Can JavaScript Accurately Measure Internet Speed, and How?

How to Measure Internet Speed with JavaScript (With Caveats)

Detecting a user's internet speed with JavaScript is possible to some extent, but it's crucial to manage expectations regarding accuracy. The approach involves:

• Image Loading with Known File Size: Load an image with a predetermined file size.
• Event-Based Measurement: In the image's "onload" event, calculate the elapsed time since the event was triggered.
• Calculation: Divide the time by the image file size to determine the connection speed.

However, this method has the following limitations:

• Inherent Inaccuracy: Network conditions, caching, and other factors can affect the accuracy of calculated speed.
• Selective Detection: It only measures the speed specific to the image file being downloaded, not the overall network speed.

Example Implementation:

A working example of the described approach can be found here: Calculate speed using JavaScript.

Test Case with Fixes:

The following code incorporates a fix to address some inaccuracies:

// ... same setup code as before ...

function showResults() {
    var duration = (endTime - startTime) / 1000;
    var duration2 = (Math.round(duration * 1000) / 1000).toFixed(2); // Round-trip time fix

    var bitsLoaded = downloadSize * 8;
    var speedBps = (bitsLoaded / duration).toFixed(2);
    var speedKbps = (speedBps / 1024).toFixed(2);
    var speedMbps = (speedKbps / 1024).toFixed(2);
    ShowProgressMessage([
        "Your connection speed is:", 
        speedBps + " bps", 
        speedKbps + " kbps", 
        speedMbps + " Mbps"
    ]);
}

This fix includes a more accurate calculation of the duration by rounding it to two decimal places.

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