I tried testing my sample app on iOS Safari Mobile browser (without Cordova), and while it almost worked on Chrome 63 –
video preview is showing but scanning occurred once during all my testing – it did not work on iOS Mobile Safari.
Then I found this comment on an existing issue for the plugin : https://github.com/bitpay/cordova-plugin-qrscanner/issues/89#issuecomment-336454494
So basically, this plugin, used as a library, won’t work without Cordova on mobile browsers 
Here is my sample app: