How to set environment variables?

[saeta-eth]

Hi, I'm having troubles when I want to set environment variables.
I think that the best way is using diferent tasks on the script field inpackage.json to add this values but I don't know.

Do you know how I can create this task to get different values depends of the environment?

Thanks.

[developit]

"scripts": {
  "start": "FOO=bar node ."
}

or:

FOO=bar npm start