[Fix] version errors should log to stderr, not stdout

Fixes #2082

Author: ljharb

lib/util/error.js

@@ -0,0 +1,14 @@
+'use strict';
+
+/**
+ * Logs out a message if there is no format option set.
+ * @param {String} message - Message to log.
+ */
+function error(message) {
+  if (!/\=-(f|-format)=/.test(process.argv.join('='))) {
+    // eslint-disable-next-line no-console
+    console.error(message);
+  }
+}
+
+module.exports = error;

lib/util/version.js

@@ -5,7 +5,7 @@
 'use strict';
 
 const resolve = require('resolve');
-const log = require('./log');
+const error = require('./error');
 
 let warnedForMissingVersion = false;
 
@@ -16,7 +16,7 @@ function detectReactVersion() {
     return react.version;
   } catch (e) {
     if (e.code === 'MODULE_NOT_FOUND') {
-      log('Warning: React version was set to "detect" in eslint-plugin-react settings, ' +
+      error('Warning: React version was set to "detect" in eslint-plugin-react settings, ' +
         'but the "react" package is not installed. Assuming latest React version for linting.');
       return '999.999.999';
     }
@@ -33,12 +33,12 @@ function getReactVersionFromContext(context) {
       settingsVersion = detectReactVersion();
     }
     if (typeof settingsVersion !== 'string') {
-      log('Warning: React version specified in eslint-plugin-react-settings must be a string; ' +
+      error('Warning: React version specified in eslint-plugin-react-settings must be a string; ' +
         `got “${typeof settingsVersion}”`);
     }
     confVer = String(settingsVersion);
   } else if (!warnedForMissingVersion) {
-    log('Warning: React version not specified in eslint-plugin-react settings. ' +
+    error('Warning: React version not specified in eslint-plugin-react settings. ' +
       'See https://github.com/yannickcr/eslint-plugin-react#configuration.');
     warnedForMissingVersion = true;
   }
@@ -52,7 +52,7 @@ function getFlowVersionFromContext(context) {
   if (context.settings.react && context.settings.react.flowVersion) {
     const flowVersion = context.settings.react.flowVersion;
     if (typeof flowVersion !== 'string') {
-      log('Warning: Flow version specified in eslint-plugin-react-settings must be a string; ' +
+      error('Warning: Flow version specified in eslint-plugin-react-settings must be a string; ' +
         `got “${typeof flowVersion}”`);
     }
     confVer = String(flowVersion);