A better coding convention for lists and object literals in JavaScript

comma-first-var.js

// See comments below.

// This code sample and justification brought to you by
// Isaac Z. Schlueter, aka isaacs

// standard style
var a = "ape",
  b = "bat",
  c = "cat",
  d = "dog",
  e = "elf",
  f = "fly",
  g = "gnu",
  h = "hat",
  i = "ibu";

// comma-first style
var a = "ape"
  , b = "bat"
  , c = "cat"
  , d = "dog"
  , e = "elf"
  , f = "fly"
  , g = "gnu"
  , h = "hat"
  , i = "ibu"
  ;

// error in standard style
var a = "ape",
  b = "bat",
  c = "cat",
  d = "dog"
  e = "elf",
  f = "fly",
  g = "gnu",
  h = "hat",
  i = "ibu";

// error in comma-first style
var a = "ape"
  , b = "bat"
  , c = "cat"
  , d = "dog"
  e = "elf"
  , f = "fly"
  , g = "gnu"
  , h = "hat"
  , i = "ibu"
  ;

// Objects:

// JSON.stringify style
var o = {
    a : "ape",
    b : "bat",
    c : "cat",
    d : "dog",
    e : "elf",
    f : "fly",
    g : "gnu",
    h : "hat",
    i : "ibu"
  },
  a = [
    [ "ape", "bat" ],
    [ "cat", "dog" ],
    [ "elf", "fly" ],
    [ "gnu", "hat" ],
    [ "ibu" ]
  ];
  

// comma-first
var o =
    { a : "ape"
    , b : "bat"
    , c : "cat"
    , d : "dog"
    , e : "elf"
    , f : "fly"
    , g : "gnu"
    , h : "hat"
    , i : "ibu"
    }
  , a = 
    [ [ "ape", "bat" ]
    , [ "cat", "dog" ]
    , [ "elf", "fly" ]
    , [ "gnu", "hat" ]
    , [ "ibu" ]
    ];

// errors in objects:

// JSON.stringify style
var o = {
    a : "ape",
    b : "bat",
    c : "cat",
    d : "dog"
    e : "elf",
    f : "fly",
    g : "gnu",
    h : "hat",
    i : "ibu"
  },
  a = [
    [ "ape", "bat" ],
    [ "cat", "dog" ],
    [ "elf", "fly" ]
    [ "gnu", "hat" ],
    [ "ibu" ]
  ];
  

// comma-first
var o =
    { a : "ape"
    , b : "bat"
    , c : "cat"
    , d : "dog"
    e : "elf"
    , f : "fly"
    , g : "gnu"
    , h : "hat"
    , i : "ibu"
    }
  , a = 
    [ [ "ape", "bat" ]
    , [ "cat", "dog" ]
    , [ "elf", "fly" ]
    [ "gnu", "hat" ]
    , [ "ibu" ]
    ];

// Addendum: effects on the return statement.
// It does not break.

return [ 1
       , 2
       , 3
       ] // returns [1,2,3]

return { a : "ape"
       , b : "bat"
       } // returns {a:"ape",b:"bat"}

// even just separating two values by commas is fine,
// though a bit silly
return 1
     , 2
     , 3
     , 4 // returns the last value, 4

// this, however is wrong:
return
    1
  , 2 // returns undefined, because of semicolon-insertion.

// so is this. otb == fail.
return
  { a : "ape"
  , b : "bat"
  } // returns undefined,
    // then creates a block with two named statements.

// this is ok:
return ( 1
       , 2
       ) // returns 2

// so is this:
return (
  { a : "ape"
  , b : "bat"
  }
) // returns {a:"ape",b:"bat"}


// Addendum 2: A function call

doSomething( aPrettyLongVariableName
           , "A string, which has some useful information"
           , "If you put these all together, it'd be too long"
           , { a: "is for antelope", b: "is for bat" }
           , 42
           )

// Addendum 3: More realistic error in standard style:

// leaks FIVE globals!
var a = "ape eat banana",
  b = "bat, allowed to fly",
  c = "cat toy",
  d = "dog chasing the mailman,"
  e = "elf lord",
  f = "fly through the air",
  g = "gnu is not unix",
  h = "hat goes on your head",
  i = "ibu isn't a cow";

// Error: Can't call method 'forEach' of undefined.
// not passing in undefined as an argument!??
mergeLists([ apple, [ penelope, granger ] ],
           [ fun ],
           [ 1, 2, 3, 4, 5, 6, 7, 8 ]
           [ "mary's store has many pies, and cookies, and eggs," ]
           [ function() { doSomething() } ]);

Is there an eslint rule that will not choke when you do something like this? My eslint configuration complains about Project being indented by 2 chars.

const {
      Project
    , Dimension
    , Task
    , View
    , Layout
    , User
    , Group
    , Organization
    , CustomField
} = db.model;

It's too bad so few people use comma (and operator) first.
There are so many bad examples here.
Comma and operator first is really useful when you nest complex data structures or code.
Here is a case where it is good at:

x = {
    a : ( b + 27 * (
        foo1(
            b
        +   (   (c + 232312)
            *   (   foo2(
                        (d + 3423423 * foo34(34))
                    *   (y+3243)
                    )
                +   foo3(
                        (234 * sdf + 23 * dfdse )
                    ,   23423 + foo45(23)
                    )
                )
            *   ( 23423 / 34)
            )
        +   (   234234 / sdf + 23423)
        +   (   foo4(234234)
            /   foo5(
                    foo34(
                        (   (   fo345(3434 + 2323423)
                            +   23423
                            )
                        /   234
                        )
                    ,   [ 23234, 23432, 234234]
                    )
                /   349
                )
            )
        )
    ))
,   b : 234
,   c : [
        34534
    ,   534634
    ,   98
    ]
,   d : {
        x1 : [
            23423
        ,   232
        ,   43+343*(
                343
            +   324
            )
        ,   23
        ]
    ,   x2: 234
    ,   x3: foo(2323 + (
                ( foo56(234) + 3434)
            /   (2342 + foo6(2323))
            ))
    ,   x4: 34534
    }
}

I don't know how I got there, probably about the trailing comma debate I had with a colleague, but honestly this hurts my eyes so much. You don't do comma-first because firstly it's ugly and unintuitive. Javascript was designed like any coding language to be human-friendly. The creators knew how to read and write english, meaning that you should respect the comma position as you would in an non programming language.

Now if you write like this ,you'll see that this is by far the dumbest way to say "Hey I'm a hipster I write code with comma first" ,that's not how you're supposed to do things and you'll be the only one to do this in a team that would dare to hire you.

Comma is not a operator so I don't see the point how saying it should be the first thing in a line and in english (or any other language), a comma is attached to the previous word and a space (or a new line) must be put after this comma.

I really think I'm in the flat earth conference.

@Mcfloy I feel you!

I cringe every time I see this comma-first garbage. And I think I have the best reason not to use comma-first: I used to work in a team where one developer, working alone on a project, had put them all around.

The guy was fired (for other reasons, the main one being that he had done a poor job). I was hired to replace him, and was then joined by two other developers. Every dev in the company, including others not working on this project - that is 15 persons roughly - kind of screamed in horror when I showed them the codebase we had inherited.

So if you want a single reason why not to use this crap, here it is: no one in his right mind does it, and no one wants to hear your explanation about why it's so brilliant, because it's just an insult to their eyes. If you do use it, you'll be looked upon by your teammates as being an insufferable know-it-all who thinks he's just invented hot water.

Thank God, common ESLint rules have banished this creature of doom from our world (hopefully).

Not a chance @bhubr, some of us like our "comma-style":[2,"first"] rule ;)

The only place I've seen it cause issues is with JSDOC blocks not being identified correctly by the IDE.

A lot of the examples in this thread are using variables values of same/same character lengths. When line length is random it becomes much more obvious that comma-first adds value.