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My route needs to pass a series of JavaScript objects to the pug template.

router.get('/edit/:itemObjectId', async function(req, res, next) 
 var itemObjectId = req.params.itemObjectId;
 var equipmentCategoryArr = ;
 var lifeExpectancyArr = ;
 var businessUnitResponsibleArr = ;
 var item = undefined;

 try 
 item = await db_item.getItem(itemObjectId);

 // Where item["result"] is either undefined or a JavaScript Object. E.g. "createdAt":"2018-11-07T04:07:44.587Z","updatedAt":"2018-11-07T04:25:18.526Z","item_name":"GM Portable","manufacturer":"GM"
 res.render('editItem', 
 title: 'Edit item details', // Give a title to our page
 item: item["result"],
 equipmentCategoryArr: req.app.locals.equipmentCategoryArr,
 lifeExpectancyArr: req.app.locals.lifeExpectancyArr,
 businessUnitResponsibleArr: req.app.locals.businessUnitResponsibleArr
 );
 catch (err) 
 return Promise.reject(JSON.stringify( "Error": err ));
 
);

Where

item["result"] could be either undefined or a JavaScript object.

On the editItem.pug page, I need to use the item in JavaScript.

block extraScript
 script.
 console.log(!JSON.stringify(item));

However, it renders like "console.log();" if item that was passed is undefined. So !JSON.stringify(item) was not rendered at all.

How can I check if item is undefined or not on the fron-end?

You can manipulate this in three places, all using the ternary operator ( condition ? ifTrue : ifFalse )

In the route you can switch it from being undefined to an empty object

item: item["result"] ? item["result"] : 

In the template you could apply the same logic:

console.log(! item ? JSON.stringify(item) : '' );

You are going to need to do one of the two things above to make sure that something is getting rendered into the client-side code, otherwise you're going to face a syntax error from an empty expression:

var item = ;