JavaScript variable definition: Commas vs. Semicolons
JavascriptVariablesJavascript Problem Overview
What are the differences and/or advantages, if any, of using commas when declaring a group of variables rather than semicolons.
For example:
var foo = 'bar', bar = 'foo';
versus
var foo = 'bar';
var bar = 'foo';
I know that if you specify the var
keyword on the first variable in the first example it persists across all of the variables, so they both produce the same end result regarding scope. Is it just personal preference, or is there a performance benefit to doing it either way?
Javascript Solutions
Solution 1 - Javascript
No performance benefit, just a matter of personal choice and style.
The first version is just more succinct.
Update:
In terms of the amount of data going over the wire, of course less is better, however you would need a hell of a lot of removed var
declarations in order to see a real impact.
Minification has been mentioned as something that the first example will help with for better minification, however, as Daniel Vassallo points out in the comments, a good minifier will automatically do that for you anyways, so in that respect no impact whatsoever.
Solution 2 - Javascript
After reading Crockford and others, I started to chain my variables with comma exclusively. Then later, I really got annoyed by the Chrome DevTools debugger that wouldn't stop at variable definitions with comma. For the debugger, variable definitions chained with comma are a single statement, while multiple var statements are multiple statements at which the debugger can stop. Therefore, I switched back from:
var a = doSomethingA,
b = doSomethignB,
c = doSomethingC;
To:
var a = doSomethingA;
var b = doSomethignB;
var c = doSomethingC;
By now, I find the second variant much cleaner, not to mention its advantage of solving the debugger issue.
The "less code through the wire" argument is not persuasive, as there are minifiers.
Solution 3 - Javascript
I prefer the var-per-variable
notation:
var a = 2
var b = 3
because the other comma-instead-of-another-var
notation have these three shortcomings:
1. Hard to maintain
Consider this code:
var a = 1,
b = mogrify(2),
c = 3
But hey, what does the mogrify do? Let's print b to find out:
var a = 1,
b = mogrify(2),
console.log(b)
c = 3
breaks stuff
2. Hard to read
The var in the begging of the line clearly communicates that there will be a new variable initiated.
var get_all_unicorn_promise = db.get_all_unicorns((unicorn) => {
unicorn.legs.map((leg) => {
leg.log('yes')
})
}).sort(),
c = 3
What the hell is the c = 3
doing there right?
3. Not consistent
Consider this:
var a = 1,
b = 2,
c = 3
With var-per-variable
every declaration follow the same structure. With comma-instead-of-another-var
the first variable is declared in different way than others. If you decide to, say, move the first variable inside a for cycle, you will have to add var to the middle of declarations
Other than preference, it seems like majority of notable projects use the var-per-variable
notation
Solution 4 - Javascript
I agree with the other answerers that this is mainly a matter of personal style. But to bring an "Authoritative" opinion into the discussion, this is what Douglas Crockford says on the website of the popular JSLint tool:
> But because JavaScript does not have block scope, it is wiser to declare all of a function's variables at the top of the function. It is recommended that a single var statement be used per function. This can be enforced with the onevar
option.
Solution 5 - Javascript
As others have noted, it is a style preference. JSLint might tell you to only have one var
per function (if you use the "Good Parts"). Thus if using JSLint to check your code (not a bad idea, IMHO), you'll end up using the first format more than the latter.
On the other hand, the same author, Douglas Crockford, says to put each variable in its own line in his coding conventions. So you may want to uncheck the "All one var
per function" checkbox in JSLint if you use it. ;-)
Solution 6 - Javascript
I don't think there's any noticeable difference, as far as I'm concerned it's just personal preference.
I hate having multiple var declarations so I usually do:
var
one
,two
,three
,four
;
As it's shorter and arguably more readable, no var
noise to look at.
Solution 7 - Javascript
Since I don't see any references to it, here is a link to the ECMA-262 specification, which is the underlying spec for JavaScript. The grammar from that page says:
12.2 Variable Statement
Syntax
VariableStatement :
var VariableDeclarationList ;
VariableDeclarationList :
VariableDeclaration
VariableDeclarationList , VariableDeclaration
VariableDeclarationListNoIn :
VariableDeclarationNoIn
VariableDeclarationListNoIn , VariableDeclarationNoIn
VariableDeclaration :
Identifier Initialiseropt
VariableDeclarationNoIn :
Identifier InitialiserNoInopt
Initialiser :
= AssignmentExpression
InitialiserNoIn :
= AssignmentExpressionNoIn
What you can glean from this is using commas or not doesn't matter. Either way, it ends up being parsed as a VariableDeclaration
and is treated exactly the same. There should be no difference to how the script engine treats the two declarations. The only differences would be ones already mentioned in other answers - saving more space and practically immeasurable differences in the amount of time it takes to apply the grammar to find all the VariableDeclarations
when the script is compiled.
Solution 8 - Javascript
The first saves a few characters--so there is a very small saving in terms of the JS filesize and therefore bandwidth consumption. The only time this would become noticable would be in extreme cases.
Solution 9 - Javascript
I prefer the second version (each has its own var
). I think that's because I come from a C++ background. In C++, you can declare variables like you do in your first example, but it is frowned upon (it easily leads to mistakes when you're trying to create pointers that way).
Solution 10 - Javascript
If you are minifying your javascript, there is a fairly large benefit:
var one, two, three, four;
becomes
var a, b, c, d;
Where as
var one;
var two;
var three;
var four;
becomes
var a;
var b;
var c;
var d;
That's an additional three instances of var
, which can add up over time.
See The "A List Apart" article series "Better Javascript Minification" Part 1 and Part 2