在 JavaScript 中循环返回函数的语句

Question

I was trying to built a heat map using google map and I write this code but now as you can see I am passing hard coded values to it, I want to use a loop in return statement because I am using MySQL and all the info will be stored in the database so when I call this function it should exactly create as much points on the map as many values are stored in the database.

function getPoints() 
{
    return [
        new google.maps.LatLng(28.81820630, 77.05907876),
        new google.maps.LatLng(28.8097835, 77.05673218),
        new google.maps.LatLng(28.8097835, 77.003688),
        new google.maps.LatLng(28.8097835, 77.002815),
        new google.maps.LatLng(28.782992, 77.002112),
        new google.maps.LatLng(28.783100, 77.001061),
        new google.maps.LatLng(28.783206, 77.000829),
        new google.maps.LatLng(28.783273, 77.000320),
        new google.maps.LatLng(28.783316, 77.000023),
        new google.maps.LatLng(28.783357, 77.039790),
        new google.maps.LatLng(28.783281, 77.039687),
        new google.maps.LatLng(28.783368, 77.039666),
        new google.maps.LatLng(28.783383, 77.039590),
        new google.maps.LatLng(28.783508, 77.039525),
        new google.maps.LatLng(28.783802, 77.039591),
        new google.maps.LatLng(28.780107, 77.039668),
        new google.maps.LatLng(28.780206, 77.039686),
        new google.maps.LatLng(28.780386, 77.039790),
        new google.maps.LatLng(28.780701, 77.039902),
        new google.maps.LatLng(28.780965, 77.039938),
        new google.maps.LatLng(28.785010, 77.039907),
        new google.maps.LatLng(28.785360, 77.039952),
        new google.maps.LatLng(28.785715, 77.000030),
        new google.maps.LatLng(28.786117, 77.000119),
        new google.maps.LatLng(28.786560, 77.000209),
        new google.maps.LatLng(28.786905, 77.000270),
        new google.maps.LatLng(29.786905, 77.000270)
    ];
}

Can we use loop statements in JavaScript in return statement ? If so then how?

Answer 1

No, it is not possible. But why you don't first build the array and afterwards return it?

function getPoints() {
    var array = [];
    for (...) {
        array.push(new google.maps.LatLng(value1, value2));
    }
    return array;
}

Answer 2

You can't in JavaScript but you just use a loop on the server to to print the values inside of your JavaScript return statement.

<script>
<%! 
        String getPointsFromData() {
            return myGetPointsLoopMethod();
        }
%>
function getPoints() 
{
    return [
       <% getPointsFromData(); %>
    ];
}
</script>

Or you can do this:

<script>

    function getPoints()

        return [
            <%
              InitialContext ctx;
              DataSource ds;
              Connection conn;
              Statement stmt;
              ResultSet rs;

              try {
                ctx = new InitialContext();
                ds = (DataSource) ctx.lookup("java:comp/env/jdbc/MySQLDataSource");
                //ds = (DataSource) ctx.lookup("jdbc/MySQLDataSource");
                conn = ds.getConnection();
                stmt = conn.createStatement();
                rs = stmt.executeQuery("SELECT lat,long * FROM table");

                while(rs.next()) {
            %>
         new google.maps.LatLng(<%= rs.getString("Lat") %>, <%= rs.getString("Lon") %>),
            <%  
                }
              }
              catch (Exception e) {              
              }

            %>
         ]; 
        }
</script>
</body>

Answer 3

The return inside the function can only be called once at a time.

function YourFuncName()
{
   var arr =[
    new google.maps.LatLng(28.81820630, 77.05907876),
    new google.maps.LatLng(28.8097835, 77.05673218),
    new google.maps.LatLng(28.8097835, 77.003688),
    new google.maps.LatLng(28.8097835, 77.002815),
    new google.maps.LatLng(28.782992, 77.002112),
    new google.maps.LatLng(28.783100, 77.001061),
    new google.maps.LatLng(28.783206, 77.000829),
    new google.maps.LatLng(28.783273, 77.000320),
    new google.maps.LatLng(28.783316, 77.000023),
    new google.maps.LatLng(28.783357, 77.039790),
    new google.maps.LatLng(28.783281, 77.039687),
    new google.maps.LatLng(28.783368, 77.039666),
    new google.maps.LatLng(28.783383, 77.039590),
    new google.maps.LatLng(28.783508, 77.039525),
    new google.maps.LatLng(28.783802, 77.039591),
    new google.maps.LatLng(28.780107, 77.039668),
    new google.maps.LatLng(28.780206, 77.039686),
    new google.maps.LatLng(28.780386, 77.039790),
    new google.maps.LatLng(28.780701, 77.039902),
    new google.maps.LatLng(28.780965, 77.039938),
    new google.maps.LatLng(28.785010, 77.039907),
    new google.maps.LatLng(28.785360, 77.039952),
    new google.maps.LatLng(28.785715, 77.000030),
    new google.maps.LatLng(28.786117, 77.000119),
    new google.maps.LatLng(28.786560, 77.000209),
    new google.maps.LatLng(28.786905, 77.000270),
    new google.maps.LatLng(29.786905, 77.000270)];
   return arr;
}

if your data isn't hard coded you can perform a loop which will add data to arr (array) instead, before passing it to return. example:

function YourFunctionName()
{
     var arr = [];
     for(var i=0; i<10; i++)
     {
         arr.push(i);
     }
     return arr;
}

the return result of the above code will be arrays from 0-10