Add javascript function as a substitute to postgresql stored procedures / functions in postgresq

Add an ability for sql.js to apply custom javascript function in the SQL.

Would it be cool to see this in sql.js

CREATE TABLE people_location(
     person_id int,
     latitude double,
     longitude double,
);

-- see people within 5 KM radius
SELECT * FROM people_location WHERE  compute_distance(latitude, longitude, my_lat, my_long) <= 5;

    function compute_distance(lat1, lon1, lat2, lon2){
         var φ1 = lat1.toRadians(), φ2 = lat2.toRadians(), Δλ = (lon2-lon1).toRadians(), R = 6371; // gives d in km
         var d = Math.acos( Math.sin(φ1)*Math.sin(φ2) + Math.cos(φ1)*Math.cos(φ2) * Math.cos(Δλ) ) * R;
        return d;
} 

The issue names no files, tests, or entry points. Start by locating how sql.js exposes SQLite functionality to JavaScript, then establish the intended registration and invocation behavior for custom SQL functions. Done means a JavaScript function can be registered, called from SQL, and covered by a focused test.