How can I calculate the distance between two coordinates using JavaScript?

I've recently come across a problem where I needed to calculate the distance between two sets of latitude and longitude coordinates. This is a common task in many applications, such as mapping services, location-based services, and geocoding.

Luckily, the complicated math for it has already been done for us. We can simply use the Haversine formula to calculate the distance between two points on the Earth's surface given their latitude and longitude coordinates. Apart from the two points, we'll also need the radius of the Earth to calculate the distance, which is approximately 6,371 kilometers or 3,959 miles.

For the formula to work, we'll first have to convert our coordinates to radians. This is a matter of simply using Math.PI and multiplying by the latitude and longitude values, then dividing by 180 (essentially the toRadians snippet). We'll also need to find the differences in latitude and longitude between the two points. Then, we can apply the Haversine formula to calculate the distance.

const coordinateDistance = (lat1, lon1, lat2, lon2) => {
  // Convert degrees to radians
  const radLat1 = lat1 * Math.PI / 180;
  const radLon1 = lon1 * Math.PI / 180;
  const radLat2 = lat2 * Math.PI / 180;
  const radLon2 = lon2 * Math.PI / 180;

  // Radius of the Earth in meters
  const R = 6_371_000; // m

  // Differences in coordinates
  const dLat = radLat2 - radLat1;
  const dLon = radLon2 - radLon1;

  // Haversine formula
  const a = Math.sin(dLat / 2) * Math.sin(dLat / 2) +
            Math.cos(radLat1) * Math.cos(radLat2) *
            Math.sin(dLon / 2) * Math.sin(dLon / 2);

  const c = 2 * Math.atan2(Math.sqrt(a), Math.sqrt(1 - a));
  const distance = R * c;

  return distance.toFixed(2); // Return distance rounded to 2 decimal places
};

// Sample usage:
const lat1 = 40.7128; // New York City
const lon1 = -74.0060;
const lat2 = 34.0522; // Los Angeles
const lon2 = -118.2437;

coordinateDistance(lat1, lon1, lat2, lon2); // 3935746.25 (meters)

The beauty of this formula is that we can tweak the R value to calculate the distance in different units. For example, if we set R to 6_371 (the Earth's radius in kilometers), the result will be in kilometers. If we set it to 3959 (the Earth's radius in miles), the result will be in miles. This makes it very versatile for different use cases.

If you want to take it a step further, you may also calculate the distance of two points on another planet or sphere by changing the R value accordingly. The Haversine formula is not limited to Earth's surface, but can be applied to any sphere. This makes it a powerful tool for a wide range of applications.