Wednesday, November 6, 2013

TypeScript: Internal vs External Modules

I only recently started looking at TypeScript, so there are still some concepts that are new to me. One the things I struggle with is the difference between internal and external modules.

As far as I understand it, you can define Namespaces in TypeScript by using internal modules:

By using the ‘module’ keyword I can create a Module. Everything that I put inside this module will only available inside the module itself. If I want to make something available outside the module I should apply the ‘export’ keyword.

So far so good. Now things start to get complex(at least for me) when you bring external modules into the mix. These are modules that use the AMD conventions and make it possible to export an entire module and then import it into another module. This defines a dependency chain that AMD and require.js can manage for you.

Something that took me some time to figure out is that you cannot mix internal and external modules. I wanted to use external modules but split one module out over multiple *.ts files. I had to take a look at the language specification to understand that this is ‘as designed’:

External modules (section 9.4) are separately loaded bodies of code referenced using external module names. An external module is written as a separate source file that contains at least one import or export declaration.

Further down it says that internal modules are open ended and can extend over multiple files:

Internal modules are “open-ended” and internal module declarations with the same qualified name relative to a common root (as defined in section 2.2) contribute to a single module.

If you want to read more about the difference, have a look at following StackOverflow post:


codeBelt said...

You should setup your External Module classes like below so you don't have do:
new shapes.Rectangle(10,4);
And you can just do:
new Rectangle(10,4);

class Rectangle {
constructor (public height: number, public width: number) {
export = Rectangle;

Check out my TypeScript Internal and External Modules for examples.

Also you should update the import to be:
import shapes = require('shapes');

Bart Wullems said...

Thx! I updated the code...

Shruti Ramalingam said...
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