Are Semicolons Necessary in JavaScript?

I've never understood this argument for always including braces in one-line if-blocks. Won't this error be immediately obvious? I mean, after making the change, won't even the most minimal test catch the mistake?

If developer #2 isn't doing the most basic test to verify that "statement2" runs when "conditional" is true, then you've got way bigger problems than your style guide.

What kind of “unpredictable subtle way” do you think leaving out semicolons can break things? The only way I know is what was mentioned in the article “Semicolons in JavaScript are optional”, where this code is incorrect:

  a = b + c
  (d + e).print()

. The author of the article says you can fix this by always putting a semicolon before any line-starting parentheses, like

  ;(d + e).print()

. Are you saying that you expect that rule to be too hard to remember for developers? If not, you should be able to give an example of another case where leaving out semicolons breaks things.

Also, I agree that showing off your understanding of JavaScript parsing is a bad reason to leave out semicolons, but you imply that that is the reason everyone leaves out semicolons, and that is false. As you said, there are aesthetic reasons to do it – but having good aesthetics of code can (slightly) help one understand code more quickly, so it’s not pointless.

I want to second that. I've had to contact numerous js library authors to warn them about missing semicolons which break minimization.

Which minifier blows up for you? Sounds like time for a bug report filed against the tool.

I’ve never personally minified JavaScript, with or without semicolons, but the article “Semicolons in JavaScript are optional” linked to in the page says the author minified jQuery without semicolons with Google Closure Compiler, and the test suite passed. Is that article wrong? Did minifying semicolons use to work, but they removed that feature for some reason? Does no one actually use Closure Compiler, and the other minifiers don’t support missing semicolons? Or is the reason the plugin you found “blows up” simply the error described in “Semicolons in JavaScript are optional” in the section “The only real pitfall when coding without semicolons”, which is starting a line with “(d + e)” instead of “;(d + e)”?

That article says that if a minifier doesn’t understand JavaScript that an actual JavaScript interpreter can understand, the minifier is buggy. I agree with it – if your minifier messed up the semicolon-less jQuery plugin, and if none of the situations I mentioned above was the case, then your minifier needs fixing, or you should start using a different one.

Unless you can name a minifier that actually breaks in this case, I'm gonna have to call BS on this.

Most of those examples are designed to push the edge cases in confusing ways (i.e. bad code). Easy answer: don’t do that.

Even if you include semicolons in all the “reasonable” places for them, it’s quite possible to get tripped up if you don’t understand what counts as a statement in JS and what doesn’t.

Edit to add: For example, don’t write code like this, with or without the autoinserted semicolons:

  var x = function()
  {
    return (function(n){
      return n
    })(n)
  }(function(){Math.exp(
    f
  )})

Someone else looking at the code is hardly going to be better off understanding it if you change that to:

  var x = function()
  {
    return (function(n){
      return n;
    })(n);
  }(function(){Math.exp(
    f
  );});

Do you write code like this? If so, you have bigger issues than ASI!

I would think they didn’t explain how it makes JS better because they were on the defensive from people who said leaving semicolons out will break code. They are saying “stop forbidding me from coding in the style I want, my style works just fine”. They are just trying to defend their style, not convert people to it.

However, that’s not to say there is no reason to omit semicolons. I personally leave out semicolons as an extension of the DRY principle – if you are putting a line break after a statement anyway, then you might as well leave off the semicolon. It’s one less character to delete if you combine two adjacent lines into one. Also, if you already think of line breaks and statement ends going together, then semicolons are just noise that you have to make an effort to ignore – the effort required may be small, but there’s little reason not to avoid that effort.

The only problem with omitting semicolons in JavaScript is this case mentioned in “Semicolons in JavaScript are optional”:

  a = b + c
  (d + e).print()

The author recommends avoiding the error in that by changing the last line to

  ;(d + e).print()

. If the extra effort of adding a semicolon before all line-starting parentheses costs more effort than you would save by getting the two advantages I described above, then maybe adding semicolons everywhere is right for you. You might have to experiment with coding in both styles to be able to accurately quantify how much those two advantages and one disadvantage would affect you.

See my reply to someone else, describing the advantages and disadvantages of omitting semicolons: http://news.ycombinator.com/item?id=2184289

As for whether you should omit the semicolons, only you can decide how much those pros and cons affect you. Depending on how much you care about others who read your code, you might also want to consider how you think those other people would choose, if they evaluated those pros and cons for themselves.

That example has nothing to do with whether semicolons are omitted or not; the rule that you need to know is don’t ever put a newline between the “return” keyword and the value to be returned, but whether you add semicolons at the end of what you want to make a statement makes absolutely no difference in this case.

In other words, whether you write

  return
    foo;

or

  return
    foo

the result is the same: an empty return, completely ignoring the foo bit.