Can't update elements which are inside the array - node.js

Question

Let's say I have this
userinfo={
userDetails:
{
username:"",
password:"",
cookie:"",
firstname:"",
lastname:"",
phonenumber:"",
postalcode:"",
country:""
},
applicationsInfo:[
{
application:"",
consumerKey:"",
accessToken:""
}
]
}
First I created user and latter I am to update applicationsInfo section when user creates an application. First I tried this way and It works
var consumerKey="asdyfsatfdtyafydsahyadsy";
var findCon={"userDetails.username":"someName"};
db.find(findCon,function(err,docs){
if(err){
console.log(err);
}else{
var updateCon={$set:{"applicationsInfo.0.consumerKey":consumerKey}};
db.update(findCon,updateCon,{},function(err,docs){
console.log(docs);
});
}
});
But actually what I want is update some selected one I tried that in this way.
........
var appNum=0;
var updateCon={$set:{"applicationsInfo."+appNum+".consumerKey":consumerKey}};
then I start my node server then I got error like this.
/home/jobs/nodeserver/routes/initusers.js:180
"applicationsInfo."+appNum+
^
SyntaxError: Unexpected token +
at Module._compile (module.js:439:25)
at Object.Module._extensions..js (module.js:474:10)
at Module.load (module.js:356:32)
at Function.Module._load (module.js:312:12)
at Module.require (module.js:364:17)
at require (module.js:380:17)

Answer 1

You need to set it the below way:
var appNum = 0;
var updateCon = {$set:{}};
updateCon.$set["applicationsInfo."+appNum+".consumerKey"] = 1;
Setting an expression ("applicationsInfo."+appNum+".consumerKey") as key of an object during initialization is not allowed in java script.