reconsider the current logic related to `babel.config.js`

[justinhelmer]

The change from this commit 0283ef9 introduced the concept of babel.config.js. However, the current logic assumes the file exports a configuration object.

The Babel Configuration Docs are now promoting the use of babel.config.js for project-wide configuration - but the format of the file is to export a function.

I submitted another PR (#111) to help provide support for this new config file type (currently blocked by #108), and it should not conflict with the current logic. However, there may be unintended consequences of the current implementation if projects use the configFile or root options to load their project-wide configuration. In this scenario, babel will be assuming that babel.config.js exports a function, which could be seen as inconsistent with the current logic.

[eddyerburgh]

This is fixed by #127, to be released in v4. We'll use babel-jest to resolve babel files.

Before v4 is released, I'll update vue-jest to export a createTransformer method that gives users a way to customize babel-jest.

[Globegitter]

I am just trying out the latest v4 beta and for me the option:

  globals: {
    'vue-jest': {
      transform: {
        'js': './custom/babel-jest.js',
      },
    },
  },

is working well. As we already have a custom babel-jest to find the babel config at our custom location, so we can just reuse that.