Javascript how to add a radius to latitude and longitude

I have a javascript function to calculate the distance between two coordinates. Now, I need to create another function that takes in a gps coordinates and add a radius to come up with a 2nd coordinates. I'm not sure how to do that and the math behind it.

Here's the function for calculating the distance between two coordinates.

function distanceInKm(lat1, lon1, lat2, lon2) {
var earthRadiusKm = 6371;

var dLat = degreesToRadians(lat2-lat1);
var dLon = degreesToRadians(lon2-lon1);

lat1 = degreesToRadians(lat1);
lat2 = degreesToRadians(lat2);

var a = Math.sin(dLat/2) * Math.sin(dLat/2) +
Math.sin(dLon/2) * Math.sin(dLon/2) * Math.cos(lat1) * Math.cos(lat2);
var c = 2 * Math.atan2(Math.sqrt(a), Math.sqrt(1-a));
return earthRadiusKm * c;
}


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