Why does a conditional variable initialization need "var"?

Question

If I remove "var", the browser(Chrome) complains : "someVar is not defined"

var someVar = someVar || {};

From what I know, it should complain only if I am not initializing the variable.

Without the var... you aren't initializing it. So it complains. — Niet the Dark Absol
– Niet the Dark Absol, Commented Dec 10, 2014 at 15:49
But if I say "someVar = 5;", I do not need a "var". Is it something to do with the conditional nature of initialization? — Paddy
– Paddy, Commented Dec 10, 2014 at 15:51
a = 5 does not try to read a. a = a || 5 tries to read a, so if it doesn't exist, an error is thrown. — Felix Kling
– Felix Kling, Commented Dec 10, 2014 at 15:56
@Paddy: It explains the difference between declared and undeclared variables. Accessing an undeclared variable will throw a reference error. — Bergi
– Bergi, Commented Dec 10, 2014 at 15:58

Felix Kling · Accepted Answer · 2014-12-10 16:03:28Z

4

From what I know, it should complain only if I am not initializing the variable.

But you are trying to read an undeclared variable. Given

someVar = someVar || {};

it first evaluates someVar || {} and tries to get the value of someVar in the process. Since someVar doesn't exist (yet), you get a reference error.

Here is an example that does not throw an error:

someVar = true || someVar;

Because || does shirt-circuiting evaluation, the second operand is never evaluated and thus no error is thrown.

Why does it work with var then?

You might think that using the var keyword wouldn't make a difference since someVar || {} is still evaluated first. However, due to hoisting, the statement

var someVar = someVar || {};

is actually evaluated as

var someVar;
someVar = someVar || {};

So at the moment someVar is read, it is already declared.

edited Dec 10, 2014 at 16:03

answered Dec 10, 2014 at 15:58

Felix Kling

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1 Comment

Paddy Over a year ago

So, I got confused with the hoisting thing, and this link discuses the concept in more mundane manner : adequatelygood.com/JavaScript-Scoping-and-Hoisting.html

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