significance of double quotes for field name

Does anybody know why double quotes are used for field name. 

The query seems to work with or without. 

For example - "_id"  below 

db.zips.aggregate({$group:{_id:"$state",totalPop:{$sum:"$pop"}}})

db.zips.aggregate({$group:{"_id":"$state",totalPop:{$sum:"$pop"}}})

Does anybody know why double quotes are used for field name. 

The query seems to work with or without. 

Hi,

When you run a query or command from the `mongo` shell interface it is being parsed by the shell's JavaScript interpreter.

Anything which isn't valid JavaScript will cause a syntax error. For example, a collection or field name starting with a digit rather than an underscore or alpha character would have to be quoted:

> db.movies.find({3dmovie: 'Jaws III'})

2014-10-31T09:46:58.457+1000 SyntaxError: Unexpected token ILLEGAL

Names like _id, $sum, and totalPop are valid JavaScript so do not have be strictly quoted in the `mongo` shell.

In general I find it easier to be in the habit of using quotes so you don't have to remember the exceptions, although you can save on a few keystrokes without ;-)

Thank you for taking time to answer.