Get elements by attribute when querySelectorAll is not available without using libraries?

You could write a function that runs getElementsByTagName('*'), and returns only those elements with a "data-foo" attribute:

function getAllElementsWithAttribute(attribute)
{
  var matchingElements = [];
  var allElements = document.getElementsByTagName('*');
  for (var i = 0, n = allElements.length; i < n; i++)
  {
    if (allElements[i].getAttribute(attribute) !== null)
    {
      // Element exists with attribute. Add to array.
      matchingElements.push(allElements[i]);
    }
  }
  return matchingElements;
}

Then,

getAllElementsWithAttribute('data-foo');

Use

//find first element with "someAttr" attribute
document.querySelector('[someAttr]')

or

//find all elements with "someAttr" attribute
document.querySelectorAll('[someAttr]') 

to find elements by attribute. It's now supported in all relevant browsers (even IE8): http://caniuse.com/#search=queryselector

I played a bit around and ended up with this crude solution:

function getElementsByAttribute(attribute, context) {
  var nodeList = (context || document).getElementsByTagName('*');
  var nodeArray = [];
  var iterator = 0;
  var node = null;

  while (node = nodeList[iterator++]) {
    if (node.hasAttribute(attribute)) nodeArray.push(node);
  }

  return nodeArray;
}

The usage is quite simple, and works even in IE8:

getElementsByAttribute('data-foo');
// or with parentNode
getElementsByAttribute('data-foo', document);

http://fiddle.jshell.net/9xaxf6jr/

But I recommend to use querySelector / All for this (and to support older browsers use a polyfill):

document.querySelectorAll('[data-foo]');