Return NaN

am trying to opt out a number but it telling me NaN

let number = document.getElementsByClassName("number");
for(var i=0;i<number.length;i++){
    number[i].addEventListener("click", function(){
        //this will display the number clicked
        let output =  reverseNumberFormat(getOutput());
        if(output!=NaN){ //if output is a number
            output=output+this.id;
            printOutput("num");
        }



    });
}