Download File with Go

Before we get started, please ensure you already understand the basic of REST API in GO
Let’s see the final result of this tutorial

Result

Basically when the user is click on the button, it will then trigger the javascript function downloadFile() to make http request to the backend server in order to download the file.

Go

func DownloadFile(w http.ResponseWriter, r *http.Request) {
	Openfile, err := os.Open("C://Users/User/Desktop/Download/Download.xlsx") //Open the file to be downloaded later
	defer Openfile.Close() //Close after function return

	if err != nil {		
		http.Error(w, "File not found.", 404) //return 404 if file is not found
		return
	}

	tempBuffer := make([]byte, 512) //Create a byte array to read the file later
	Openfile.Read(tempBuffer) //Read the file into  byte
	FileContentType := http.DetectContentType(tempBuffer) //Get file header

	FileStat, _ := Openfile.Stat() //Get info from file
	FileSize := strconv.FormatInt(FileStat.Size(), 10) //Get file size as a string

	Filename := "demo_download"

	//Set the headers
	w.Header().Set("Content-Type", FileContentType+";"+Filename)
	w.Header().Set("Content-Length", FileSize)

	Openfile.Seek(0, 0) //We read 512 bytes from the file already so we reset the offset back to 0
	io.Copy(w, Openfile) //'Copy' the file to the client
}

Explanation

Open the file from it’s directory.
Create a Byte variable and Read the file data
Get the content type based on the file data by using http.DetectContentType(< byte >)
Get the file infomation such as size under the method *File.Openfile.Stat()
Define the file name and set into the Content-Type Header together with it’s content type
Set Content-Length Header based on the file size
Copy and response the file back to the client size by using io.Copy(< HTTP Response >, < file >)

Javascript

function downloadFile() {
    let url = "http://localhost:3000/downloadFile"
    let options = {
        method: 'GET',
        headers: new Headers({
            'Content-Type': 'application/json',
        }),
        mode: 'cors',
        cache: 'default'
    };
    let strMimeType;
    let strFileName;

    //Perform a GET Request to server
    fetch(url, options)
    .then(function (response) {
        let contentType = response.headers.get("Content-Type"); //Get File name from content type
        strMimeType = "application/vnd.openxmlformats-officedocument.spreadsheetml.sheet";
        strFileName = contentType.split(";")[1];
        return response.blob();

    }).then(function (myBlob) {
        let downloadLink = window.document.createElement('a');
        downloadLink.href = window.URL.createObjectURL(new Blob([myBlob], { type: strMimeType }));
        downloadLink.download = strFileName;
        document.body.appendChild(downloadLink);
        downloadLink.click();
        document.body.removeChild(downloadLink);
    }).catch((e) => {
        console.log("e", e);
    });
}

Explanation

Make a GET request to the server
Get the file info like File name from the Reponse Content-Type Header
Define the MIME type of the file. Please refer here to get the common MIME type
Return the response in BLOB object
Create a URL.createObjectURL() inside the window and start downloading by firing the anchor link with the method .click()