general javascript

Question

i havent written in js in awhile and am a bit rusty apparently. trying to understand the following problem. the alert in getCurrentPosition successCallback shows the latitude correctly, but the last line alerts undefined. why isnt my client_location function returning the latitude when call outside the function?

client_location = function() {
  if (navigator.geolocation) {
    return navigator.geolocation.getCurrentPosition(function(position) {
      alert(position.coords.latitude);  ## RETURNS LATITUDE CORRECTLY ##
      return position.coords.latitude;
    });
  }
};
alert(client_location());               ## RETURNS UNDEFINED ##

mu is too short · Accepted Answer · 2011-11-12 03:08:44Z

3

You're passing a callback to getCurrentPosition and your alert is inside that callback. Your return position.coords.latitude is also inside that callback. Your client_location function returns whatever getCurrentPosition returns and getCurrentPosition doesn't return anything.

If you want to do something with the latitude, you'll have to do it inside your callback; you could hand client_location a callback like this:

client_location = function(callback) {
  if (navigator.geolocation) {
    return navigator.geolocation.getCurrentPosition(function(position) {
      callback(position.coords.latitude);
    });
  }
};

client_location(function(lat) {
    alert(lat);
});

answered Nov 12, 2011 at 3:08

mu is too short

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4 Comments

Ray Toal Over a year ago

+1 for the link to the interface GeoLocation. Shows exactly why the final alerted value is undefined and not something arbitrary.

mu is too short Over a year ago

@Ray: I prefer to supply an authoritative reference rather than expect people to trust me as an authority.

brewster Over a year ago

hm. im trying to write a simple function i can call that gets handed back the latitude, but i want to avoid having to pass any arguments to that function. how could i internalize passing in a callback into the client_location function?

mu is too short Over a year ago

@brewster: That's not going to work very well since getCurrentPosition is asynchronous; so, you don't know when you'll get your result from getCurrentPosition. I suggest that you get used to the async callback approach, you have to deal with it all the time when using AJAX and a lot of other APIs.

Bruce.Norton · Accepted Answer · 2011-11-12 03:16:40Z

0

If the device does not return a geolocation it is undefined. Add an else statement to deal with no geolocation.

client_location = function() {
  if (navigator.geolocation) {
    return navigator.geolocation.getCurrentPosition(function(position) {
      alert("clientLocation: "+position.coords.latitude);
      return position.coords.latitude;
    });
  }else{
    return("no navigator.geolocation");  
  }
};
alert(client_location());

answered Nov 12, 2011 at 3:16

Bruce.Norton

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