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HTML

              
                <div class="container p-5">
  <h2 class="mb-5 text-center">File Upload with JavaScript</h2>
  
  <label class="text-center mb-5 image-preview-wrapper" for="file-uploader">
    <img src="https://via.placeholder.com/300x300/efefef?text=Avatar" alt="image-placehoder" class="img-thumbnail" data-target="image-preview">
    <div class="spinner-wrapper position-absolute" data-target="spinner">
      <div class="spinner-border text-secondary" role="status">
        <span class="sr-only">Loading...</span>
      </div>
    </div>
  </label>
  
  <div class="row">
    <div class="col-12 col-md-6 mx-auto">
      <div class="custom-file">
        <input type="file" class="custom-file-input" data-target="file-uploader" id="file-uploader">
        <label class="custom-file-label" for="customFile">Choose file</label>
      </div>
    </div>
  </div>
</div>
              
            
!

CSS

              
                .image-preview-wrapper{
  display: block;
  max-width: 310px;
  max-height: 310px;
  width: 100%;
  border: 2px solid #cccccc;
  margin: 0 auto;
  position: relative;
  cursor: pointer;
}

.spinner-wrapper {
  opacity: 0;
  margin: 0;
  padding: 0;
  left: 50%;
  top: 50%;
  transform: translate(-50%, -50%);
}

.opacity-1 {
  opacity: 1;
}
              
            
!

JS

              
                // STEP 1: select element and register change event
const imagePreview = document.querySelector('[data-target="image-preview"]');
const spinner = document.querySelector('[data-target="spinner"]');
const fileUploader = document.querySelector('[data-target="file-uploader"]');
fileUploader.addEventListener("change", handleFileUpload);

async function handleFileUpload(e) {
  try {
    const file = e.target.files[0];
    setUploading(true);
    if (!file) return;

    const beforeUploadCheck = await beforeUpload(file);
    if (!beforeUploadCheck.isValid) throw beforeUploadCheck.errorMessages;

    const arrayBuffer = await getArrayBuffer(file);
    const response = await uploadFileAJAX(arrayBuffer);
    
    alert("File Uploaded Success");
    showPreviewImage(file);
  } catch (error) {
    alert(error);
    console.log("Catch Error: ", error);
  } finally {
    e.target.value = '';  // reset input file
    setUploading(false);
  }
}

// STEP 2: showPreviewImage with createObjectURL
// If you prefer Base64 image, use "FileReader.readAsDataURL"
function showPreviewImage(fileObj) {
  const image = URL.createObjectURL(fileObj);
  imagePreview.src = image;
}

// STEP 3: change file object into ArrayBuffer
function getArrayBuffer(fileObj) {
  return new Promise((resolve, reject) => {
    const reader = new FileReader();
    // Get ArrayBuffer when FileReader on load
    reader.addEventListener("load", () => {
      resolve(reader.result);
    });

    // Get Error when FileReader on error
    reader.addEventListener("error", () => {
      reject("error occurred in getArrayBuffer");
    });

    // read the blob object as ArrayBuffer
    // if you nedd Base64, use reader.readAsDataURL
    reader.readAsArrayBuffer(fileObj);
  });
}

// STEP 4: upload file throguth AJAX
// - use "new Uint8Array()"" to change ArrayBuffer into TypedArray
// - TypedArray is not a truely Array,
//   use "Array.from()" to change it into Array
function uploadFileAJAX(arrayBuffer) {
  // correct it to your own API endpoint
  return fetch("https://jsonplaceholder.typicode.com/posts/", {
    headers: {
      version: 1,
      "content-type": "application/json"
    },
    method: "POST",
    body: JSON.stringify({
      imageId: 1,
      icon: Array.from(new Uint8Array(arrayBuffer))
    })
  })
    .then(res => {
      if (!res.ok) {
        throw res.statusText;
      }
      return res.json();
    })
    .then(data => data)
    .catch(err => console.log("err", err));
}

// STEP 5: Create before upload checker if needed
function beforeUpload(fileObject) {
  return new Promise(resolve => {
    const validFileTypes = ["image/jpeg", "image/png"];
    const isValidFileType = validFileTypes.includes(fileObject.type);
    let errorMessages = [];

    if (!isValidFileType) {
      errorMessages.push("You can only upload JPG or PNG file!");
    }

    const isValidFileSize = fileObject.size / 1024 / 1024 < 2;
    if (!isValidFileSize) {
      errorMessages.push("Image must smaller than 2MB!");
    }

    resolve({
      isValid: isValidFileType && isValidFileSize,
      errorMessages: errorMessages.join("\n")
    });
  });
}

function setUploading(isUploading) {
  if (isUploading === true) {
    spinner.classList.add("opacity-1");
  } else {
    spinner.classList.remove("opacity-1");
  }
}

              
            
!
999px

Console