for each element type in body

Solution 1:

var elements = document.getElementsByTagName('div');

for (var i = 0; i < elements.length; i++) {
    elements[i].innerHTML = "foo";
}

Live DEMO

If you want to look only in the <body>:

var elements = document.body.getElementsByTagName('div');

for (var i = 0; i < elements.length; i++) {
    elements[i].innerHTML = "foo";
}

NodeList.forEach Chrome icon Firefox icon Edge icon Opera icon Safari icon ¹⁰

querySelectorAll returns a static non-live NodeList exposing a forEach method:

const elements = document.querySelectorAll('div');

elements.forEach( el => {
  el.innerHTML = "foo";
});

Document.querySelectorAll^MDN
NodeList.prototype.forEach()^MDN

Array.from Chrome icon Firefox icon Edge icon Opera icon Safari icon ⁹

const elements = document.querySelectorAll('div');

Array.from(elements).forEach( (el) => {
  el.innerHTML = "foo";
});

Array.from()^MDN

Array Destructuring Chrome icon Firefox icon Edge icon Opera icon Safari icon ⁹

const elements = document.querySelectorAll('div');

[...elements].forEach( el => {
  el.innerHTML = "foo";
});

Destructuring assignment^MDN
Document.querySelectorAll()^MDN

Array forEach.call Chrome icon Firefox icon Edge icon IExplor icon ⁹ Opera icon Safari icon

var elements = document.querySelectorAll('div');

[].forEach.call(elements, function( el ) {
  el.innerHTML = "foo";
});

Array.prototype.forEach()^MDN

For loop

var elements = document.getElementsByTagName('div');

for (var i=0; i<elements.length; i++) {
  elements[i].innerHTML = "foo";
}

Element.getElementsByTagName()^MDN