example.js

module.exports = {
  dashboarddistance: function(a, b, c, d, callback) {
    var ds = 0;
    var distance = require('google-distance');
    distance.get({
      index: 1,
      origin: a + ',' + b,
      destination: c + ',' + d
    }, function(err, data) {
      if (err) {
        console.log("a");
        ds = 0;
        return callback(err, ds);
      } else {
        console.log("b");
        ds = data.distance;
        return callback(null, ds);
      }
    });
  },
};

main.js

var example = require('./example.js');

example.dashboarddistance(a, b, c, d, function(err, distance) {
  if (err) {
    // dosomething and return
    return ;
  }
  console.log(distance);
});

What I understood is that you want to store the distance returned from the dashboarddistance() in a variable.
But what is the scope of the distance variable, ie where do you want to use it?

As You will see in the above code by @dca Hsu it returns distance in variable in distance.
When I use console.log(distance); inside function it gives an output.

But if I want to store this distance in a variable 'finaldistance' and add it to a response what should i do.

I think the scope of distance is only inside callback function that i have passed inside function dashboarddistance.