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]