your code and wrap blocks around it so you can easily see what each section is doing. He also said labeled breaks and continues are much easier to read than unlabeled ones because you can know exactly what you are breaking out of. I've never seen any code patterns like this, so I'm not sure if this is true.

I've been using Javascript for a while and there are a few things in here that I've never seen before and some things that I still don't understand. I thought that the break keyword was specifically meant for breaking out of loops. The higher and lower blocks are not loops, but apparently you can still break out of it. How is that possible? It seems odd to me to break out of something that doesn't loop. Can you also break out of functions using the break keyword?

Upvotes: 5

Views: 2029

Answers (4)

Brad Christie
Brad Christie

Reputation: 101614

Breaks can in fact have labels (and do so accept them). However, I'm not sure who "He" is, but I would go as far as to say "He" is conveying his ideals of programming, rather than a specific standard. That is to say, there's no need to use labels, it just makes it more legible to that particular person. (And, IMHO, labels are reminiscent of the BASIC/GOTO days which usually results in Spaghetti Code).

Extra Credit: Ask your friend if they used to write in BASIC; I'm betting you'll get a "yes" (along with a lot of bad habits for the duration of the course--that's not to profile, I've just never had a good experience with BASIC/VB programmers following [current] coding patterns))

The break command is commonly used to exit loops, however if the code block is nested within a label, you're establishing yet another block which you can break from. It also gives you a bit more flexibility as to where the break is meant to exit from. For example:

for (;;){ // "for a"
  for(;;){ // "for b"
    break; // breaks "for b"
  }
}

In this instance, break is only meant to exit the nested ("for b") loop. However:

myblock: {
  for(;;){
    for(;;){
      break mybock; // breaks label "myblock"
    }
  }
}

In this case, break is actually exiting both loops because you're instructing it to quit the label block entirely. This would be almost like having:

function myblock(){
  for(;;){
    for(;;){
      return; // exits function "myblock"
    }
  }
}

Where return exits the block similar to the way break myblock acts.

By the way, not for nothing, I find this a tad easier to read:

var again = true;
while (again){
    var num = (new Date()).getMilliseconds() % 100,
        guess = +prompt("I'm thinking of a number between 0 and 100. Try to guess it.", "1"),
        guesses = 1;
    while (num !== guess){
        guesses++;
        guess = +prompt((guess < num ? "Too low." : "Too high.") + " Try again.", guess);
    }
    alert("You got it in " + guesses + " guesses! The number is " + num);
    again = prompt("Do you want to guess again? (y/n)", "y") == "y";
}

Upvotes: 3

bfavaretto
bfavaretto

Reputation: 71918

I can understand the code after a little research on label and break in javascript. However, I personally don't like that style of coding, mainly because the labelled block syntax is too similar to the literal object notation - which I use a lot. And even Mozilla's documentation on labels tell us to avoid them:

Labels are not very commonly used in JavaScript since they make programs harder to read an understand. As much as possible, avoid using labels and, depending on the cases, prefer calling functions or throwing an error.

Regarding your objective questions:

  1. break can be used to exit for loops, while loops and case statements, as well as labelled blocks.

  2. You cannot use break to exit a function, you must use return.

Upvotes: 1

user142951
user142951

Reputation:

Those break statements are being used in the same way that goto is used in older languages. And as Shark pointed out: you can also use them in switch statements, but not in functions.

It's probably confusing to you because he is using the labeled blocks in conjunction with infinite for(;;) loops.

Look at this article here for more information: http://james.padolsey.com/javascript/labelled-blocks-useful/

Upvotes: 0

Wayne
Wayne

Reputation: 60414

From MDN (bold emphasis mine):

The break statement includes an optional label that allows the program to break out of a labeled statement. The break statement needs to be nested within this labelled statement. The labelled statement can be any block statement; it does not have to be preceded by a loop statement.

So, yes, labels can be used outside of loops, but, no, you can't use them to return from a function.

Upvotes: 0

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