JS: iterating over result of getElementsByClassName using Array.forEach

No. As specified in DOM4, it's an HTMLCollection (in modern browsers, at least. Older browsers returned a NodeList).

In all modern browsers (pretty much anything other IE <= 8), you can call Array's forEach method, passing it the list of elements (be it HTMLCollection or NodeList) as the this value:

var els = document.getElementsByClassName("myclass");

Array.prototype.forEach.call(els, function(el) {
    // Do stuff here
    console.log(el.tagName);
});

// Or
[].forEach.call(els, function (el) {...});

If you're in the happy position of being able to use ES6 (i.e. you can safely ignore Internet Explorer or you're using an ES5 transpiler), you can use Array.from:

Array.from(els).forEach((el) => {
    // Do stuff here
    console.log(el.tagName);
});

You can use Array.from to convert collection to array, which is way cleaner than Array.prototype.forEach.call:

Array.from(document.getElementsByClassName("myclass")).forEach(
    function(element, index, array) {
        // do stuff
    }
);

In older browsers which don't support Array.from, you need to use something like Babel.

---

ES6 also adds this syntax:

[...document.getElementsByClassName("myclass")].forEach(
    (element, index, array) => {
        // do stuff
    }
);

Rest destructuring with ... works on all array-like objects, not only arrays themselves, then good old array syntax is used to construct an array from the values.

---

While the alternative function querySelectorAll (which kinda makes getElementsByClassName obsolete) returns a collection which does have forEach natively, other methods like map or filter are missing, so this syntax is still useful:

[...document.querySelectorAll(".myclass")].map(
    (element, index, array) => {
        // do stuff
    }
);

[...document.querySelectorAll(".myclass")].map(element => element.innerHTML);

Or you can use querySelectorAll which returns NodeList:

document.querySelectorAll('.myclass').forEach(...)

Supported by modern browsers (including Edge, but not IE):
Can I use querySelectorAll
NodeList.prototype.forEach()

MDN: Document.querySelectorAll()