Unable to get redis key-value pair using Node.js (ERR_HTTP_HEADERS_SENT)

Trying to fetch key-value pair from my localhost redis client using Node.js.

1. If I include the res.json() inside the function/for loop, I get HTTP_HEADER error for sending response more than once.

2. If I define it outside the function/loop, I get an empty array.

Error:

_http_outgoing.js:485
    throw new ERR_HTTP_HEADERS_SENT('set');
    ^
Error [ERR_HTTP_HEADERS_SENT]: Cannot set headers after they are sent to the client

Below is the code for app.js

router.get('/route1', function(req, res)
{ 
    var valarr=[];
    client.keys('*', function(err, keys) {
        console.log(keys);
        for(var i=0;i<keys.length;i++){
            client.get(keys[i], function (error, value) {
                if (err) return console.log(err);
                console.log(value);
                valarr.push(value);      
                // res.json({result1:keys, result2:valarr});
                //I get ERR_HTTP_HEADERS_SENT if res.json() is defined here
            });  
        }
        console.log(valarr); //Empty
        res.json({result1:keys, result2:valarr});
        //Returns an empty array (valarr)
    });
});

router.get('/route1', function(req, res)
{ 
    var valarr=[];
    client.keys('*', function(err, keys) {
        console.log(keys);
        for(var i=0;i<keys.length;i++){
            client.get(keys[i], function (error, value) {
                if (err) return console.log(err);

                if(value){
                  console.log(value);
                  valarr.push(value);      
                  res.json({result1:keys, result2:valarr});
                }
            });  
        }
        console.log(valarr); //Empty
        res.json({result1:keys, result2:valarr});
        //Returns an empty array (valarr)
    });
});