In webpack, how can I have different output directories for multiple entry points?

A bit hacky, but this should do the trick.

module.exports = {
 entry: {
  "somePage": "./scripts/someDir/somePage.js",
  "someSubDir/anotherPage": "./scripts/someDir/someSubDir/anotherPage.js"
 },
 output: {
   path: path.resolve(__dirname, 'out/someDir'),
   filename: '[name].js'
 },
 // Etc.
}

You cannot set the path to a function, webpack won't call it for you.

You can return an array of configurations for webpack to execute. I think that will give you enough control over the output path to achieve what you need.