javascript - Uncaught TypeError: Cannot read property 'toUpperCase' of null -

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my code seems right don't know why getting error:

uncaught typeerror: cannot read property 'touppercase' of null

here code:

    //the function executed after clicks "take quiz"      function startquiz() {         //the variable first question         var firstanwser = prompt("who posted first youtube video?");         //the if statement first question         if (firstanwser.touppercase()  === 'jawed karim') {             //if person correct dialog box says correct pops            alert("correct");             //the variable second question             var secondanwser = prompt("when domain name youtube.com     activated?");             if (secondanwser.touppercase() === 'febuary 14, 2005') {                 alert("correct");                 var thirdanwser = prompt("what first video on youtube called?");                 if (thirdanwser.touppercase() === 'me @ zoo') {                     alert("correct");                 } else {                     alert("sorry, wrong");                 }              } else {                 alert("sorry, wrong");             }         } else {             //if person wrong dialog box pops says "sorry,  wrong"             alert("sorry, wrong");         }     }

the error on line says if (secondanwser.touppercase() === 'febuary 14, 2005') {

the prompt() method returns input value if user clicks "ok". if user clicks "cancel" method returns null , script report error, because there no function on null object.

solution: check if answer isn't null, before call touppercase()

if (secondanswer != null && secondanwser.touppercase() === 'febuary 14, 2005')

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