How do I import other TypeScript files?

From TypeScript version 1.8 you can use simple import statements just like in ES6:

import { ZipCodeValidator } from "./ZipCodeValidator";

let myValidator = new ZipCodeValidator();

https://www.typescriptlang.org/docs/handbook/modules.html

Old answer: From TypeScript version 1.5 you can use tsconfig.json: http://www.typescriptlang.org/docs/handbook/tsconfig-json.html

It completely eliminates the need of the comment style referencing.

Older answer:

You need to reference the file on the top of the current file.

You can do this like this:

/// <reference path="../typings/jquery.d.ts"/>
/// <reference path="components/someclass.ts"/>

class Foo { }

etc.

These paths are relative to the current file.

Your example:

/// <reference path="moo.ts"/>

class bar extends moo.foo
{
}

Typescript distinguishes two different kinds of modules: Internal modules are used to structure your code internally. At compile-time, you have to bring internal modules into scope using reference paths:

/// <reference path='moo.ts'/>

class bar extends moo.foo {
}

On the other hand, external modules are used to refernence external source files that are to be loaded at runtime using CommonJS or AMD. In your case, to use external module loading you have to do the following:

moo.ts

export class foo {
    test: number;
} 

app.ts

import moo = module('moo');
class bar extends moo.foo {
  test2: number;
}

Note the different way of brining the code into scope. With external modules, you have to use module with the name of the source file that contains the module definition. If you want to use AMD modules, you have to call the compiler as follows:

tsc --module amd app.ts

This then gets compiled to

var __extends = this.__extends || function (d, b) {
    function __() { this.constructor = d; }
    __.prototype = b.prototype;
    d.prototype = new __();
}
define(["require", "exports", 'moo'], function(require, exports, __moo__) {
    var moo = __moo__;

    var bar = (function (_super) {
        __extends(bar, _super);
        function bar() {
            _super.apply(this, arguments);

        }
        return bar;
    })(moo.foo);
})    

If you're using AMD modules, the other answers won't work in TypeScript 1.0 (the newest at the time of writing.)

You have different approaches available to you, depending upon how many things you wish to export from each .ts file.

Multiple exports

Foo.ts

export class Foo {}
export interface IFoo {}

Bar.ts

import fooModule = require("Foo");

var foo1 = new fooModule.Foo();
var foo2: fooModule.IFoo = {};

---

Single export

Foo.ts

class Foo
{}

export = Foo;

Bar.ts

import Foo = require("Foo");

var foo = new Foo();

If you are looking to use modules and want it to compile to a single JavaScript file you can do the following:

tsc -out _compiled/main.js Main.ts

Main.ts

///<reference path='AnotherNamespace/ClassOne.ts'/>
///<reference path='AnotherNamespace/ClassTwo.ts'/>

module MyNamespace
{
    import ClassOne = AnotherNamespace.ClassOne;
    import ClassTwo = AnotherNamespace.ClassTwo;

    export class Main
    {
        private _classOne:ClassOne;
        private _classTwo:ClassTwo;

        constructor()
        {
            this._classOne = new ClassOne();
            this._classTwo = new ClassTwo();
        }
    }
}

ClassOne.ts

///<reference path='CommonComponent.ts'/>

module AnotherNamespace
{
    export class ClassOne
    {
        private _component:CommonComponent;

        constructor()
        {
            this._component = new CommonComponent();
        }
    }
}

CommonComponent.ts

module AnotherNamespace
{
    export class CommonComponent
    {
        constructor()
        {
        }
    }
}

You can read more here: http://www.codebelt.com/typescript/javascript-namespacing-with-typescript-internal-modules/