Javascript Uncaught TypeError: Cannot set property of undefined

Question

I have a problem with my code in JavaScript, probably some stupid mistake but I can't find it... an error occurs.

Uncaught TypeError: Cannot set property 'innerHTML' of undefined

Even though the code seems to do what it should (creates tasks which have a header of Task 1, Task 2, Task 3 and so on).. How can I fix that? Maybe there is a shorter way to give my tasks numbered headers?

document.addEventListener("DOMContentLoaded", function(){

  var add=document.querySelector("#addTaskButton");
  var tasklist=document.querySelector("#taskList");
  var clickcount=0;

  add.addEventListener("click",function(event){
   var newTask=document.createElement("li");
   tasklist.appendChild(newTask);
   var newh1=document.createElement("h1");
   newTask.appendChild(newh1);

  clickcount+=1;
   if(clickcount==1){
     var h1=document.querySelector("h1");
     h1.innerText="Task 1";
   }else if(clickcount>1){
     var hmore=document.getElementsByTagName("h1");
     for(var i=1;i<=hmore.length;i++){
       hmore[i].innerHTML="Task "+(i+1);
      }
   }

 })

});

I think it should be for(var i=0;i<hmore.length;i++){ rather than for(var i=1;i<=hmore.length;i++){ — Olivier Boissé
– Olivier Boissé, Commented Aug 11, 2016 at 12:27

Quentin · Accepted Answer · 2016-08-11 12:30:50Z

2

for(var i=1;i<=hmore.length;i++){

Array-like objects in JavaScript are zero indexed.

The length is the number of items in the object.

This means that if you have an array with 3 items in it, they exist at indexes 0, 1, and 2.

So when i == hmore.length you have gone one beyond the end.

You need to test for < not <=

answered Aug 11, 2016 at 12:30

Quentin

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