Deraileur for 42 sprocket cassette?
 

What deraileur do I need for this kind of cassette on a frame for 8 speed cassette? or can I get a longer hanger?
8 Speed Cassette 11-42T MTB Cassette

odd that no one knows .....

I'd probably try a long or medium cage deraileur with possibly a wolfs tooth extender. There's calculators out there which can tell you what the total spread of your gears can be front and rear. You need to stay within that limit to remain within MFG specs...

Wished I could help more but I know there's others who have more details on this.

What's rest of your drivetrain? Shifter(s) and crankset specifically.

probably need to get one designed to handle a 42 rear and of course make sure shifters are compatible

like a sram apex at $70 https://www.jensonusa.com/SRAM-Apex-...RoCkPEQAvD_BwE

shimano at similar $70 https://www.jensonusa.com/Shimano-XT...BoCbokQAvD_BwE

The only crank sprocket I use is a 32t heavy duty for electric bicycles. there is a problem of wearing out my sprockets too fast because of the motor is so strong. I may need to go to a heavy chain single speed.

There was a seller on Amazon who was packing these cassettes with a knockoff of Wolftooth's Roadlink.