Using the Geolocation API

The Geolocation API is available through the navigator.geolocation object.

If the object exists, geolocation services are available. You can test for the presence of geolocation thusly:

if ("geolocation" in navigator) {
  /* geolocation is available */
} else {
  /* geolocation IS NOT available */
}

To obtain the user's current location, you can call the getCurrentPosition() method. This initiates an asynchronous request to detect the user's position, and queries the positioning hardware to get up-to-date information. When the position is determined, the defined callback function is executed. You can optionally provide a second callback function to be executed if an error occurs. A third, optional, parameter is an options object where you can set the maximum age of the position returned, the time to wait for a request, and if you want high accuracy for the position.

navigator.geolocation.getCurrentPosition((position) => {
  doSomething(position.coords.latitude, position.coords.longitude);
});

The above example will cause the doSomething() function to execute when the location is obtained.

If the position data changes (either by device movement or if more accurate geo information arrives), you can set up a callback function that is called with that updated position information. This is done using the watchPosition() function, which has the same input parameters as getCurrentPosition(). The callback function is called multiple times, allowing the browser to either update your location as you move, or provide a more accurate location as different techniques are used to geolocate you. The error callback function, which is optional just as it is for getCurrentPosition(), can be called repeatedly.

const watchID = navigator.geolocation.watchPosition((position) => {
  doSomething(position.coords.latitude, position.coords.longitude);
});

The watchPosition() method returns an ID number that can be used to uniquely identify the requested position watcher; you use this value in tandem with the clearWatch() method to stop watching the user's location.

navigator.geolocation.clearWatch(watchID);

Both getCurrentPosition() and watchPosition() accept a success callback, an optional error callback, and an optional options object.

This object allows you to specify whether to enable high accuracy, a maximum age for the returned position value (up until this age it will be cached and reused if the same position is requested again; after this the browser will request fresh position data), and a timeout value that dictates how long the browser should attempt to get the position data for, before it times out.

A call to watchPosition could look like:

function success(position) {
  doSomething(position.coords.latitude, position.coords.longitude);
}

function error() {
  alert("Sorry, no position available.");
}

const options = {
  enableHighAccuracy: true,
  maximumAge: 30000,
  timeout: 27000,
};

const watchID = navigator.geolocation.watchPosition(success, error, options);

The user's location is described using a GeolocationPosition object instance, which itself contains a GeolocationCoordinates object instance.

The GeolocationPosition instance contains only two things, a coords property that contains the GeolocationCoordinates instance, and a timestamp property that contains a timestamp, given as Unix time in milliseconds, at which the position data was retrieved.

The GeolocationCoordinates instance contains a number of properties, but the two you'll use most commonly are latitude and longitude, which are what you need to draw your position on a map. Hence many Geolocation success callbacks look fairly simple:

function success(position) {
  const latitude = position.coords.latitude;
  const longitude = position.coords.longitude;

  // Do something with your latitude and longitude
}

You can however get a number of other bits of information from a GeolocationCoordinates object, including altitude, speed, what direction the device is facing, and an accuracy measure of the altitude, longitude, and latitude data.

The error callback function, if provided when calling getCurrentPosition() or watchPosition(), expects a GeolocationPositionError object instance as its first parameter. This object type contains two properties, a code indicating what type of error has been returned, and a human-readable message that describes what the error code means.

You could use it like so:

function errorCallback(error) {
  alert(`ERROR(${error.code}): ${error.message}`);
}

In the following example the Geolocation API is used to retrieve the user's latitude and longitude. If successful, the available hyperlink is populated with an openstreetmap.org URL that will show their location.

body {
  padding: 20px;
  background-color: #ffffc9;
}

button {
  margin: 0.5rem 0;
}