How to display error message when using a grade calculator in JavaScript? -

below grade calculator attach letter grade number entered, using if/else statements. having trouble finding way display error message if out of ranger number or non numeric value entered. suggestions? thanks!

    var entry;     var lettergrade;      entry = prompt("enter number grade 0 through 100\n" +                    "or enter 999 end entries", 999);     entry = parseint(entry);  if (entry <= 59)     lettergrade = "f"; else if (entry >= 60 && entry <= 69)     lettergrade = "d"; else if (entry >= 70 && entry <= 79)     lettergrade = "c"; else if (entry >= 80 && entry <= 89)     lettergrade = "b"; else if (entry >= 90 && entry <= 100)     lettergrade = "a";      alert("number grade = " + entry + "\n" +           "letter grade = " + lettergrade); 

well out of range >100 can cover else in end:

[..] else if (entry >= 90 && entry <= 100)     lettergrade = "a"; else     alert("error, number " + entry + " out of range (>100)"); 

as not being number, can use isnan():

if(isnan(entry)){     alert("that not number!"); else {     if (entry <= 59)         lettergrade = "f";     .... } 

edit: see input >0<101 catch negative numbers need add first if statement:

if (entry >= 0 && entry <= 59)