Why so many semicolons in JavaScript?

Question

I tend to be a prolific user of semicolons in my JavaScript:

var x = 1;
var y = 2;
if (x==y){do something};

I recently noticed that I was looking at a lot of JavaScript that doesn't have semicolons following if statements. It then occurred to me that I don't even know the preferred syntax for semicolons in JS and after some googling learned (rather surprisingly) that there is no need for semicolons at all aside from splitting statements that are on one line.

So, the question...where did this habit of people using semicolons come from? Is it a remnant from some popular language that was in use at the time JavaScript came into play? Just good practice in general? Is it just me?

I'm probably going to stick with it for no other reason that it's nice when writing jQuery chains to easily spot the end.

UPDATE:

Thanks for all the answers, everyone! It looks like, to summarize things, the reason we see a lot of semicolons in JS even when not needed comes from various variables:

• older JS minimizer's would produce broken code if you did not insert semicolons
• many other languages use them, so it's a carried-over habit
• on occasion, a semicolon CAN change the logic
• some prefer to use them to make the code more human-readable

Answer 1

Many computer languages use semicolons to denote the end of a statement. C, C++, and Java are popular examples of this.

As for why people use them despite them being optional, they improve the readability of your code. In most cases it's simply done out of habit, but occasionally you need semicolons in your code to remove possible ambiguity. It's always better safe (and consistent) than sorry.

Here is an example taken from Do you recommend using semicolons after every statement in JavaScript?

// 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 () {
    //...
})();

Additionally, semicolons allow Javascript to be packed/minified properly. Otherwise all the statements will be mushed together into one big mess.

Answer 2

JavaScript requires semicolons. But it does have Automatic Semi-colon Insertion.

The majority of JavaScript programmers are smarter than ASI and prefer to be explicit with their semi colons.

Answer 3

The habit comes from fear. Fear of "broken" js compressors (I've had some problems with Plone compressors with legacy js that didn't have the var statement for example), fear of possible broken browser implementations, the list goes on. So, why not just use ; everywhere and avoid all of these pitfalls?

The problem with this approach (just always use semicolons without further explanation) is that you don't try to understand why are you doing it in the first place, you just do because someone said you to do it, and, IMHO, this can be harmful as well. And the broken compressors you had in the past are now fixed, and you keep inserting semicolons... breaking habits is hard. We just keep doing things because "we always did it".

Many new JavaScript programmers are advised to just use semicolons everywhere, and expect that if they do not intentionally use the semicolon insertion rules, they can safely ignore the existence of this entire language feature. This is not the case, because of the restricted productions described above, notably the return statement. When becoming aware of the restricted production issue, programmers may then become overly wary of linebreaks, and avoid them even when they would increase clarity. It is best to be familiar with all the rules for ASI so as to be able to read any code regardless of how it is written, and to write code that is as clear as it can be.

Here is a great resource about this subject and the source of the quote. A great read.

So which style is better? To the extent that there is an objectively “better” choice, it appears to me that the minimal-semicolon/comma-first style is slightly superior, both because it is fundamentally more scannable and because it encourages programmers to better understand the language they use.

... and the 2nd quote from a more optionated article, against always using semicolons. A great read as well.

Answer 4

Whether to use semicolons or not is largely a matter of choice. However, in javascript, unlike many other languages, not using them can lead to unexpected results. For example,

return
    {'foo':'bar'};

will, because of javascript's automatic semicolon insertion, return nothing.

Answer 5

This is probably going to remain a debatable question as to the origin of how this this manner of coding came to be, but it could have come from the ability to differentiate between 1 and many statements within an if conditional, which is common in most languages. And also, people could have confused syntax with JavaScript's object literal

First, you have the single statement if:

if (someNumber == 2)
  doSomething();
else
  doSomethingElse();

Then, you have the multi-statement if (no semi-colon needed):

if (someNumber == 3) {
  doThisFirst();
  doThisSecond();
} else
  doSomethingElse();

This syntax could also be confused with JavaScript's object literal, which does need a semi-color:

var SomeObjectLiteral = {
  property: 2,
  doSomething: function() {
  }
};

Basically, the semi-colon is important in Javascript to determine where a command, or definition, ends. And Braces are just as important to determine blocks of these commands. But, you can have as many semi-colons as you want. If in doubt, use a semi-colon, but be prepared if someone calls you out for not needing one :)

Answer 6

The C language uses semicolons at the end of statements, and also uses braces.

Answer 7

PHP uses semicolons at the end of lines, and that's why i use them in javascript usually.

Answer 8

Semicolons mark the end of statements in JS, but I do not put semicolons after a close-brace, only after a statement within a brace and ALWAYS on a return statement.

In your example I dont find it necessary.

Answer 9

you can write multiple statement within the function with help of semicolon; semicolon used to separate the statement. easy to understand.

Answer 10

Apart from code readability, the use of semi-colons comes into picture when you actually try to minify your code. After minification, if there are no semicolons in your code, javascript engine might fail to know which statement ends where as there will be no longer be any new lines in the code.