Using Closures in Event Listeners

  • This example demonstrates creating a Google Map with 5 randomly placed markers within a defined boundary in Australia.

  • Each marker, when clicked, reveals a part of a secret message using an info window.

  • The code is provided in both TypeScript and JavaScript, utilizing the Google Maps JavaScript API.

  • The sample can be run locally with Git and Node.js or tried online using JSFiddle or Google Cloud Shell.

  • CSS is included to ensure the map fills the browser window and has an explicit height.

This example creates a map with tappable markers displaying a secret message.

Read the documentation.

TypeScript

async function init() {
    // Request needed libraries.
    const [{ AdvancedMarkerElement }] = await Promise.all([
        google.maps.importLibrary('marker'),
        google.maps.importLibrary('maps'),
    ]);

    const mapElement = document.querySelector('gmp-map')!;
    const innerMap = mapElement.innerMap;

    const bounds: google.maps.LatLngBoundsLiteral = {
        north: -25.363882,
        south: -31.203405,
        east: 131.044922,
        west: 125.244141,
    };

    // Display the area between the location southWest and northEast.
    innerMap.fitBounds(bounds);

    // Add 5 markers to map at random locations.
    // For each of these markers, give them a title with their index, and when
    // they are clicked they should open an infoWindow with text from a secret
    // message.
    const secretMessages = ['This', 'is', 'the', 'secret', 'message'];
    const lngSpan = bounds.east - bounds.west;
    const latSpan = bounds.north - bounds.south;

    for (const secretMessage of secretMessages) {
        const marker = new AdvancedMarkerElement({
            position: {
                lat: bounds.south + latSpan * Math.random(),
                lng: bounds.west + lngSpan * Math.random(),
            },
            map: innerMap,
            gmpClickable: true,
        });

        void attachSecretMessage(marker, secretMessage);
    }
}

// Attaches an info window to a marker with the provided message. When the
// marker is clicked, the info window will open with the secret message.
async function attachSecretMessage(
    marker: google.maps.marker.AdvancedMarkerElement,
    secretMessage: string
) {
    const { InfoWindow } = await google.maps.importLibrary('maps');

    const infoWindow = new InfoWindow({
        content: secretMessage,
    });

    marker.addEventListener('gmp-click', () => {
        infoWindow.open(marker.map, marker);
    });
}

void init();

JavaScript

async function init() {
    // Request needed libraries.
    const [{ AdvancedMarkerElement }] = await Promise.all([
        google.maps.importLibrary('marker'),
        google.maps.importLibrary('maps'),
    ]);

    const mapElement = document.querySelector('gmp-map');
    const innerMap = mapElement.innerMap;

    const bounds = {
        north: -25.363882,
        south: -31.203405,
        east: 131.044922,
        west: 125.244141,
    };

    // Display the area between the location southWest and northEast.
    innerMap.fitBounds(bounds);

    // Add 5 markers to map at random locations.
    // For each of these markers, give them a title with their index, and when
    // they are clicked they should open an infoWindow with text from a secret
    // message.
    const secretMessages = ['This', 'is', 'the', 'secret', 'message'];
    const lngSpan = bounds.east - bounds.west;
    const latSpan = bounds.north - bounds.south;

    for (const secretMessage of secretMessages) {
        const marker = new AdvancedMarkerElement({
            position: {
                lat: bounds.south + latSpan * Math.random(),
                lng: bounds.west + lngSpan * Math.random(),
            },
            map: innerMap,
            gmpClickable: true,
        });

        void attachSecretMessage(marker, secretMessage);
    }
}

// Attaches an info window to a marker with the provided message. When the
// marker is clicked, the info window will open with the secret message.
async function attachSecretMessage(marker, secretMessage) {
    const { InfoWindow } = await google.maps.importLibrary('maps');

    const infoWindow = new InfoWindow({
        content: secretMessage,
    });

    marker.addEventListener('gmp-click', () => {
        infoWindow.open(marker.map, marker);
    });
}

void init();

CSS

/* 
 * Optional: Makes the sample page fill the window. 
 */
html,
body {
    height: 100%;
    margin: 0;
    padding: 0;
}

HTML

<html>
    <head>
        <title>Using Closures in Event Listeners</title>

        <link rel="stylesheet" type="text/css" href="./style.css" />
        <script type="module" src="./index.js"></script>
        <script>
            // prettier-ignore
            (g=>{var h,a,k,p="The Google Maps JavaScript API",c="google",l="importLibrary",q="__ib__",m=document,b=window;b=b[c]||(b[c]={});var d=b.maps||(b.maps={}),r=new Set,e=new URLSearchParams,u=()=>h||(h=new Promise(async(f,n)=>{await (a=m.createElement("script"));e.set("libraries",[...r]+"");for(k in g)e.set(k.replace(/[A-Z]/g,t=>"_"+t[0].toLowerCase()),g[k]);e.set("callback",c+".maps."+q);a.src=`https://maps.${c}apis.com/maps/api/js?`+e;d[q]=f;a.onerror=()=>h=n(Error(p+" could not load."));a.nonce=m.querySelector("script[nonce]")?.nonce||"";m.head.append(a)}));d[l]?console.warn(p+" only loads once. Ignoring:",g):d[l]=(f,...n)=>r.add(f)&&u().then(()=>d[l](f,...n))})({
                key: "GOOGLE_MAPS_API_KEY"
            });
        </script>
    </head>
    <body>
        <gmp-map
            center="-25.363882, 131.044922"
            zoom="4"
            map-id="DEMO_MAP_ID"></gmp-map>
    </body>
</html>

Clone Sample

Git and Node.js are required to run this sample locally. Follow these instructions to install Node.js and NPM. You will also need a Google Maps API key.

The following commands clone the repository, install dependencies, and copy the environment file. 

  git clone https://github.com/googlemaps-samples/js-api-samples.git
  cd js-api-samples
  npm i
  cp .env.template .env

Once you copy the environment file, make sure to open the .env file and replace <Insert your own development API key here> with your actual API key before starting the sample. 

  cd samples/event-closure
  npm start