Thank you very much, that did the trick!
So my new JS transformation function “filter_ltminus100_gt100.js” which filters all values above 100 degrees and below -100 degrees now looks like this:
(function(i) {
var val = parseFloat(i);
if (val >= 100 || val <= -100 )
{ return 'UNDEF'; }
else
{ return val; }
})(input)
and the channel definition in the .things file like this:
Type number : TE_MJHTV1_5C8E "TE_MJHTV1_5C8E " [ stateTopic="OHAB/TE_MJHTV1_5C8E", transformationPattern="JS:filter_ltminus100_gt100.js" ]
Again thank you very much @rlkoshak , that solves my problem!