rsnext/packages/next/build/compiler.ts

import webpack, {Stats} from 'webpack'

export type CompilerResult = {
  errors: Error[]
  warnings: Error[]
}

function generateStats(result: CompilerResult, stat: Stats): CompilerResult {
  const { errors, warnings } = stat.toJson({
    all: false,
    warnings: true,
    errors: true,
  })
  if (errors.length > 0) {
    result.errors.push(...errors)
  }

  if (warnings.length > 0) {
    result.warnings.push(...warnings)
  }

  return result
}

export function runCompiler(
  config: webpack.Configuration | webpack.Configuration[]
): Promise<CompilerResult> {
  return new Promise(async (resolve, reject) => {
    // @ts-ignore webpack allows both a single config or array of configs
    const compiler = webpack(config)
    compiler.run((err: Error, statsOrMultiStats: any) => {
      if (err) {
        return reject(err)
      }

      if(statsOrMultiStats.stats) {
        const result: CompilerResult = statsOrMultiStats.stats.reduce(
          generateStats,
          { errors: [], warnings: [] }
        )
        return resolve(result)
      }

      const result = generateStats({ errors: [], warnings: [] }, statsOrMultiStats)
      return resolve(result)
    })
  })
}