Cannot find namespace NodeJS when using NodeJS.Timer in Ionic 2

Open src/tsconfig.app.json*.

Add "node" to the "types" array.

Example:

{
  "extends": "../tsconfig.json",
  "compilerOptions": {
    "outDir": "../out-tsc/app",
    "baseUrl": "./",
    "module": "es2015",
    "types": [
      "node"
    ]
  },
  "exclude": [
    "test.ts",
    "**/*.spec.ts"
  ]
}

*if this file does not exist add the specified part to tsconfig.json in root folder.