app/javascript/custom/image_upload.js

// Prevent uploading of big images.
document.addEventListener("change", function(event) {
  let image_upload = document.querySelector("#micropost_image");
  const size_in_megabytes = image_upload.files[0].size/1024/1024;
  if (size_in_megabytes > 5) {
    alert("Maximum file size is 5MB. Please choose a smaller file.");
    image_upload.value = "";
  }
});