If you're on Windows the previous example may have failed with an error that DEBUG is not a known command. The problem is that the Windows shell, the cmd.exe program, does not support the Bash command-line structure.
Adding VARIABLE=value at the beginning of a command-line is specific to some shells, like Bash, on Linux and macOS. It sets that environment variable only for the command-line being executed, and is a very convenient way to temporarily override environment variables for a specific command.
Clearly a solution is required if your package.json is to be usable across different operating systems.
The best solution appears to be the cross-env package in the npm repository, ...
