Do you recommend using semicolons after every statement in JavaScript?

In many situations, JavaScript parsers will insert semicolons for you if you leave them out. My question is, do you leave them out?

If you're unfamiliar with the rules, there's a description of semicolon insertion on the Mozilla site. Here's the key point:

>If the first through the n^th tokens of a JavaScript program form are grammatically valid but the first through the n+1^st tokens are not and there is a line break between the n^th tokens and the n+1^st tokens, then the parser tries to parse the program again after inserting a virtual semicolon token between the n^th and the n+1^st tokens.

That description may be incomplete, because it doesn't explain @Dreas's example. Anybody have a link to the complete rules, or see why the example gets a semicolon? (I tried it in JScript.NET.)

This stackoverflow question is related, but only talks about a specific scenario.

Yes, you should use semicolons after every statement in JavaScript.

An ambiguous case that breaks in the absence of a semicolon:

// define a function
var fn = function () {
    //...
} // semicolon missing at this line

// then execute some code inside a closure
(function () {
    //...
})();

This will be interpreted as:

var fn = function () {
    //...
}(function () {
    //...
})();

We end up passing the second function as an argument to the first function and then trying to call the result of the first function call as a function. The second function will fail with a "... is not a function" error at runtime.

Yes, you should always use semicolons. Why? Because if you end up using a JavaScript compressor, all your code will be on one line, which will break your code.

Try http://www.jslint.com/; it will hurt your feelings, but show you many ways to write better JavaScript (and one of the ways is to always use semicolons).

What everyone seems to miss is that the semi-colons in JavaScript are not statement terminators but statement separators. It's a subtle difference, but it is important to the way the parser is programmed. Treat them like what they are and you will find leaving them out will feel much more natural.

I've programmed in other languages where the semi-colon is a statement separator and also optional as the parser does 'semi-colon insertion' on newlines where it does not break the grammar. So I was not unfamiliar with it when I found it in JavaScript.

I don't like noise in a language (which is one reason I'm bad at Perl) and semi-colons are noise in JavaScript. So I omit them.

I'd say consistency is more important than saving a few bytes. I always include semicolons.

On the other hand, I'd like to point out there are many places where the semicolon is not syntactically required, even if a compressor is nuking all available whitespace. e.g. at then end of a block.

if (a) { b() }

JavaScript automatically inserts semicolons whilst interpreting your code, so if you put the value of the return statement below the line, it won't be returned:

Your Code:

return
5

JavaScript Interpretation:

return;
5;

Thus, nothing is returned, because of JavaScript's auto semicolon insertion

I think this is similar to what the last podcast discussed. The "Be liberal in what you accept" means that extra work had to be put into the Javascript parser to fix cases where semicolons were left out. Now we have a boatload of pages out there floating around with bad syntax, that might break one day in the future when some browser decides to be a little more stringent on what it accepts. This type of rule should also apply to HTML and CSS. You can write broken HTML and CSS, but don't be surprise when you get weird and hard to debug behaviors when some browser doesn't properly interpret your incorrect code.

The article Semicolons in JavaScript are optional makes some really good points about not using semi colons in Javascript. It deals with all the points have been brought up by the answers to this question.

This is the very best explanation of automatic semicolon insertion that I've found anywhere. It will clear away all your uncertainty and doubt.

No, only use semicolons when they're required.

I use semicolon, since it is my habit. Now I understand why I can't have string split into two lines... it puts semicolon at the end of each line.