Jer Jer - 7 months ago 43
Javascript Question

Why does my call to forEach on an array result in "Uncaught TypeError: undefined is not a function"

Fiddle

In the above fiddle, 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:

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


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;
}

Answer

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=>{
   ...