feat: expose loadConfigFromFile API

[antfu]

For advanced usage like https://github.com/antfu/vite-ssg to have better controls over user configs

[yyx990803]

Is resolveConfig not enough for the use case?

[antfu]

I found a workaround by asking users to have this line in their config and inject the env on the go:

Vue({
  ssr: !!process.env.VITE_SSG
})

My use case is fulfilled and I am ok with closing. However, I think there are still some limitations toward programmatic usages:

Both build() and resolveConfig() accept a inlined UserConfig for their first argument. While resolveConfig() returns ResolvedConfig which also contains Vite's internal plugins. There is no easy way to read users' raw config to have some context-aware operations without writing a custom loader for the config.

For example:

const config = await resolveConfig()

// manipulate the config, injecting plugins, etc.

await build(config) // errors on both type and runtime

By exposing loadConfigFromFile() we can have something like:

const config = await loadConfigFromFile('vite.config.js')

// manipulate the config, injecting plugins, etc.

await build(config, '') // assuming passing an empty string to disable the config file merging.

[yyx990803]

Yeah makes sense.