node_modules/webpack-cli/bin/config/webpackConfigurationSchema.json

{
  "anyOf": [
    {
      "type": "object",
      "description": "A webpack configuration object."
    },
    {
      "type": "array",
      "description": "An array of webpack configuration objects.",
      "items": {
        "description": "A webpack configuration object.",
        "type": "object"
      }
    },
    {
      "instanceof": "Promise",
      "description": "A promise that resolves with a configuration object, or an array of configuration objects."
    }
  ]
}