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, see: https://www.npmjs.com/package/cross-env  With this package installed, commands in the scripts section in package.json can set environment variables just as in Bash on Linux/macOS.  The usage looks like so:

"scripts": { 
    "start": "cross-env DEBUG=fibonacci:* node ./bin/www" 
}, 
"dependencies": {
    ...
    "cross-env": "5.1.x"
}

Then the command is executed as so:

C:\Users\david\Documents\chap04\fibonacci>npm install
... output from installing packages
C:\Users\david\Documents\chap04\fibonacci>npm run start

> fibonacci@0.0.0 start C:\Users\david\Documents\chap04\fibonacci
> cross-env DEBUG=fibonacci:* node ./bin/www

fibonacci:server Listening on port 3000 +0ms
GET / 304 90.597 ms - -
GET /stylesheets/style.css 304 14.480 ms - -
GET /fibonacci 200 84.726 ms - 503
GET /stylesheets/style.css 304 4.465 ms - -
GET /fibonacci?fibonum=22 500 1069.049 ms - 327
GET /stylesheets/style.css 304 2.601 ms - -