what's difference multiple blank variables declare

Question

literal object methods:

var objectA = {}; 
var objectB = {};

vs

var objectA = objectB = {};

constructor object methods:

var objectA = new Object();
var objectB = new Object();

vs

var objectA = objectB = new Object();

Answer 1

Actually, when you do

var objectA = {}; 
var objectB = {};

or

var objectA = new Object();
var objectB = new Object();

You are creating two different JavaScript objects and they are referred by objectA and objectB. But when you do

var objectA = objectB = {};

or

var objectA = objectB = new Object();

You are actually creating only one object and making both objectA and objectB refer the same object.

You can confirm this by checking if both the objects are one and the same or not, like this

var objectA = {}, objectB = {};
console.log(objectA === objectB);
// false
var objectC = objectD = {};
console.log(objectC === objectD);
// true

Note:

var objectC = objectD = {};

will be evaluated like this

var objectC = (objectD = {});

That is why both objectC and objectD refer the same object.

Important: As dfsq mentions in the comment, in the last example, objectD will be leaked to the global scope. So, avoid using this pattern.