How to download single file with multiple chunks in javascript using fetch API

user_12912755 0
Viewed 2734

I am using fetch API in my react app to download a file from a URL. Each URL provides one chunk as the response. When the file has only one chunk, i can use the response.blob() to download the file. However if there are multiple chunks for a single file,i have to fetch the chunks from n different urls, say url1,url2,..so on. How can I combine their responses to generate a single blob and download the file.

Below is the code to download the file with only one chunk(fetched from url-1).

let downloadUrl = `https://url-1`;
      fetch(downloadUrl, {
        method: 'GET'
      })
        .then(
          (res: any) => {
            if (res.ok && res.status === 200) {
              //File response loaded
            } else {
              return null;
            }
            return res.blob();
          },
          error => {
            // error in downloading
          }
        )
        .then(fileData => {
          if (fileData) {
            let url = window.URL.createObjectURL(fileData);
            let a = document.createElement('a');
            a.href = url;
            a.download = file.fileName;
            a.click();
          }
        });

I can make multiple requests in the fetch API and use Promise.all(responseArr). But how to combine their responses into single blob response ?

reactjs fetch blob download javascript
2 Answers

You could get the individual chunks with fetch() and then get each response as an array buffer with .arrayBuffer().

Then you can use the Blob(array, options) constructor to create a new blob consisting of all the arraybuffers you just downloaded.

Example:

// fetch chunks from somewhere
let urls = ["http://foo.bar/chunk/1", "http://foo.bar/chunk/2"];
let chunkPromises = urls.map(url => {
  return fetch(url).then(res => {
    return res.arrayBuffer();
  });
});

Promise.all(chunkPromises).then(chunks => {
  // combine chunks into a single blob
  let blob = new Blob(chunks, {
    // Optional: specify the MIME Type of your file here
    type: "text/plain" 
  });

  // download the blob
  let url = window.URL.createObjectURL(blob);
  let a = document.createElement('a');
  a.href = url;
  a.download = "Filename.txt";
  a.click();
});
user_84114060

I would loop through the URLs to download the chunks in order and combine them as they download

try{
    var urls = ['https://url-1','https://url-2.0','https://url-3.1415']
    var fullFile = new Blob();
    for (let x = 0; x < urls.length; x++){
        await fetch(urls[x]).then((res) => {
            if (!res.ok || res.status !== 200) {
                throw('Download failed');
            }
            return res.blob();
        }).then(data => fullFile = new Blob([fullFile, data]));
    }
    let url = window.URL.createObjectURL(fullFile);
    let a = document.createElement('a');
    a.href = url;
    a.download = file.fileName;
    a.click();
} 
catch(e){console.log(e)};
user_13530110
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