And for anyone who isn’t already aware (like me until relatively recently), this means you can set the environment on each mix command.

Unix-like:

MIX_ENV=prod mix {task}

Windows:

set "MIX_ENV=prod" && mix {task}

https://github.com/elixir-lang/elixir/blob/50293b46f13a86328f0ffabdcbb8592e29ac24c6/lib/mix/lib/mix/state.ex#L11

it gets set here. It’s a compile-time variable, gets fetched fron shell environment, and default value is “dev”.

A related concept: you may have noticed that when you run mix test, it assumes MIX_ENV=test, but most tasks assume MIX_ENV=dev unless otherwise specified.

To set the default environment for a Miix task (eg, a task to run JS tests) or task alias, you can use :preferred_cli_env. For example:

defmodule MyProject.Mixfile do
  use Mix.Project

  def project do
    [
      # ...
      aliases: aliases(),
      preferred_cli_env: [
        # note that all task names are atoms
        test: :test,
        jstest: :test,
        "test.all": :test
      ]
    ]
  end

  # ...

  defp aliases do
    [
      "test.all": ["test", "jstest"]
    ]
  end
end