feat: Allow clients to access messages from eslint

  Adds another export `analyze` from this package, which behaves
  exactly like `format` except that it returns a simple object
  with properties `output` giving the formatted text and `messages`
  giving the eslint messages produced in processing the input text.
  Clients can call `analyze` instead of `format` if they wish, for
  example, to capture any errors that esling may have encountered.

  Resolves prettier#704.

.eslintrc.js

@@ -12,6 +12,9 @@ const config = {
       }
     ],
     'import/no-import-module-exports': 'off'
+  },
+  settings: {
+    'import/ignore': ['node_modules', 'src'] // Using CommonJS in src
   }
 };
 

src/__tests__/index.js

@@ -5,7 +5,7 @@ import stripIndent from 'strip-indent';
 import eslintMock from 'eslint';
 import prettierMock from 'prettier';
 import loglevelMock from 'loglevel-colored-level-prefix';
-import format from '../';
+import {format, analyze} from '../';
 
 jest.mock('fs');
 
@@ -256,6 +256,22 @@ tests.forEach(({ title, modifier, input, output }) => {
   });
 });
 
+test('analyze returns the messages', async () => {
+  const text = 'var x = 0;';
+  const result = await analyze({
+    text,
+    eslintConfig: {
+      rules: { 'no-var': 'error' }
+    }
+  })
+  expect(result.output).toBe(`${text}\n`);
+  expect(result.messages).toHaveLength(1);
+  const msg = result.messages[0];
+  expect(msg.ruleId).toBe('no-var');
+  expect(msg.column).toBe(1);
+  expect(msg.endColumn).toBe(11);
+})
+
 test('failure to fix with eslint throws and logs an error', async () => {
   const { lintText } = eslintMock.mock;
   const error = new Error('Something happened');

src/index.js

@@ -41,9 +41,25 @@ module.exports = format;
  * @return {Promise<String>} - the formatted string
  */
 async function format(options) {
+  const {output} = await analyze(options);
+  return output;
+}
+
+/**
+ * Analyzes and formats text with prettier and eslint, based on the
+ * identical options as for the `format` function. It differs from
+ * `format` only in that the return value is a simple object with
+ * properties `output` giving the formatted code and `messages` giving
+ * any error messages generated in the analysis.
+ * @param {Object} identical to options parameter of `format`
+ * @returns {Promise<Object>} the return value is an object `r` such that
+ *  `r.output` is the formatted string and `r.messages` is an array of
+ *  message specifications from eslint.
+ */
+async function analyze(options) {
   const { logLevel = getDefaultLogLevel() } = options;
   logger.setLevel(logLevel);
-  logger.trace('called format with options:', prettyFormat(options));
+  logger.trace('called analyze with options:', prettyFormat(options));
 
   const {
     filePath,
@@ -127,11 +143,17 @@ async function format(options) {
     const eslintFixed = await eslintFix(text, filePath);
     return prettify(eslintFixed);
   }
-  return eslintFix(prettify(text), filePath);
+  return eslintFix(prettify(text).output, filePath);
 }
 
 function createPrettify(formatOptions, prettierPath) {
-  return function prettify(text) {
+  return function prettify(param) {
+    let text = param
+    let messages = []
+    if (typeof param !== 'string') {
+      text = param.output
+      messages = param.text
+    }
     logger.debug('calling prettier on text');
     logger.trace(
       stripIndent`
@@ -152,7 +174,7 @@ function createPrettify(formatOptions, prettierPath) {
         ${indentString(output, 2)}
       `
       );
-      return output;
+      return {output, messages};
     } catch (error) {
       logger.error('prettier formatting failed due to a prettier error');
       throw error;
@@ -207,7 +229,7 @@ function createEslintFix(eslintConfig, eslintPath) {
       );
       // default the output to text because if there's nothing
       // to fix, eslint doesn't provide `output`
-      const [{ output = text }] = await report;
+      const [{ output = text, messages }] = await report;
       logger.trace('eslint --fix: output === input', output === text);
       // NOTE: We're ignoring linting errors/warnings here and
       // defaulting to the given text if there are any
@@ -220,7 +242,7 @@ function createEslintFix(eslintConfig, eslintPath) {
         ${indentString(output, 2)}
       `
       );
-      return output;
+      return {output, messages};
     } catch (error) {
       logger.error('eslint fix failed due to an eslint error');
       throw error;
@@ -324,3 +346,8 @@ function getModulePath(filePath = __filename, moduleName) {
 function getDefaultLogLevel() {
   return process.env.LOG_LEVEL || 'warn';
 }
+
+// Allow named imports of either `analyze` or `format` from this module,
+// while leaving `format` in place as the default import:
+module.exports.format = format
+module.exports.analyze = analyze