Allow vue.config.js to export a function

[jkzing]

What problem does this feature solve?

Currently vue.config.js only allowed to export an object containing options.

When user want to have more complicated configuration, they have to do:

const baseConfig = {
  // basic configurations
}

if (something) {
  module.exports = {
    ...baseConfig,
    // specific configurations
    baseUrl: '/foo/'
  }
} else {
  module.exports = {
    ...baseConfig,
    // specific configurations
    baseUrl: '/bar/'
  }
}

Moreover, we have received several issues asking about how to build with different configurations. (Like this one: #2861) Although this can be done with current api, I thinks it's not intutive enough so that people with limited nodejs knowledge couldn't figure it out.

What does the proposed API look like?

Like what babel.config.js did, let user use function to generate configuration:

module.exports = function () {
  const baseConfig = {
    // basic configurations
  }
  if (something) {
    return {
      ...baseConfig,
      // specific configurations
      baseUrl: '/foo/'
    }
  } else {
    return {
      ...baseConfig,
      // specific configurations
      baseUrl: '/foo/'
    }
  }
}

Further more, we can pass some useful information/api as arguments into the configuration function, for example:

module.exports = function (options, api) {
  const {
    args, // command line arguments parsed with cli
    env // project default ENV
    // any other ones ?
  } = options

  // like chainWebpack and configureWebpack inside PluginAPI
  api.chainWebpack(fn)
  api.configureWebpack(config)

  // this could solve the problem met in:
  // https://github.com/vuejs/vue-cli/issues/2506
  // https://github.com/vuejs/vue-cli/pull/3135
  api.loadEnvDir(pathToDir)
  api.loadEnvFile(pathToFile)
  // then user can use ENV variables below
}

[haoqunjiang]

While I appreciate this idea very much, I think the API should be minimal as compared to the proposed ones in the OP.

We already have a project local plugin mechanism, so it seems redundant to replicate this logic in the config file.
Also, vue.config.js is a higher abstraction layer over webpack, we should distinguish it from plain webpack config IMHO.
As for the loadEnv* API, it's against out guideline as stated by Evan: #2506 (comment)

So I think we can divide this proposal into 2 PRs: one for support of functional export, one for the minimal API interface (yet to be designed and discussed).

[jkzing]

@sodatea , nice to see progress on this FR, support for functional export would be a good start!