Missing letter help

JavaScript
1 
function fearNotLetter(str){
    let myarrfull="abcdefghijklmnopqrstuvwxyz".split("");
    let mygivearr=str.split("");
    if(myarrfull!=mygivearr){
   
   return  myarrfull.slice(myarrfull.indexOf(mygivearr[0]),myarrfull.indexOf(mygivearr[mygivearr.length-1])+1).filter(x=>!mygivearr.includes(x)).join("");
    }
 
return undefined;
}

why i am not getting undefined when mygivearr and myarrfull are equal

2

Solved

 function fearNotLetter(str){
        let myarrfull="abcdefghijklmnopqrstuvwxyz".split("");
        let mygivearr=str.split("");
        let a=myarrfull.slice(myarrfull.indexOf(mygivearr[0]),myarrfull.indexOf(mygivearr[mygivearr.length-1])+1).filter(x=>!mygivearr.includes(x)).join("");
    if(a!=""){
       return a
    }
      return undefined;
    
}