Fetch API to force download file

Question

I'm calling an API to download excel file from the server using the fetch API but it didn't force the browser to download, below is my header response:

HTTP/1.1 200 OK Content-Length: 168667 
Content-Type: application/vnd.openxmlformats-officedocument.spreadsheetml.sheet 
Server: Microsoft-IIS/8.5 
Content-Disposition: attachment; filename=test.xlsx 
Access-Control-Allow-Origin: http://localhost:9000 
Access-Control-Allow-Credentials: true 
Access-Control-Request-Method: POST,GET,PUT,DELETE,OPTIONS 
Access-Control-Allow-Headers: X-Requested-With,Accept,Content-Type,Origin 
Persistent-Auth: true 
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Date: Wed, 24 May 2017 20:18:04 GMT

Below my code that I'm using to call the API :

this.httpClient.fetch(url, {
    method: 'POST',
    body: JSON.stringify(object),
    headers: {
        'Accept': 'application/json',
        'Content-Type': 'application/json'
    }
})

Answer 1

The browser won't show the usual interaction for the download (display Save As... dialog, etc.), only if you navigate to that resource. It is easier to show the difference with an example:

1. window.location='http://mycompany.com/'
2. Load http://mycompany.com/ via XHR/Fetch in the background.

In 1., the browser will load the page and display its content. In 2., the browser will load the raw data and return it to you, but you have to display it yourself.

You have to do something similar with files. You have the raw data, but you have to "display" it yourself. To do this, you need to create an object-URL for your downloaded file and navigate to it:

this.httpClient
    .fetch(url, {method, body, headers})
    .then(response => response.blob())
    .then(blob => URL.createObjectURL(blob))
    .then(url => {
        window.open(url, '_blank');
        URL.revokeObjectURL(url);
    });

This fetches the response, reads it as a blob, creates an objectURL, opens it (in a new tab), then revokes the URL.

More about object-URLs: https://developer.mozilla.org/en-US/docs/Web/API/URL/createObjectURL