How to process output of a i2c sensor to get proper values.

               i2c.writeTo(TMP117_Address, Temp_Reg);
  let a = i2c.writeTo(TMP117_Address, 0);

Is there any specific reason to write twice to the register 0 / Temp_Reg?

I'd say for this device it should be only one line, like this

i2c.writeTo({address:TMP117_Address, stop:false}, Temp_Reg);