If I am reading the code correctly the heater is turned on the moment the temperature goes below the target and turns back off the moment it goes above. I'm concerned that this will lead to turning the heater on and off very often, which seems inefficient and possibly bad for the heater.

Suggest implementing a minimum deviation from target temperature before toggling the heater on/off and/or a minimum time period between toggles. A more complicated solution would be to use a PID controller such ashttps://www.npmjs.com/package/node-pid-controller.