implement flow version detection

lib/util/version.js

@@ -46,11 +46,29 @@ function getReactVersionFromContext(context) {
   return confVer.split('.').map(part => Number(part));
 }
 
+function detectFlowVersion() {
+  try {
+    const flowPackageJsonPath = resolve.sync('flow-bin/package.json', {basedir: process.cwd()});
+    const flowPackageJson = require(flowPackageJsonPath);
+    return flowPackageJson.version;
+  } catch (e) {
+    if (e.code === 'MODULE_NOT_FOUND') {
+      error('Warning: Flow version was set to "detect" in eslint-plugin-react settings, ' +
+        'but the "flow-bin" package is not installed. Assuming latest Flow version for linting.');
+      return '999.999.999';
+    }
+    throw e;
+  }
+}
+
 function getFlowVersionFromContext(context) {
   let confVer = '999.999.999';
   // .eslintrc shared settings (http://eslint.org/docs/user-guide/configuring#adding-shared-settings)
   if (context.settings.react && context.settings.react.flowVersion) {
-    const flowVersion = context.settings.react.flowVersion;
+    let flowVersion = context.settings.react.flowVersion;
+    if (flowVersion === 'detect') {
+      flowVersion = detectFlowVersion();
+    }
     if (typeof flowVersion !== 'string') {
       error('Warning: Flow version specified in eslint-plugin-react-settings must be a string; ' +
         `got “${typeof flowVersion}”`);

tests/util/version.js

@@ -26,12 +26,13 @@ describe('Version', () => {
   });
 
   describe('Detect version', () => {
-    const context = {settings: {react: {version: 'detect'}}};
+    const context = {settings: {react: {version: 'detect', flowVersion: 'detect'}}};
 
     it('matches detected version', () => {
       process.chdir('detect-version');
       assert.equal(versionUtil.testReactVersion(context, '1.2.3'), true);
       assert.equal(versionUtil.testReactVersion(context, '1.2.4'), false);
+      assert.equal(versionUtil.testFlowVersion(context, '0.92.0'), true);
     });
 
     it('assumes latest version if react is not installed', () => {
@@ -41,6 +42,14 @@ describe('Version', () => {
         ['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.']
       ];
     });
+
+    it('assumes latest version if flow-bin is not installed', () => {
+      assert.equal(versionUtil.testFlowVersion(context, '999.999.999'), true);
+
+      expectedErrorArgs = [
+        ['Warning: Flow version was set to "detect" in eslint-plugin-react settings, but the "flow-bin" package is not installed. Assuming latest Flow version for linting.']
+      ];
+    });
   });
 
   describe('string version', () => {