diff --git a/.travis.yml b/.travis.yml
index a57a50a..c34c211 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -1,6 +1,6 @@
 language: node_js
-  - 9
-  - 8
+  - "node"
+
 dist: trusty
 
 sudo: false
diff --git a/package.json b/package.json
index 3de9185..408085b 100644
--- a/package.json
+++ b/package.json
@@ -21,6 +21,7 @@
     "@types/faker": "4.1.2",
     "@types/jasmine": "2.8.6",
     "@types/jasmine-ajax": "3.1.37",
+    "@types/node": "9.6.17",
     "faker": "4.1.0",
     "fork-ts-checker-webpack-plugin": "0.4.1",
     "jasmine": "3.1.0",
@@ -55,6 +56,10 @@
     "xmlhttprequest",
     "superagent"
   ],
+  "engines": {
+    "node": ">=9.9.0",
+    "npm": ">=5.6.0"
+  },
   "license": "MIT",
   "types": "dist/SimpleRestClients.d.ts"
 }
diff --git a/src/typings/assert.d.ts b/src/typings/assert.d.ts
deleted file mode 100644
index 513695f..0000000
--- a/src/typings/assert.d.ts
+++ /dev/null
@@ -1,58 +0,0 @@
-// Type definitions for assert and power-assert
-// Project: https://github.com/Jxck/assert
-// Project: https://github.com/twada/power-assert
-// Definitions by: vvakame <https://github.com/vvakame>
-// Definitions: https://github.com/DefinitelyTyped/DefinitelyTyped
-
-// copy from assert external module in node.d.ts
-
-declare function assert(value:any, message?:string):void;
-declare namespace assert {
-    export class AssertionError implements Error {
-        name:string;
-        message:string;
-        actual:any;
-        expected:any;
-        operator:string;
-        generatedMessage:boolean;
-
-        constructor(options?:{message?: string; actual?: any; expected?: any; operator?: string; stackStartFunction?: Function});
-    }
-
-    export function fail(actual?:any, expected?:any, message?:string, operator?:string):void;
-
-    export function ok(value:any, message?:string):void;
-
-    export function equal(actual:any, expected:any, message?:string):void;
-
-    export function notEqual(actual:any, expected:any, message?:string):void;
-
-    export function deepEqual(actual:any, expected:any, message?:string):void;
-
-    export function notDeepEqual(acutal:any, expected:any, message?:string):void;
-
-    export function strictEqual(actual:any, expected:any, message?:string):void;
-
-    export function notStrictEqual(actual:any, expected:any, message?:string):void;
-
-    export var throws:{
-        (block:Function, message?:string): void;
-        (block:Function, error:Function, message?:string): void;
-        (block:Function, error:RegExp, message?:string): void;
-        (block:Function, error:(err:any) => boolean, message?:string): void;
-    };
-
-    export var doesNotThrow:{
-        (block:Function, message?:string): void;
-        (block:Function, error:Function, message?:string): void;
-        (block:Function, error:RegExp, message?:string): void;
-        (block:Function, error:(err:any) => boolean, message?:string): void;
-    };
-
-    export function ifError(value:any):void;
-}
-
-// duplicate to node.d.ts
- declare module "assert" {
-     export = assert;
- }