Allow passing custom env instead of process.env

[ecraig12345]

Mostly for testing purposes, it would be nice if ParseOptions included an env override, to be used instead of process.env. I know there are various mocking approaches that could work, but it would be simpler in my case to just pass through the desired value to parse().

If this sounds like a reasonable feature, I'd be happy to implement it.

[shadowspawn]

Commander does env tests low-tech, just add and delete the environment variable in the test. (There is an implicit assumption the testing harness does not run tests in the same process at the same time.)

https://github.com/tj/commander.js/blob/master/tests/options.env.test.js

[shadowspawn]

I don't think there is enough value to add custom environment variable override/source. However, I'll leave the issue open for a while if you want a chance for other people to show interest.

The only previous issue I can find that overlaps is #1762

[Errr0rr404]

Opened #2558 adding parseOptions.env so .env() options can use a custom env bag in tests.

[Errr0rr404]

Closed the premature PR (#2558). Agreed with the earlier assessment that mutating process.env in tests is enough for most cases; no need to expand the parse API without clearer demand.

[ecraig12345]

I agree it's possible to just overwrite/restore process.env in tests, but I think it would be cleaner to be able to explicitly pass it in. That pattern seems common in other libraries and (like in that PR) would be simple to add to ParseOptions.

[shadowspawn]

Clarification: ParseOptions mention in this issue refers to the optional extra parameter passed to parse() and parseAsync(). (Not to the low level .parseOptions() routine.)

That pattern seems common in other libraries

What other libraries? (I have not noticed it in other CLI parsing libraries, but might be common in some other areas.)

[shadowspawn]

There was a similar request for yargs to support injecting env, but it only got a couple of responses in eight years.

• Allow to inject environment yargs/yargs#1202

[shadowspawn]

Have a new example in our own tests where it would be a convenience: #2608