Can't understand why service is asking for datashape for output?

Hi I'm trying to make one service where I was using following code

service output should be boolean but when service is run it ask for datashape to show output. Please help me to understand this if I'm missing something

SERVICE CODE

SERVICE INPUTS

a --> number

b --> number

c--> number

formula --> string

try{
var formu = formula;
var formulaArray = formu.split("");
var check = formulaArray.forEach(function(value){
if(value === "a"||"b"||"c"||"+"||"-"||"/"||"*"||")"||"("){
return true;
} else{
return false;
}
});
logger.debug("formulaMaker "+ check);
// function evil(fn) {
// return new Function('return ' + fn)();
//}
//var calculation = evil(formula);
result = check;
//
//
//
//
//
}
catch(err){
logger.error(err);
}