Using the Geolocation API
=========================

**Secure context:** This feature is available only in secure contexts (HTTPS), in some or all supporting browsers.

The Geolocation API is used to retrieve the user's location, so that it can for example be used to display their position using a mapping API. This article explains the basics of how to use it.

The geolocation object
----------------------

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:


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

### Getting the current position

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.

**Note:** By default, `getCurrentPosition()` tries to answer as fast as possible with a low accuracy result. It is useful if you need a quick answer regardless of the accuracy. Devices with a GPS, for example, can take a minute or more to get a GPS fix, so less accurate data (IP location or Wi-Fi) may be returned to `getCurrentPosition()`.


```js
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.

### Watching the current position

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.

**Note:** You can use `watchPosition()` without an initial `getCurrentPosition()` call.


```js
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.


```js
navigator.geolocation.clearWatch(watchID);
```

### Fine tuning the response

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:


```js
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);
```

Describing a position
---------------------

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:


```js
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.

Handling errors
---------------

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:


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

Examples
--------

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;
}
```

### HTML


```html
<button id="find-me">Show my location</button><br />
<p id="status"></p>
<a id="map-link" target="\_blank"></a>
```

### JavaScript


```js
function geoFindMe() {
  const status = document.querySelector("#status");
  const mapLink = document.querySelector("#map-link");

  mapLink.href = "";
  mapLink.textContent = "";

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

    status.textContent = "";
    mapLink.href = `https://www.openstreetmap.org/#map=18/${latitude}/${longitude}`;
    mapLink.textContent = `Latitude: ${latitude} °, Longitude: ${longitude} °`;
  }

  function error() {
    status.textContent = "Unable to retrieve your location";
  }

  if (!navigator.geolocation) {
    status.textContent = "Geolocation is not supported by your browser";
  } else {
    status.textContent = "Locating…";
    navigator.geolocation.getCurrentPosition(success, error);
  }
}

document.querySelector("#find-me").addEventListener("click", geoFindMe);
```

### Result