Modules

Global Module

By default when you start typing code in a new TypeScript file your code is in a global namespace. As a demo consider a file foo.ts:

var foo = 123;

If you now create a new file bar.ts in the same project, you will be allowed by the TypeScript type system to use the variable foo as if it was available globally:

var bar = foo; // allowed

Needless to say having a global namespace is dangerous as it opens your code up for naming conflicts. We recommend using file modules which are presented next.

File Module

Also called external modules. If you have an import or an export at the root level of a TypeScript file then it creates a local scope within that file. So if we were to change the previous foo.ts to the following (note the export usage):

export var foo = 123;

We will no longer have foo in the global namespace. This can be demonstrated by creating a new file bar.ts as follows:

var bar = foo; // ERROR: "cannot find name 'foo'"

If you want to use stuff from foo.ts in bar.ts you need to explicitly import it. This is shown in an updated bar.ts below:

import { foo } from "./foo";
var bar = foo; // allowed

Using an import in bar.ts not only allows you to bring in stuff from other files, but also marks the file bar.ts as a module and therefore, declarations in bar.ts don't pollute the global namespace either.

What JavaScript is generated from a given TypeScript file that uses external modules is driven by the compiler flag called module.