Using forEach on an array from getElementsByClassName results in “TypeError: undefined is not a function”

Question

In my JSFiddle, I’m simply trying to iterate over an array of elements. The array is non-empty, as the log statements prove. Yet the call to forEach gives me the (not so helpful) “Uncaught TypeError: undefined is not a function” error.

I must be doing something stupid; what am I doing wrong?

My code:

var arr = document.getElementsByClassName('myClass');
console.log(arr);
console.log(arr[0]);
arr.forEach(function(v, i, a) {
  console.log(v);
});

.myClass {
  background-color: #FF0000;
}

<div class="myClass">Hello</div>

Answer 1

That's because document.getElementsByClassName returns a HTMLCollection, not an array.

Fortunately it's an "array-like" object (which explains why it's logged as if it was an object and why you can iterate with a standard for loop), so you can do this :

[].forEach.call(document.getElementsByClassName('myClass'), function(v,i,a) {})

With ES6 (on modern browsers or with Babel), you may also use Array.from which builds arrays from array-like objects:

Array.from(document.getElementsByClassName('myClass')).forEach( v=> {})

or spread the array-like object into an array:

[...document.getElementsByClassName('myClass')].forEach( v=> {})