EnvironmentPlugin: function for defaultValues

[simon04]

EnvironmentPlugin: allow defaultValues to be computed from a function.

This is useful when the computation of the defaultValue is expensive, such as CI_COMMIT_TAG requiring an invocation of git rev-parse HEAD.

What kind of change does this PR introduce?

feature

Did you add tests for your changes?

not yet

Does this PR introduce a breaking change?

no

What needs to be documented once your changes are merged?

• https://webpack.js.org/plugins/environment-plugin

[webpack-bot]

For maintainers only:

• This needs to be documented (issue in webpack/webpack.js.org will be filed when merged)
• This needs to be backported to webpack 4 (issue will be created when merged)

[sokra]

please add an test case and fill the PR template

[webpack-bot]

@simon04 Thanks for your update.

I labeled the Pull Request so reviewers will review it again.

@sokra Please review the new changes.

[webpack-bot]

Thank you for your pull request! The most important CI builds succeeded, we’ll review the pull request soon.

[sokra]

This adds complexity that's often not needed. Please use DefinePlugin.runtimeValue for your usecase instead:

https://webpack.js.org/plugins/define-plugin/#runtime-values-via-runtimevalue