feat(lint): add output file option (close #4849)

Adds an `output-file` option to the ESLint plugin that saves lint report results to the specified file path.

This mimics the [ESLint option](https://eslint.org/docs/user-guide/command-line-interface#options) of the same name.

Author: ataylorme

packages/@vue/cli-plugin-eslint/README.md

@@ -15,6 +15,7 @@
     --no-fix             do not fix errors
     --max-errors         specify number of errors to make build failed (default: 0)
     --max-warnings       specify number of warnings to make build failed (default: Infinity)
+    --output-file        specify file to write report to
   ```
 
   Lints and fixes files. If no specific files are given, it lints all files in `src` and `test`.

packages/@vue/cli-plugin-eslint/__tests__/eslintPlugin.spec.js

@@ -140,3 +140,58 @@ test('should not throw when src folder is ignored by .eslintignore', async () =>
   // should not throw
   await run('vue-cli-service lint')
 })
+
+test('should save report results to file with --output-file option', async () => {
+  const project = await create('eslint-output-file', {
+    plugins: {
+      '@vue/cli-plugin-babel': {},
+      '@vue/cli-plugin-eslint': {
+        config: 'airbnb',
+        lintOn: 'commit'
+      }
+    }
+  })
+  const { read, write, run } = project
+  // should've applied airbnb autofix
+  const main = await read('src/main.js')
+  expect(main).toMatch(';')
+  // remove semicolons
+  const updatedMain = main.replace(/;/g, '')
+  await write('src/main.js', updatedMain)
+
+  // result file name
+  const resultsFile = 'lint_results.json'
+
+  try {
+    // lint in JSON format to output-file
+    await run(`vue-cli-service lint --format json --output-file ${resultsFile} --no-fix`)
+  } catch (e) {
+    // lint with no fix should fail
+    expect(e.code).toBe(1)
+    expect(e.failed).toBeTruthy()
+  }
+
+  let resultsFileContents = ''
+
+  // results file should exist
+  try {
+    resultsFileContents = await read(resultsFile)
+  } catch (e) {
+    expect(e.code).toBe(0)
+    expect(e.failed).toBeFalsy()
+  }
+
+  // results file should not be empty
+  expect(resultsFileContents.length).toBeGreaterThan(0)
+
+  // results file is valid JSON
+  try {
+    JSON.parse(resultsFileContents)
+  } catch (e) {
+    expect(e.code).toBe(0)
+    expect(e.failed).toBeFalsy()
+  }
+
+  // results file should show "Missing semicolon" errors
+  expect(resultsFileContents).toEqual(expect.stringContaining('Missing semicolon'))
+})

packages/@vue/cli-plugin-eslint/index.js

@@ -72,7 +72,9 @@ module.exports = (api, options) => {
         '--max-errors [limit]':
           'specify number of errors to make build failed (default: 0)',
         '--max-warnings [limit]':
-          'specify number of warnings to make build failed (default: Infinity)'
+          'specify number of warnings to make build failed (default: Infinity)',
+        '--output-file [file_path]':
+          'specify file to write report to'
       },
       details:
         'For more options, see https://eslint.org/docs/user-guide/command-line-interface#options'

packages/@vue/cli-plugin-eslint/lint.js

@@ -15,7 +15,8 @@ const renamedArgs = {
   rule: 'rules',
   eslintrc: 'useEslintrc',
   c: 'configFile',
-  config: 'configFile'
+  config: 'configFile',
+  'output-file': 'outputFile'
 }
 
 module.exports = function lint (args = {}, api) {
@@ -83,6 +84,16 @@ module.exports = function lint (args = {}, api) {
 
   const formatter = engine.getFormatter(args.format || 'codeframe')
 
+  if (config.outputFile) {
+    const outputFilePath = path.resolve(config.outputFile)
+    try {
+      fs.writeFileSync(outputFilePath, formatter(report.results))
+      log(`Lint results saved to ${chalk.blue(outputFilePath)}`)
+    } catch (err) {
+      log(`Error saving lint results to ${chalk.blue(outputFilePath)}: ${chalk.red(err)}`)
+    }
+  }
+
   if (config.fix) {
     CLIEngine.outputFixes(report)
   }