Update: September 2016

You need to make sure that the type definitions for Node are available. The way to do this depends on which version of TypeScript you are using.

TypeScript 1

Use Typings to install the definitions.

typings install --save --global env~node

Be sure to include typings/index.d.ts in your tsconfig.json file. Either include it in the files array:

files: [typings/index.d.ts]

Or ensure it is omitted from the exclude array.

TypeScript 2

In TypeScript 2, definitions can be installed via npm under the @types scope.

npm install --save-dev @types/node

Original Answer: February 2016

You have to make sure the appropriate type definitions are available. Use the typings package manager for this. Install the definitions for node as follows:

typings install --save --ambient node

Now, there are a few ways you can make sure the definitions are available to the compiler. The preferred method is to set up your tsconfig file like so:

{

  exclude: [

    typings/browser.d.ts,

    typings/browser,

    node_modules

  ]

}

Or, alternatively:

{

  files: [

    typings/main.d.ts

  ]

}

If you are not using a tsconfig file, you can use a reference at the top of your main entry file, like so:

/// <reference path=path/to/typings/main.d.ts />