Use of commas versus semicolons?

Question

Given the following code:

var fn = function () {
    var x = 'x',
    y = 'y';
    this.a = 'a',
    this.b = 'b',
    this.c = 'c';
    this.d = 'd',
    this.e = 'e';   
}

As can be seen, there is a mix of both.

What would be the benefit of using one or the other?

My understanding is that the semicolon is to end the statement. And comma should be used to string together multiple declarations.

So is it safe to say that with this example then there should only be two semicolon?

var fn = function () {
    var x = 'x',
    y = 'y';
    this.a = 'a',
    this.b = 'b',
    this.c = 'c',
    this.d = 'd',
    this.e = 'e';   
}

@SteveValliere this was a javascript question. The syntax would be invalid java — Ben McCormick, Mar 18 '13 at 20:01
Note as an endline token, they are not always equivalent. This snippet: `var a=1,b=2` will declare 2 variables with one var statement. But in `var a=1;b=2` now `b` is no longer part of the var statement, and creates an implicit global `b`. — Alex Wayne, Mar 18 '13 at 20:02
@The_asMan- One could interpret this question as a "what's better?" question, which is typically frowned upon in the SO community. We like questions with definitive yes or no answers. Asking for best practices often falls into the category of "that's a great question, but it doesn't belong on this site." (Also, that wasn't my close vote.) — templatetypedef, Mar 18 '13 at 20:04
It might get closed but I had been wondering about the same thing. A timely post. Thanks. — Neil Thompson, Mar 18 '13 at 20:07

score 88 · Accepted Answer · answered Mar 18 '13 at 20:02

The comma operator is an operator that can be used inside an expression. It is used to separate out multiple different expressions and has the meaning "evaluate all of the following expressions, then produce the value of the final expression." For example:

a = 1, b = 2, c = 3

means "evaluate a = 1, then b = 2, then c = 3, then evaluate to the value of the expression c = 3.

The semicolon is not an operator and cannot be used inside an expression. It is used as part of JavaScript syntax to mark the end of an expression that is being treated as a statement. For example, you could say

a = 1; b = 2; c = 3;

And this would mean "there are three statements to do in sequence: evaluate the first expression as the first statement, the second expression as the second statement, and the third expression as the third statement."

In this regard, the two are not completely interchangeable. For example, you cannot write

var a = 1, var b = 2;

Because var a = 1 and var b = 2 are statements, not expressions, and thus can't be separated by commas. You would have to use a semicolon here.

(A note: you could say

var a = 1, b = 2;

because the language specifically permits this use of comma as a part of the syntax of a declaration statement. Here, comma is not used as an operator.)

Similarly, you can't say

a = (b = 1; c = 2);

Because here the right-hand side of the expression must be an expression, not a statement, and ; is used to separate statements. The inner semicolon would have to be a comma instead. (Then again, this code is pretty awkward and unusual in the first place, so you probably shouldn't do this at all!)

From a stylistic perspective, the comma operator is rarely used and is obscure enough that it might trip up reasonably competent JavaScript coders. As a result, I would strongly suggest not using it and instead following the established conventions in JavaScript about using semicolons to terminate statements, even if it would be equivalent and syntactically legal to use commas to separate out expressions that are each used as statements.

Hope this helps!

How is it possible to write all this in 5 minutes? `O_O` – VisioN Mar 18 '13 at 20:03 — VisioN, Mar 18 '13 at 20:03

score 32 · Answer 2 · edited May 23 '17 at 12:10

32

No, comma has three meanings.

With variable declartions, it just let you omit the var before each variable.
With expressions, "The comma operator evaluates both of its operands (from left to right) and returns the value of the second operand."
Separate arguments in function declarations and function calls, but that should be obvious to every programmer.

Example for two;

alert((2,3)); //3

Note that the comma in the var statement is not the comma operator, because it doesn't exist within an expression. Rather, it is a special character in var statements to combine multiple of them into one. Practically, that comma behaves almost the same as the comma operator, though.

MDN

When is the comma operator useful?

edited May 23 '17 at 12:10

Community

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answered Mar 18 '13 at 20:00

gdoron is supporting Monica

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3. Separate arguments in function declarations and function calls. – VisioN Mar 18 '13 at 20:02
Yes, but that is yet the third meaning. – VisioN Mar 18 '13 at 20:04
1

If we're being picky, it's also used in object and array literal notation as well as possibly additional things in future ES6+ data structures. In order to not have to keep this answer up-to-date with the language I'd personally change it to say something like "outside of the obvious usage of commas in data structures and functions..." – Philip Walton Jan 23 '14 at 19:58

score 10 · Answer 3 · answered Mar 18 '13 at 19:59

It really doesn't matter. There's no speed benefit to using commas as far as I know.

Just use whatever you prefer :)

This is what JavaScript - The Good Parts has to say:

The comma operator can lead to excessively tricky expressions. It can also mask some programming errors.

JSLint expects to see the comma used as a separator, but not as an operator (except in the initialization and incrementation parts of the for statement). It does not expect to see elided elements in array literals. Extra commas should not be used. A comma should not appear after the last element of an array literal or object literal because it can be misinterpreted by some browsers.

score 3 · Answer 4 · answered Mar 18 '13 at 19:59

3

It's best to always remain consistent with convention. It's either all semi-colons or all commas, and I'm sure you'd prefer to use semi-colons rather than commas everywhere.

There is no speed gain as well, so there's nothing to worry about.

answered Mar 18 '13 at 19:59

user123

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Use of commas versus semicolons?

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