Get all the button tag types

As the answer from @jorgesys points out, use:

document.getElementsByTagName("button");

That returns an HTMLCollection instance. In modern JavaScript you can convert to an array and use map:

Array.from(document.getElementsByTagName("button"))
  .map(b => b.getAttribute('type'))

That will return an array like ["submit", "submit", null, null, ...]"

I recently needed to disable all buttons, which could be done using forEach:

Array.from(document.getElementsByTagName("button"))
  .forEach(b => b.disabled = true)

Or to avoid the temporary array (which probably isn't something you should worry about), you could create this:

function forEachButton(func) {
    let elements = document.getElementsByTagName("button");
    for(let i = 0, len = elements.length; i < len; i++) {
        func(elements[i])
    }
}

Then disable all buttons with:

forEachButton(b => b.disabled = true)

var buttons = document.querySelectorAll(".classname").length;
for (var i = 0; i < buttons; i++) {
  document.querySelectorAll(".classname")[i].addEventListener("click", function () {
    alert("clicked!");
  });
}