# [File Format Verification] With Elixir

 
 

%[https://www.youtube.com/watch?v=7gfzNfI9NAg]

 
 


Let's handle file name extensions with Elixir, useful to validate the type of format of files that we would like to allow.

### Step 1:

Let's create a new Elixir project by going to the command line and typing the following in your desired directory:

`$ mix new file_validator && cd file_validator`

### Step 2:

Open your test file `test/file_validator_test.exs`, remove the autogenerated test and add the following test:


```elixir
  test "file validation should be false" do
    file = "somefile.exe"
    assert FileValidator.valid?(file) == false
  end

  test "file validation should be true" do
    file = "somefile.png"
    assert FileValidator.valid?(file) == true
  end
```

Go to the command line and run the test `mix test`

### Step 3:

Let's make our test pass by opening our main project file `lib/file_validator.ex`, remove the autogenerated hello world function and add the following:

```elixir
  @extension_whitelist ~w(.jpg .jpeg .gif .png .pdf)

  def valid?(file) do
    file_extension = get_ext(file)

    @extension_whitelist
    |> Enum.member?(file_extension)
  end

  defp get_ext(file) do
    file
    |> Path.extname()
    |> String.downcase()
  end
```

Get back to the command line and run the test `mix test` now our test should pass with no failures.

### Code Breakdown

#### @extension_whitelist ~w(.jpg .jpeg .gif .png .pdf)

The `@` symbol means that is a module attribute, that can be used as a constant throughout our module.

The ~w sigil is used to generate lists of words(words are just regular strings). Inside the ~w sigil, words are separated by whitespace.

#### Path.extname(path)

Returns the extension of the last component of the path of a string.

#### Enum.member?(list, element)

Checks if an element exists within the list.

## Conclusion

That's a pretty simple and straightforward way to validate a file format, by getting the extension from the string with `Path.extname(path)` and passing it to `Enum.member?(list, element)` along with a whitelisted list of allowed file extensions. Thank you so much for your time, I really appreciate it.

 
 


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