[medusa-interfaces] : Adds decorator functionality to BaseService (#39)

Plugins and projects can add decorators in services. E.g. if a plugin needs to load some additional information on carts the plugin can register a decorator via: `cartService.addDecorator(someFunc)` which will be available later through `cartService.runDecorators()`.

packages/medusa-interfaces/src/__tests__/base-service.js

@@ -0,0 +1,66 @@
+import BaseService from "../base-service"
+
+describe("BaseService", () => {
+  describe("addDecorator", () => {
+    const baseService = new BaseService()
+
+    it("successfully adds decorator", () => {
+      baseService.addDecorator(obj => {
+        return (obj.decorator1 = true)
+      })
+
+      expect(baseService.decorators_.length).toEqual(1)
+    })
+
+    it("throws if decorator is not a function", () => {
+      expect(() => baseService.addDecorator("not a function")).toThrow(
+        "Decorators must be of type function"
+      )
+    })
+  })
+
+  describe("runDecorators_", () => {
+    it("returns success when passwords match", async () => {
+      const baseService = new BaseService()
+
+      baseService.addDecorator(obj => {
+        obj.decorator1 = true
+        return obj
+      })
+      baseService.addDecorator(obj => {
+        obj.decorator2 = true
+        return obj
+      })
+
+      const result = await baseService.runDecorators_({ data: "initial" })
+      expect(result).toEqual({
+        data: "initial",
+        decorator1: true,
+        decorator2: true,
+      })
+    })
+
+    it("skips failing decorator", async () => {
+      const baseService = new BaseService()
+
+      baseService.addDecorator(obj => {
+        obj.decorator1 = true
+        return obj
+      })
+      baseService.addDecorator(obj => {
+        return Promise.reject("fail")
+      })
+      baseService.addDecorator(obj => {
+        obj.decorator3 = true
+        return Promise.resolve(obj)
+      })
+
+      const result = await baseService.runDecorators_({ data: "initial" })
+      expect(result).toEqual({
+        data: "initial",
+        decorator1: true,
+        decorator3: true,
+      })
+    })
+  })
+})