index.js

const CLIEngine = require("eslint").CLIEngine

function createLinter (cli) {
  const fmt = cli.getFormatter()
  return (file) => {
    const report = cli.executeOnFiles([file])
    const msg = fmt(report.results)

    if (msg.length > 0) {
      console.warn(msg)
    }

    return {
      errorCount: parseInt(report.errorCount),
      warningCount: parseInt(report.warningCount)
    }
  }
}

function sumProperty(key) {
  return (count, obj) => count += obj[key]
}

function LinterError(message) {
  this.name = "LinterError"
  this.message = message || ""
}

LinterError.prototype = Object.create(Error.prototype)

module.exports = function () {
  this.eslint = function (opts) {
    const lint = createLinter(new CLIEngine(opts))

    return this.unwrap((files) => {
      const problems = files.map(file => lint(file))
        .filter((problem) => problem.errorCount > 0 || problem.warningCount > 0)

      const errorCount = problems.reduce(sumProperty("errorCount"), 0)
      const warningCount = problems.reduce(sumProperty("warningCount"), 0)

      if (errorCount > 0 || warningCount > 0) {
        throw new LinterError(errorCount + " errors and " + warningCount + " warnings in " + problems.length + " files.")
      }
    })
  }
}