I'm curious how some website prevent you to download its video ( only for study purpose ).

I start by wrap window.MediaSource.addSourceBuffer. But some how the website detect it. Here's the wrapper code :

let _addSourceBuffer = window.MediaSource.prototype.addSourceBuffer
window.MediaSource.prototype.addSourceBuffer = function(mime) {
    return _addSourceBuffer.call(this, mime)
}

How do you detect such wrapper in javascript?