How to define multiple variables on a single line?

0 votes
asked Nov 12, 2010 by heatherk

Reading documentation online, I'm getting confused how to properly define multiple JavaScript variables on a single line.

If I want to condense the following code, what's the proper JavaScript "strict" way to define multiple javascript variables on a single line?

var a = 0;
var b = 0;

Is it:

var a = b = 0;

or

var a = var b = 0; 

etc...

5 Answers

0 votes
answered Nov 12, 2010 by meder-omuraliev

You want to rely on commas because if you rely on the multiple assignment construct, you'll shoot yourself in the foot at one point or another.

An example would be:

>>> var a = b = c = [];
>>> c.push(1)
[1]
>>> a
[1]

They all refer to the same object in memory, they are not "unique" since anytime you make a reference to an object ( array, object literal, function ) it's passed by reference and not value. So if you change just one of those variables, and wanted them to act individually you will not get what you want because they are not individual objects.

There is also a downside in multiple assignment, in that the secondary variables become globals, and you don't want to leak into the global namespace.

(function() {  var a = global = 5 })();
alert(window.global) // 5

It's best to just use commas and preferably with lots of whitespace so it's readable:

var a = 5
  , b = 2
  , c = 3
  , d = {}
  , e = [];
0 votes
answered Nov 12, 2010 by matthew-flaschen

There is no way to do it in one line with assignment as value.

var a = b = 0;

makes b global. A correct way (without leaking variables) is the slightly longer:

var a = 0, b = a;
0 votes
answered Nov 8, 2014 by pda

note you can only do this with Numbers and Strings

you could do...

var a, b, c; a = b = c = 0; //but why?

c++;
// c = 1, b = 0, a = 0;
0 votes
answered Nov 15, 2016 by nick-l

Why not doing it in two lines?

var a, b, c, d;    //All in the same scope
a = b = c = d = 1; // Set value to all.

The reason why, is to preserve the local scope on variable declarations, as this:

var a = b = c = d = 1;

will lead to the implicit declarations of b, c and d on the window scope.

0 votes
answered Nov 25, 2016 by lucas

Using Javascript's es6 or node, you can do the following:

var [a,b,c,d] = [0,1,2,3]

And if you want to easily print multiple variables in a single line, just do this (using template literals):

console.log(`${a} ${b} ${c} ${d}`)

0 1 2 3

This is similar to @alex gray 's answer here, but this example is in Javascript instead of CoffeeScript.

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