Download File With Javascript Fetch (Simple Example)

Welcome to a tutorial and example of how to download a file using Javascript Fetch. Want to initiate a file download using Javascript Fetch? Yes, it is possible.

That should cover the basics, but if you need more concrete examples – Read on!

 

 

TLDR – QUICK SLIDES

Fullscreen Mode – Click Here

 

TABLE OF CONTENTS

 

 

 

FETCH FILE DOWNLOAD

<script>
function fetchDown (url, saveas) {
  // (A) FETCH FILE
  fetch(url)
 
  // (B) RETURN AS BLOB
  .then(res => {
    if (res.status != 200) { throw new Error("Bad server response"); }
    return res.blob();
  })
 
  // (C) BLOB DATA
  .then(data => {
    // (C1) "FORCE DOWNLOAD"
    var url = window.URL.createObjectURL(data),
        anchor = document.createElement("a");
    anchor.href = url;
    anchor.download = saveas;
    anchor.click();
 
    // (C2) CLEAN UP
    window.URL.revokeObjectURL(url);
    document.removeChild(anchor);
  })
 
  // (D) HANDLE ERRORS - OPTIONAL
  .catch(err => console.error(err));
}
</script>
 
<input type="button" value="Download"
       onclick="fetchDown('orange.png', 'demo.png')">

This is the “full version” of the introduction snippet. This should be self-explanatory, but a quick walkthrough:

1. Captain Obvious, the fetch() request itself.
2. Return the server response as a blob object. Also, take note of the result != 200 check here. That’s right, fetch will consider it a success as long as there is a server response – Even when there are server errors such as 404 (not found) or 403 (unauthorized). So it is best to do a manual 200 (OK) check here.
3. Create a download link to the blob object, and revoke it after the download is initiated.
4. Handle errors. Optional, but highly recommended.

 

 

 

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EXAMPLE CODE DOWNLOAD

Click here for the source code on GitHub gist, just click on “download zip” or do a git clone. I have released it under the MIT license, so feel free to build on top of it or use it in your own project.

 

 

IT WORKS, BUT…

I can understand the use of fetch to download a file that is protected or requires parameters to be generated dynamically.

// DATA TO BE ATTACHED
var form = new FormData();
form.append("KEY", "VALUE");
 
// FETCH - MAYBE GENERATE A DYNAMIC REPORT
fetch(url, { method:"post", body:form })
... SAME AS ABOVE ...

But for you guys who just want to offer a download on a static file, just use the HTML anchor tag – <a href="URL" download>. Yep, fetch download is cool, but don’t waste your time doing roundabout stuff.

 

LINKS & REFERENCES

• Fetch API – MDN
• Fetch Basic Auth – Code Boxx
• Fetch POST – Code Boxx
• Fetch GET – Code Boxx

 

 

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