variable in payload message

[accateo]

Hi, after different hours of research, I found how to make a find.toArray with projection:

msg.payload = [{timestamp: {$gt: date_from.getTime() , $lt: date_to.getTime() }},{"fields": { "name": 1,timestamp:1}}]

Now the problem is, if I want to substitute "name" with a variable? How can I do? I tried:

msg.payload = [{timestamp: {$gt: date_from.getTime() , $lt: date_to.getTime() }},{"fields": { var: 1,timestamp:1}}]

but it doesn't work. Any idea?

Thanks

[accateo]

Solution in ES6:

msg.payload = [{timestamp: {$gt: date_from.getTime() , $lt: date_to.getTime() }},{"fields": { [var]: 1,timestamp:1}}]