.envrc files are consumed by nix-direnv, and will install software and environment variables defined in the shell.nix in the nodejs/node repository.

To add an image variant, add a new file starting with use nix with the flags such as:

• --impure: to make sure user can still access non-Nix software.
• -I nixpkgs=/home/developer/nodejs/node/tools/nix/pkgs.nix: that defines which version of the NixOS/nixpkgs repository it should load.
• --arg <key> <nix-value>: Override the default values defines in the shell.nix with some custom value. E.g., pass --arg extraConfigFlags '["--enable-asan" "--enable-ubsan"]' to enable ASAN and UBSAN.
• --argstr <key> <string-value>: Useful to avoid the double quoting, e.g. instead of --arg icu '"small"', it is equivalent to pass --argstr icu small.

It is possible to add custom environment variables, or to override Nix-defined ones by adding export NAME_OF_THE_VAR=value lines at the end of the file, however it is preferred to keep all environment variable definitions in one place.