Crazy easy if/else script drive me crazy

Hello new Kirupa friend! I am jakartaguy from jakarta indonesia with crazy simple question…

code below is okay IF people put in only number… but if they put in LETTER… result is NaN (non number)…

the code i use (from tutorial here) is:

if (ques1 > 100 || ques1 < 0 || ques1 == undefined) {
ValidationMessage = “For statement 1 please insert a number between 0 and 100%”;
}

But question - how do I add if/else for Letter??? The script below NOT WORK… can someone please help??

if (ques1 > 100 || ques1 < 0 || ques = NaN || ques1 == undefined) {
ValidationMessage = “For statement 1 please insert a number between 0 and 100%”;
}

thankyou(terima kasih)

jakartaguy