TypeScript: Interface vs Class vs Modules vs Program vs Function

I would like to explain the Interface,Class,Module, Program and Fucntion in TypeScript per comparing C# and hopefully the answers are useful to people coming to TypeScript from similar languages too.

Interface

An interface in TypeScript is similar to those you have come across in C#. It is a contract - if one of your classes implements an interface, it promises to have certain properties or methods that the interface documents.

In TypeScript an interface can inherit from another interface in order to extend it and from a class to capture its implementation.

Whenever something seems impossible in TypeScript, you can usually solve it with an interface!

Class

This is very similar to the concept of a class in C#. You can inherit from other classes to extend or specialise the behaviour.

Module

Modules are analogous to C# namespaces. They allow you to group a number of classes together into a logical group. A module can also have functions and variables alongside classes.

Program

A program is a collection of modules, classes. This is essentially the thing you have written using TypeScript.

Function

A TypeScript function is just like a C# method.

Declare vs. var

var

creates a new variable.

declare

is used to tell TypeScript that the variable has been created elsewhere. If you use

declare

, nothing is added to the JavaScript that is generated - it is simply a hint to the compiler.

For example, if you use an external script that defines

var externalModule

, you would use

declare externalModule

to hint to the TypeScript compiler that

externalModule

has been correctly set up.