HTML5QrCode camera detection and initialization: Avoid misuse of getCameras() method

When using the HTML5QrCode library, developers often encounter Uncaught TypeError: html5QrCode.getCameras is not a function error. This article aims to clarify the correct usage of the getCameras() method, pointing out that it should be called as a static method of the Html5Qrcode class rather than an instance method, and provides a complete code example to guide users how to correctly detect available cameras and initialize the QR code scanning function, thereby effectively avoiding this type of error and ensuring that the camera function is functioning normally.

Understand the correct way to call getCameras() method

HTML5QrCode is a powerful JavaScript library used to implement QR code scanning function in web pages. When trying to get a list of available cameras on the device, many developers habitually call the getCameras() method on the created Html5Qrcode instance, such as html5QrCode.getCameras(). However, this results in a TypeError, because getCameras() is not an instance method, but a static method of the Html5Qrcode class.

A static method is a method that belongs directly to the class itself, not any specific instance of the class. This means you should call it directly by the class name, namely Html5Qrcode.getCameras(). It can be called before you create an Html5Qrcode instance, as it is responsible for providing device-level camera information without requiring a specific scanner instance.

Correctly detect available cameras and initialize scanners

In order to correctly detect the device camera and initialize or display the corresponding UI based on the results, we need to follow the following steps:

1. Introduce library files : Make sure html5-qrcode.min.js has been introduced correctly.
2. Call static method : Use Html5Qrcode.getCameras() to get the camera list.
3. Handling asynchronous results : getCameras() returns a Promise, you need to use .then() and .catch() to handle successful acquisition of camera list and failed acquisition (such as no camera or permission denied).
4. Initialize the scanner : If the camera is detected, create an Html5Qrcode instance and start the scan.
5. Error handling and UI feedback : If the camera is not detected, or the user has rejected the camera permission, the user should be displayed with a friendly prompt message.

Here is a complete code example that demonstrates how to implement this process correctly:

 


    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>HTML5QrCode Camera Detection and Scanning</title>
    <style>
        /* Simple CSS style for controlling display/hide*/
        .notVisible { display: none; }
        .hide { display: none; }
        #scanner-container { width: 300px; height: 300px; border: 1px solid #ccc; margin-bottom: 10px; }
        .message { color: red; font-weight: bold; }
    </style>


    <h1>HTML5QrCode QR code scanning tutorial</h1>

    <div id="scanner-container"></div>
    <div class="scan-dom">
        <p>Scanning the QR code...</p>
    </div>
    <div class="noCameraMessage hide message">
        <p>No available cameras were detected or camera permissions were denied. </p>
        <p>Please make sure your device has a camera and has granted browser camera access. </p>
    </div>
    <div class="decoded-result hide">
        <p>Scan result: <span id="decodedText"></span></p>
    </div>

    <!-- Introducing HTML5QrCode library-->
    <script src="https://unpkg.com/html5-qrcode@2.3.8/html5-qrcode.min.js"></script>
    <!-- Or according to your project path: <script src="/js/html5-qrcode.min.js"></script> -->

    <script>
        // Define the ID of the scanner container
        const scannerContainerId = "scanner-container";
        let html5QrCode; // Declare a variable to store the Html5Qrcode instance// Scan successfully to callback function const qrCodeSuccessCallback = (decodedText, decodedResult) => {
            console.log(`QR Code detected: ${decodedText}`, decodedResult);
            // Stop scanning html5QrCode.stop().then(() => {
                console.log("QR Code scanning stopped.");
                // Display scan results document.getElementById(&#39;decodedText&#39;).textContent = decodedText;
                document.querySelector(&#39;.decoded-result&#39;).classList.remove(&#39;hide&#39;);
                document.querySelector(&#39;.scan-dom&#39;).classList.add(&#39;notVisible&#39;);
            }).catch((err) => {
                console.error("Error stopping QR Code scanning:", err);
            });
        };

        // Scan the configuration const config = {
            fps: 10, // Frame rate qrbox: { width: 250, height: 250 }, // Scan box size// preferFrontCamera: false, // Default use of rear camera};

        // Core logic: detect the camera and start scanning Html5Qrcode.getCameras().then(devices => {
            if (devices && devices.length > 0) {
                // At least one camera is available to console.log("Available cameras:", devices);

                // Make sure that the scan-related UI is visible to document.querySelector(".scan-dom").classList.remove("notVisible");
                document.querySelector(".noCameraMessage").classList.add("hide");

                // Create an Html5Qrcode instance html5QrCode = new Html5Qrcode(scannerContainerId);

                // Start scanning// You can specify the camera ID, or use faceMode: "environment" (back) or "user" (front)
                // Here we try to use the rear camera html5QrCode.start({ faceMode: "environment" }, config, qrCodeSuccessCallback)
                    .catch(err => {
                        console.error("Cannot start scanning, the camera may be occupied or permission problems:", err);
                        document.querySelector(".scan-dom").classList.add("notVisible");
                        document.querySelector(".noCameraMessage").classList.remove("hide");
                    });

            } else {
                // No camera console.log("No Camera Found on this device.");
                document.querySelector(".scan-dom").classList.add("notVisible");
                document.querySelector(".noCameraMessage").classList.remove("hide");
            }
        }).catch(err => {
            // Failed to get the camera list, which may be due to permission issues or the browser does not support console.error("Error getting cameras:", err);
            document.querySelector(".scan-dom").classList.add("notVisible");
            document.querySelector(".noCameraMessage").classList.remove("hide");
        });
    </script>

Notes and best practices

1. Permission Management : The browser will request user authorization when accessing the camera. If the user refuses authorization, the getCameras() or start() method will throw an error. Be sure to handle these errors in the .catch() block and provide clear guidance to the user.
2. UI Feedback : It is crucial to provide clear user interface feedback during camera detection and scanning. For example, display prompt information when there is no camera, display load status during scanning, and display results after successful scanning.
3. Stop scanning : When scanning is no longer needed, be sure to call html5QrCode.stop() to free up the camera resources. This is very important for user privacy and device performance.
4. Select camera : The devices array returned by Html5Qrcode.getCameras() contains the id and label of each camera. In the html5QrCode.start() method, you can pass in a specific deviceId to select which camera to use, or use faceMode: "environment" or "user" to automatically select.
5. Error debugging : Use the browser's developer tool (Console) to view any JavaScript errors or warnings, which is very helpful for debugging problems.

Summarize

The getCameras() method of the HTML5QrCode library is a static method that detects available cameras on the device before creating a scanner instance. The correct way to call is Html5Qrcode.getCameras(). By following the examples and best practices provided in this article, developers can effectively avoid common TypeErrors and build robust and user-friendly QR code scanning applications. Always remember to handle the Promise results of asynchronous operations and provide appropriate user feedback and error handling mechanisms.

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