docs: EN translation for style guide doc (#2239)

docs/source/en/style-guide.md

@@ -1 +1,63 @@
-this document is still waiting for translation, see [Chinese Version](/zh-cn/style-guide.html)
+## Title: Code Style Guide
+
+Developers are advised to use `egg-init --type=simple showcase` to generate and observe the recommended project structure and configuration.
+
+## Classify
+
+```js
+// old style
+module.exports = app => {
+  class UserService extends app.Service {
+    async list() {
+      return await this.ctx.curl('https://eggjs.org');
+    }
+  }
+  return UserService;
+};
+```
+
+change to:
+
+```js
+const Service = require('egg').Service;
+class UserService extends Service {
+  async list() {
+    return await this.ctx.curl('https://eggjs.org');
+  }
+}
+module.exports = UserService;
+```
+
+Additionally, the `framework developer` needs to change the syntax as follows, otherwise the `application developer` will have problems customizing base classes such as Service:
+
+```js
+const egg = require('egg');
+
+module.exports = Object.assign(egg, {
+  Application: class MyApplication extends egg.Application {
+    // ...
+  }
+  // ...
+});
+```
+
+## Private properties & Lazy Initialization
+
+* Private properties are mounted with `Symbol`.
+* The description of Symbol follows the rules of jsdoc, describing the mapped class name + attribute name.
+* Delayed initialization.
+
+```js
+// app/extend/application.js
+const CACHE = Symbol('Application#cache');
+const CacheManager = require('../../lib/cache_manager');
+
+module.exports = {
+  get cache() {
+    if (!this[CACHE]) {
+      this[CACHE] = new CacheManager(this);
+    }
+    return this[CACHE];
+  }
+};
+```