Understanding how to retrieve the FileType from a Document Object Model using JavaScript

Question

Understanding how to retrieve the FileType from a Document Object Model using JavaScript

Looking at this DOM structure, we have an image with the following details:


<img id="this-is-the-image" src="http://192.168.1.100/Image_tmp/2016-06/d4eb8d">

The task at hand is to click a button, execute a JavaScript function, and download the image file.

The button implementation and the downloading script are already completed.

Below is a snippet of the code:

function downloadFile(fileName, url) {
        var aLink = document.createElement('a');
        var blob = new Blob([url]);
        var evt = document.createEvent("HTMLEvents");
        getImageType(blob);
        evt.initEvent("click", false, false);
        aLink.download = fileName;
        aLink.href = URL.createObjectURL(blob);
        aLink.dispatchEvent(evt);
    }

I've encountered an issue where I can only extract the source

"http://192.168.1.100/Image_tmp/2016-06/d4eb8d"

or the filename d4eb8d, while in reality, the image could be in the format of .png or .jpg. Although the browser displays it correctly, when saved, the file name defaults to just d4eb8d instead of including the extension like d4eb8d.png or d4eb8d.jpg. Is there a method to accurately determine the image type so that the downloaded file includes the correct extension?

javascript image download file-type

Answer 1

Answer №1

If you need to retrieve an image file using JavaScript, you can utilize the XMLHttpRequest() function while setting the .responseType property to "blob". This will allow you to extract the image type from the blob using blob.type.split("/")[1], where type represents the MIME type of the Blob object. The result after splitting would typically be either jpg, jpeg, png, or another image format.

  window.onload = function() {
    var link = document.querySelector("a");
    var filename = "image";
    var req = new XMLHttpRequest();
    req.responseType = "blob";
    req.open("GET", "http://example.com/d4eb8d");
    req.onload = function() {
      var imgBlob = this.response;
      var imgType = imgBlob.type.split("/")[1];
      console.log(imgType);
      var event = document.createEvent("HTMLEvents");
      event.initEvent("click", false, false);
      link.download = filename + "." + imgType;
      link.href = URL.createObjectURL(imgBlob); 
      link.dispatchEvent(event);
    }
    req.send()
  }

See it in action on plnkr: http://plnkr.co/edit/To4uZXL8PUph9qG3azvZ?p=preview

Answer 2

If you need to retrieve an image file using JavaScript, you can utilize the XMLHttpRequest() function while setting the .responseType property to "blob". This will allow you to extract the image type from the blob using blob.type.split("/")[1], where type represents the MIME type of the Blob object. The result after splitting would typically be either jpg, jpeg, png, or another image format.

  window.onload = function() {
    var link = document.querySelector("a");
    var filename = "image";
    var req = new XMLHttpRequest();
    req.responseType = "blob";
    req.open("GET", "http://example.com/d4eb8d");
    req.onload = function() {
      var imgBlob = this.response;
      var imgType = imgBlob.type.split("/")[1];
      console.log(imgType);
      var event = document.createEvent("HTMLEvents");
      event.initEvent("click", false, false);
      link.download = filename + "." + imgType;
      link.href = URL.createObjectURL(imgBlob); 
      link.dispatchEvent(event);
    }
    req.send()
  }

See it in action on plnkr: http://plnkr.co/edit/To4uZXL8PUph9qG3azvZ?p=preview

Answer 3

Answer №2

Can we be certain that using new Blob([url]) will result in a blob containing image data, and not a text file with the content of

http://192.168.1.100/Image_tmp/2016-06/d4eb8d

?

It appears that converting an image link to a blob can only be achieved by reading it using XHR => How to get a file or blob from an object URL?

Once you have the blob, you can determine the file type by checking blob.type. Afterward, append the appropriate extension.

var ext ="";
switch(blob.type){
   case "image/jpeg":
        ext=".jpg";
        break;
   case "image/png":
        ext=".png";
        break;
   case "image/gif":
        ext=".gif";
        break;
}
aLink.download = fileName + ext;

Answer 4

Can we be certain that using new Blob([url]) will result in a blob containing image data, and not a text file with the content of

http://192.168.1.100/Image_tmp/2016-06/d4eb8d

?

It appears that converting an image link to a blob can only be achieved by reading it using XHR => How to get a file or blob from an object URL?

Once you have the blob, you can determine the file type by checking blob.type. Afterward, append the appropriate extension.

var ext ="";
switch(blob.type){
   case "image/jpeg":
        ext=".jpg";
        break;
   case "image/png":
        ext=".png";
        break;
   case "image/gif":
        ext=".gif";
        break;
}
aLink.download = fileName + ext;

Answer 5

Answer №3

When dealing with an instance of an HTMLImageElement, it seems that there is no built-in property in the documentation to determine its image type.

One approach could be to extract the URL, initiate an AJAX request to fetch the Content-Type header, which can help identify the file extension. However, this method relies on the remote server providing such information. Alternatively, you can utilize the XMLHttpRequest object and its .getResponseHeader() method.

Another solution involves changing the window.location to the image URL but keep in mind that downloading directly may not be possible without user interaction.

Finally, creating a simple server-side script to handle file downloads and processing details can also be an effective strategy.

Answer 6

When dealing with an instance of an HTMLImageElement, it seems that there is no built-in property in the documentation to determine its image type.

One approach could be to extract the URL, initiate an AJAX request to fetch the Content-Type header, which can help identify the file extension. However, this method relies on the remote server providing such information. Alternatively, you can utilize the XMLHttpRequest object and its .getResponseHeader() method.

Another solution involves changing the window.location to the image URL but keep in mind that downloading directly may not be possible without user interaction.

Finally, creating a simple server-side script to handle file downloads and processing details can also be an effective strategy.

Understanding how to retrieve the FileType from a Document Object Model using JavaScript

Answer №1

Answer №2

Answer №3

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