Why use commas between JavaScript declarations and initializations?

Question

What is the purpose of separating code using commas instead of moving to the next line?

For example, I found this code:

var fm = $.NSFileManager.defaultManager,
oURL = $.NSURL.fileURLWithPathIsDirectory(strPath, true),
lstFiles = ObjC.unwrap(
    fm.contentsOfDirectoryAtURLIncludingPropertiesForKeysOptionsError(
        oURL, [], 1 << 2, null
    )
),
lstHarvest = [];

I don't see it often but it feels like a run on sentence.

Update:
I think I see what is going on but I rarely ever see this. Is it ES5, ES6 thing?

It's just initializing multiple variables on one line, in one statement. Nothing to do with ES5 or 6. — Alex, Jan 11 '18 at 18:06
And personally i would indent the following lines so that all variable identifiers are on the same height. — Jonas Wilms, Jan 11 '18 at 18:17
Comment: All the answers are correct. Who thinks SO should let people select multiple answers? — 1.21 gigawatts, Jan 12 '18 at 08:19

Pac0 · Answer 1 · 2018-01-11T22:31:56.913

This is the syntax for var :

var varname1 [= value1] [, varname2 [= value2] ... [, varnameN [= valueN]]];

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/var

note that the presence of initializations (=) is irrelevant.

Why use this syntax ?

to avoid repeating var. Compare :

var a, b, c, d, e

or

var a,
    b,
    c,
    d,
    e

versus

 var a
 var b
 var c
 var d 
 var e

sometimes a matter of style and preference
in some places you must use this syntax to declare several variables because you can't use several lines, (e.g. in a for loop header : for (var a = 0, b = 3; ... ; ...))

Note that this is not the same thing as having new lines ! (the other lines after first would not be var variable declarations, see What is the purpose of the var keyword and when to use it (or omit it)? for the differences).

Your example would however be equivalent without commas at the end and with var at beginning of each others.

Is it ES5, ES6 thing?

It is the syntax since ECMAScript 1st edition (1997)

https://www.ecma-international.org/publications/files/ECMA-ST-ARCH/ECMA-262,%201st%20edition,%20June%201997.pdf

(page 63 / 110 of the pdf file, section 12.2, variable statement)

score 2 · Answer 2 · answered Jan 11 '18 at 18:09

There is just one var used for initializing multiple Variables. You can also do this (I think this is much better and more readable):

var fm = $.NSFileManager.defaultManager;
var oURL = $.NSURL.fileURLWithPathIsDirectory(strPath, true);
var lstFiles = ObjC.unwrap(
    fm.contentsOfDirectoryAtURLIncludingPropertiesForKeysOptionsError(
    oURL, [], 1 << 2, null));
var lstHarvest = [];

score 2 · Answer 3 · answered Jan 11 '18 at 18:33

2

Like most other languages like c,c++ we can declare

int a,b,c;
char x='1',y='2';

like this javascript also allow us to this . Simple :)

answered Jan 11 '18 at 18:33

RaM PrabU

415
4
16

score 1 · Accepted Answer · answered Jan 11 '18 at 18:10

1

Using the comma allows the programmer to omit var at the beginning of each declaration/definition. It's up to your preferred style to decide which form is used but be consistent.

answered Jan 11 '18 at 18:10

Brandon H. Gomes

808
2
9
24

Why use commas between JavaScript declarations and initializations?

4 Answers4

Why use this syntax ?