Dynamically Insert NASA Picture Below The Page Body Tag Using Content Scripts

Completed Extension Package Source for This Part

Overview

In part 2, you learn to update your pop-up menu to not show the static stars image you had before, but to replace that image with a title and a standard HTML button. That button, when selected, passes that stars image, which is embedded in the Extension, to the content page. That image, is inserted into the active browser tab.

  • Extension technologies covered in this guide
    • Injecting JavaScript libraries into Extension
    • Exposing Extension assets to browser tabs
    • Including content pages in existing browser tabs
    • Having content pages listen for messages from pop-ups and respond

Remove the image from the pop-up and replace it with a button

First, update your popup.html file with some straight forward markup that displays a title and a button. You program that button shortly, but for now, just include a reference to an empty JavaScript file popup.js. Here is update HTML.

<html>
    <head>
        <meta charset="utf-8" />
        <style>
            body {
                width: 500px;
            }
            button {
                background-color: #336dab;
                border: none;
                color: white;
                padding: 15px 32px;
                text-align: center;
                font-size: 16px;
            }
        </style>
    </head>
    <body>
        <h1>Show the NASA Picture of the Day</h1>
        <h2>(select the image to remove)</h2>
        <button id="sendmessageid">Display</button>
        <script src="popup.js"></script>
    </body>
</html>

After updating your Extension and selecting the Extension launch icon, the have the following pop-up includes a display button.

popup.html display after pressing the Extension icon

popup.html display after pressing the Extension icon

Updated strategy to display image at the top of the browser tab

After adding the button, the next task is to make it bring up the images/stars.jpeg image file at the top of the active tab page.

Remember, each tab page has a unique own thread and the Extension has a separate thread. So, you must first create a content script and inject that content script into the tab page. Once you do that, you must send a message from your pop-up to that content script running on the tab page telling that content script what image to show, and how to show it.

Creating the pop-up JavaScript to send a message

First, create popup/popup.js and add code to send a message to your not-yet-created content script that you must momentarily create and inject into your browser tab. To do that, the following code adds an onclick event to your pop-up display button.

const sendMessageId = document.getElementById("sendmessageid");
if (sendMessageId) {
  sendMessageId.onclick = function() {
    // do something
  };
}

In the onclick event, what you must do is find the current browser tab (if there is only one open it is that one). Then, once you find that tab, use the chrome.tabs.sendmessage Extension API to send a message to that tab.

In that message you must include the URL to the image you want to display, and you want to send a unique ID that should be assigned to that inserted image. You may choose to let the content insertion JavaScript generate that, but for reasons that become apparent later, generate that unique ID here in popup.js and pass it to the not-yet-created content script.

Here is your updated popup/popup.js file. Also, pass in the current tab ID which you should need in a later section but for now, is not be used.

const sendMessageId = document.getElementById("sendmessageid");
if (sendMessageId) {
    sendMessageId.onclick = function() {
        chrome.tabs.query({ active: true, currentWindow: true }, function(tabs) {
            chrome.tabs.sendMessage(
                tabs[0].id,
                {
                    url: chrome.extension.getURL("images/stars.jpeg"),
                    imageDivId: `${guidGenerator()}`,
                    tabId: tabs[0].id
                },
                function(response) {
                    window.close();
                }
            );
            function guidGenerator() {
                const S4 = function () {
                    return (((1 + Math.random()) * 0x10000) | 0).toString(16).substring(1);
                };
                return (S4() + S4() + "-" + S4() + "-" + S4() + "-" + S4() + "-" + S4() + S4() + S4());
            }
        });
    };
}

Making your stars.jpeg available from any browser tab

You are probably wondering why, when you pass the images/stars.jpeg must you use the chrome.extension.getURL chrome Extension API instead of just passing in the relative URL without the extra prefix like in the previous section. By the way, that extra prefix, returned by getUrl with the image attached looks something like the following.

extension://inigobacliaghocjiapeaaoemkjifjhp/images/stars.jpeg

The reason is that you are injecting the image using the src attribute of the img element into the content page. The content page is running on a unique thread that is not the same as the thread running the Extension. You must expose the static image file as a web asset for it to work correctly.

To do that, you must add another entry in the manifest.json file. You must declare the image to be accessible from any browser tab. That entry is as follows (you should see it in the full manifest.json file below when you add the content script declaration coming up).

"web_accessible_resources": [
    "images/*.jpeg"
]

You have now written the code in your popup.js file to send a message to the content page that is embedded on the current active tab page, but you have not created and injected that content page. Do that now.

Updating your manifest.json for content and web access

The updated manifest.json that includes the content-scripts and web_accessible_resources is as follows.

{
    "name": "NASA Picture of the day viewer",
    "version": "0.0.0.1",
    "manifest_version": 2,
    "description": "A Chromium Extension to show the NASA Picture of the Day.",
    "icons": {
        "16": "icons/nasapod16x16.png",
        "32": "icons/nasapod32x32.png",
        "48": "icons/nasapod48x48.png",
        "128": "icons/nasapod128x128.png"
    },
    "browser_action": {
        "default_popup": "popup/popup.html"
    },
    "content_scripts": [
        {
            "matches": [
              "<all_urls>"
            ],
            "js": ["lib/jquery.min.js","content-scripts/content.js"]
        }
    ],
    "web_accessible_resources": [
        "images/*.jpeg"
    ]
}

The section you added is content_scripts. The attribute matches set to <all_urls> means that all the files mention in the content_scripts section is injected into all browser tab pages when each are loaded. The allowable types of files that are able to be injected here are javascript and css. You also added libjquery.min.js. You are able to include that from the download mentioned at the top of the section.

Adding jQuery and understanding the associated thread

In the content scripts you are injecting, plan on using jQuery ($). You added a minified version of jQuery and put it in your Extension package as lib\jquery.min.js. These content scripts run in an individual sandbox of sorts, which means that the jQuery injected into the popup.js page does not share with the content.

Keep in mind that even if the browser tab has JavaScript running on it on the loaded web page, any content injected does not have access to that. That injected JavaScript just has access to the actual DOM loaded in that browser tab.

Adding the content script message listener

Here is that content-scripts\content.js file that gets injected into every browser tab page based on your manifest.json content-scripts section.

chrome.runtime.onMessage.addListener(function(request, sender, sendResponse) {
    $("body").prepend(
        `<img  src="${request.url}" id="${request.imageDivId}"
               class="slide-image" /> `
    );
    $("head").prepend(
        `<style>
          .slide-image {
              height: auto;
              width: 100vw;
          }
        </style>`
    );
    $(`#${request.imageDivId}`).click(function() {
        $(`#${request.imageDivId}`).remove(`#${request.imageDivId}`);
    });
    sendResponse({ fromcontent: "This message is from content.js" });
});

Notice that all the above JavaScript does is to register a listener using the chrome.runtime.onMessage.addListener Extension API method. This listener waits for messages like the one you sent from the popup.js described earlier with the chrome.tabs.sendMessage Extension API method.

The first parameter of the addListener method is a function whose first parameter, request, is the details of the message being passed in. Remember, from popup.js, when you used the sendMessage method, those attributes of the first parameter are url and imageDivId.

When an event is processed by the listener, the function that is the first parameter is run. The first parameter of that function is an object that has attributes as assigned by sendMessage. That function simply processes the three jQuery script lines.

  • The first dynamically inserts into the DOM header a <style> section that you must assign as a slide-image class to your img element.
  • The second, appends an img element right below the body of your browser tab that has the slide-image class assigned as well as the imageDivId as the ID of that image element.
  • The third adds a click event that covers the entire image allowing the user to select any place on the image and that image is be removed from the page (along with it is event listener).

Adding functionality to remove the displayed image when selected

Now, when you browse to any page and select your Extension icon, the pop-up menu is displayed as follows.

popup.html display after pressing the Extension icon

popup.html display after pressing the Extension icon

When you select the Display button, you get what is below. If you select anywhere on the stars.jpeg image, that image element is removed and tab pages collapses back to what was originally displayed.

The image showing in browser

The image showing in browser

You have now created an Extension that successfully sends a message from the Extension icon pop-up, to the dynamically inserted JavaScript running as content on the browser tab. That injected content set the image element to display your static stars jpeg.