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!

ⓘ I have included a zip file with all the source code at the start of this tutorial, so you don’t have to copy-paste everything… Or if you just want to dive straight in.

 

 

TLDR – QUICK SLIDES

Fullscreen Mode – Click Here

 

TABLE OF CONTENTS

 

 

QUICK NOTES

If you spot a bug, feel free to comment below. I try to answer short questions too, but it is one person versus the entire world… If you need answers urgently, please check out my list of websites to get help with programming.

 

EXAMPLE CODE DOWNLOAD

Click here to download all the example source code, I have released it under the MIT license, so feel free to build on top of it or use it in your own project.

 

 

 

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(error));
}
</script>
 
<input type="button" value="Download"
       onclick="fetchDown('orange.jpg', 'demo.jpg')">

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.

 

 

 

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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