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↬ How to upload files in Golang ?30 May 2023

In this blog I will be using the native Golang http package to upload files, no third party packages. We will see how to upload single and multiple files.

The HTML form

The form is simple and straight forward. The only thing to note is the enctype="multipart/form-data" attribute. This is required for uploading files.

<form action="/upload" method="POST" enctype="multipart/form-data">
  <input name="name" type="text" placeholder="name" />
  <input name="ok" type="checkbox" placeholder="ok" value="aaaaa" /> aaaaa
  <input name="ok" type="checkbox" placeholder="ok" value="bbbbbb" /> bbbbbb
  <input type="file" name="file" id="file" />
  <button type="submit">SUBMIT!</button>
</form>

Single file upload

func uploadSingleFile(w http.ResponseWriter, r *http.Request){
  file, header, _ := r.FormFile("file") // FormFile automatically called ParseMultipartForm
  defer file.Close()
 
  // create a destination file
  dst, _ := os.Create(filepath.Join("./", header.Filename))
  defer dst.Close()
 
  // upload the file to destination path
  nb_bytes, _ := io.Copy(dst, file)
 
  fmt.Println("File uploaded successfully")
}

💡Note: All of the _ underscore variables are errors, I just ignored them for the sake of simplicity. so don't do that in production.

As you noticed we get the uploaded file with the r.FormFile("input_name_here") function, it returns 3 variables, file which is the uploaded file itself, header which contains information about the file like the name, size, MIME type etc..., you can use these information to validate the file before uploading it, and finally _ which is the error if any.

Multiple files upload

First of all before you can select multiple files, you need to add the multiple attribute to the file input

func uploadMultipleFiles(w http.ResponseWriter, r *http.Request){
  // key = "file" in the form
  //first of all ParseMultipartForm must be called 
  r.ParseMultipartForm(32 << 20)
  for key, files := range r.MultipartForm.File {
    for _, file := range files {
      dst, _ := os.Create(filepath.Join("./", file.Filename))
      f, _ := file.Open()
      io.Copy(dst, f)
    }
  }
}

Just FYI, the r.MultipartForm.File is a map[string][]*multipart.FileHeader where the key is the name of the input in the form. The file variable contains the same information as the header variable in the single file upload example, (size, name, MIME type etc...)

The other fields

To get the other fields, there are two options, which at some point you will need to use both of them.

// OPTION 1: returns a string which is the first value of the key
r.FormValue("name") // string
 
// OPTION 2: returns a slice of strings of all the values of the key
r.MultipartForm.Value["name"] // map[string][]string

You can use the first option when you want to get a single value, and the second option when you want to get multiple values, in our case the checkbox can have multiple values.

So r.FormValue("name") is like doing r.MultipartForm.Value["name"][0].

That's it, I hope you enjoyed this tutorial, if you have any questions or suggestions, you can find all of this on GitHub.

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