using a variable in mongodb update

Using Meteor, I'm trying to perform an update like the following:

Items.update(Session.get('selectedItem'), {'$set': {'directions.0.name': area.value}})

But I'm struggling with how to set the array index of directions dynamically, with something like this:

var index = //a value determined dynamically
Items.update(Session.get('selectedItem'), {'$set': {'directions[index]name': area.value}})

This doesn't work because [index] is wrapped in a string. I also tried to form a custom string, like this:

var string = 'directions.'+itemIndex+'.name'
Items.update(Session.get('selectedItem'), {'$set': {string: area.value}})

But that doesn't work. Any idea on how to do this?

You need to build up your $set object programmatically:

var setModifier = { $set: {} };
setModifier.$set['directions.' + index + '.name'] = area.value;
Items.update(Session.get('selectedItem'), setModifier);

Update

If your JavaScript environment supports computed property names (e.g. node.js 4+), you can do this in one step:

Items.update(Session.get('selectedItem'), { $set: {
    ['directions.' + index + '.name']: area.value
}});