addEventListener在简单的for循环中工作，但在for-in循环中不工作

Question

When I use simple for loop, addEventListener works well in for loop.

But when I use for-in loop, it makes error like

Uncaught TypeError: checklist[i].addEventListener is not a function

This is my work-well code.

var checklist = document.querySelectorAll(".checklist");
for (var i = 0, len = checklist.length; i < len; i += 1) {
  checklist[i].addEventListener('change', function (event) {
    alert('test');
  });
}

This is my Error code.

var checklist = document.querySelectorAll(".checklist");
for (var i in checklist) {
  checklist[i].addEventListener('change', function (event) {
    alert('test');
  });
}

I don't know what is difference between two codes. Please Help me. Thanks!

Answer 1

The problem is that for-in loop iterates over all enumerable properties of an array or object. So, if you log your variable in console you'll see that along with the indexes of the elements you also get other properties like length , keys , values of the array and checklist[length] or checklist[keys] are not DOM elements. So you can't add an event listener to them.