javascript - Uncaught TypeError: Cannot set property 'onfocus' of null -

i trying learn javascript , i'm building basic tutorial. in trying demonstrate onfocus , onblur, error message in javascript console: uncaught typeerror: cannot set property 'onfocus' of null.

here code. new learning javascript , use help.

//alert("hello, world!");  // javascript alert button  // var year = 2014; var useremail = ""; var todaysdate = ""; /*var donation = 20;  if (donation < 20) {     alert("for $20 cookie. change donation?"); } else {     alert("thank you!"); } */  var mainfile = document.getelementbyid("maintitle"); console.log("this element of type: ", maintitle.nodetype); console.log("the inner html ", maintitle.innerhtml); console.log("child nodes: ", maintitle.childnodes.length);  var mylinks = document.getelementsbytagname("a"); console.log("links: ", mylinks.length);  var mylistelements = document.getelementsbytagname("li"); console.log("list elements: ", mylistelements.length);  var myfirstlist = document.getelementbyid("2 paragraphs"); /* can use: var limitedlist = myfirstlist.getelementsbytagname("li"); dig deeper dom */ var myelement = document.createelement("li"); var mynewelement = document.createelement("li"); //mynewelement.appendchild(mynewelement);   var mytext = document.createtextnode("new list item"); mynewelement.appendchild(mytext);   // creating elements var newlistitem = document.createelement("li"); var newpara = document.createelement("p");  // add content, either use inner html // or create child nodes manually so: // newpara.innerhtml = "blah blah blah..."; var paratext = document.createtextnode("and beginner level intro javascript! yay!"); newpara.appendchild(paratext);  //and still need attach them document document.getelementbyid("basic").appendchild(newpara);  var mynewelement = document.createelement("li"); var seconditem = myelement.getelementsbytagname("li")[1]; myelement.insertbefore(mynewelement, seconditem);  // example of using anonymous function: onclick. //when click anywhere on page, alert appears. //document.onclick = function() { //  alert("you clicked somewhere in document"); //}  // , example of restricting click alert  // element on page. var myimage = document.getelementbyid("mainimage"); myimage.onclick = function() {     alert("you clicked on picture!"); }  function prepareeventhandlers() {     var myimage = document.getelementbyid("mainimage");     myimage.onclick = function() {       alert("you clicked on picture!");     }     //onfocus , onblur event handler illustration     var emailfield = document.getelementbyid("email");     emailfield.onfocus = function() {     if (emailfield.value == "your email") {         emailfield.value = "";     } };      emailfield.onblur = function() {     if (emailfield.value == "") {         emailfield.value = "your email";     } };  }   window.onload = function() {     // preps , ensures      // other js functions don't     // called before document has     // loaded.     prepareeventhandlers(); // named function call nested inside anonymous function. }    //sometimes want js run later or call // function in 60 seconds or every 5 sec, etc. // 2 main methods timers: settimeout , setinterval // these timer functions in milliseconds  var myimage = document.getelementbyid("mainimage"); var imagearray = ["images/blue-roses.jpg", "images/purple-rose.jpg", "images/white-rose.jpg", "images/orange-rose.jpg", "images/pink-roses.jpg", "images/red-roses.jpg", "images/yellow-roses.jpg", "images/murdock.jpg", "images/dorothy-red-ruby-slippers.jpg"]; var imageindex = 0;  function changeimage(){     myimage.setattribute("src",imagearray[imageindex]);     imageindex++;     if (imageindex >= imagearray.length) {         imageindex = 0;     } } setinterval(changeimage, 5000);  //sometimes may want random alert // pop x-number of seconds later. //so use settimeout, so: /*function simplemessage() {     alert("get ready learn!"); } settimeout(simplemessage, 5000); */  /*var_dump($_post);   if var_dump($_post) = "";   return var($_get);    error_log($_post); */ 

if it's giving error, means document.getelementbyid("email") evaluates null, means no element exists id email.

that's can tell without seeing html js connected to.