add method to check if a plugin is installed

__mocks__/fs.js

@@ -1,5 +1,16 @@
 const fs = jest.genMockFromModule('fs');
 
+let _files = [];
+
+/**
+ * Set files
+ *
+ * @param {String[]}
+ */
+fs.__setFiles = (files) => {
+  _files = files;
+};
+
 // Mocks fs.link
 fs.link = (src, dest, callback) => {
   callback.call(null);
@@ -10,4 +21,12 @@ fs.unlink = (dest, callback) => {
   callback.call(null);
 };
 
+/**
+ * Returns true if the file path exists in the _files list
+ *
+ * @param {String} filePath
+ * @return {Boolean}
+ */
+fs.existsSync = filePath => _files.indexOf(filePath) > -1;
+
 module.exports = fs;

__tests__/api/plugins.js

@@ -0,0 +1,16 @@
+import path from 'path';
+import plugins from '../../src/api/plugins';
+import { PLUGIN_PATH } from '../../src/utils/paths';
+
+jest.mock('fs');
+
+describe('plugins', () => {
+  it('should check if a plugin is installed', () => {
+    // eslint-disable-next-line global-require
+    require('fs').__setFiles([
+      path.resolve(PLUGIN_PATH, 'foobar'),
+    ]);
+    expect(plugins.isInstalled('foobar')).toBeTruthy();
+    expect(plugins.isInstalled('bazqux')).toBeFalsy();
+  });
+});

src/api/plugins.js

@@ -1,4 +1,7 @@
+const path = require('path');
+const fs = require('fs');
 const Conf = require('../utils/conf');
+const { PLUGIN_PATH } = require('../utils/paths');
 
 // initialize a new config file
 const config = new Conf();
@@ -14,6 +17,14 @@ const getAll = () => config.get('plugins') || [];
 // eslint-disable-next-line no-bitwise
 const isEnabled = plugin => ~getAll().indexOf(plugin);
 
+/**
+ * Checks if the plugin is installed
+ *
+ * @param {String} plugin - The plugin name
+ * @return {Boolean}
+ */
+const isInstalled = plugin => fs.existsSync(path.resolve(PLUGIN_PATH, plugin));
+
 /**
  * Enables the plugin by adding it to the config
  *
@@ -39,6 +50,7 @@ const disable = (plugin) => {
 module.exports = {
   getAll,
   isEnabled,
+  isInstalled,
   enable,
   disable,
 };