Javascript .map() is not a function

getElementsByClassName() returns an HTMLCollection not an Array. You have to convert it into a JavaScript array first :

allImgs = Array.prototype.slice.call(allImgs);

// or
allImgs = [].slice.call(allImgs);

// or (thanks @athari)
allImgs = Array.from(allImgs);

// or (thanks @eliaz-bobadilla)
allImgs = [...allImgs]

Another option would be to use map directly:

[].map.call(allImages, function() { ... });

However, what you are doing is better achieved with Array.prototype.forEach.

By using getElementsByClassName, I get an array.

No, you don't.

You get a live HTMLCollection. This is array-like but is not an array.

Since it is sufficiently array-like, you can apply the map method from a real array.

    var text_content = [].map.call(
      document.getElementsByClassName("sdf"),
      function (currentValue, index, collection) {
        return currentValue.innerHTML;
      }
    );
    console.log(text_content);

  <p class="sdf">foo</p>
  <p class="sdf"></p>
  <p class="sdf">bar</p>
  <p class="sdf"></p>
  <p class="sdf"></p>