document.getElementsByClassName doesn't work

Question

function getHeight(element) {
    console.log(element);
    var offsetHeight = document.getElementsByClassName(element).offsetHeight;
    console.log(offsetHeight);
         


        

Answer 1

according to documentation https://developer.mozilla.org/en-US/docs/Web/API/document.getElementsByClassName

document.getElementsByClassName(classNames)

returns a collection of html elements (actually instance of HTMLCollection) So If you run this code it should work

    var collection = document.getElementsByClassName(classNames),
      offset = -1;

    if (collection.length > 0) {
      offset = collection[0].offsetHeight;
    }

    return offset;

Answer 2

document.getElementsByClassName(element) returns a HTMLCollection of elements. The collection doesn' t have an offsetHeight property, each element in the collection has.

Now if you want to get the offsetHeight of the first matched element you can simply use
document.getElementsByClassName(element)[0].offsetHeight

If you wanted the max or min offsetHeight of all the matched elements you'd need to iterate over the collection.