How to get child element by class name?

Use doc.childNodes to iterate through each span, and then filter the one whose className equals 4:

var doc = document.getElementById("test");
var notes = null;
for (var i = 0; i < doc.childNodes.length; i++) {
    if (doc.childNodes[i].className == "4") {
      notes = doc.childNodes[i];
      break;
    }        
}

​

Use querySelector and querySelectorAll

var testContainer = document.querySelector('#test');
var fourChildNode = testContainer.querySelector('.four');

IE9 and upper

The accepted answer only checks immediate children. Often times we're looking for any descendants with that class name.

Also, sometimes we want any child that contains a className.

For example: <div class="img square"></div> should match a search on className "img", even though it's exact className is not "img".

Here's a solution for both of these issues:

Find the first instance of a descendant element with the class className

   function findFirstChildByClass(element, className) {
        var foundElement = null, found;
        function recurse(element, className, found) {
            for (var i = 0; i < element.childNodes.length && !found; i++) {
                var el = element.childNodes[i];
                var classes = el.className != undefined? el.className.split(" ") : [];
                for (var j = 0, jl = classes.length; j < jl; j++) {
                    if (classes[j] == className) {
                        found = true;
                        foundElement = element.childNodes[i];
                        break;
                    }
                }
                if(found)
                    break;
                recurse(element.childNodes[i], className, found);
            }
        }
        recurse(element, className, false);
        return foundElement;
    }