Reference - Namespaces - 《TypeScript v4.2 Documentation》

Namespacing
Splitting Across Files
Aliases
Ambient Namespaces

This post outlines the various ways to organize your code using namespaces (previously “internal modules”) in TypeScript. As we alluded in our note about terminology, “internal modules” are now referred to as “namespaces”. Additionally, anywhere the module keyword was used when declaring an internal module, the namespace keyword can and should be used instead. This avoids confusing new users by overloading them with similarly named terms.

Let’s start with the program we’ll be using as our example throughout this page. We’ve written a small set of simplistic string validators, as you might write to check a user’s input on a form in a webpage or check the format of an externally-provided data file.

Validators in a single file

Namespacing

As we add more validators, we’re going to want to have some kind of organization scheme so that we can keep track of our types and not worry about name collisions with other objects. Instead of putting lots of different names into the global namespace, let’s wrap up our objects into a namespace.

In this example, we’ll move all validator-related entities into a namespace called Validation. Because we want the interfaces and classes here to be visible outside the namespace, we preface them with export. Conversely, the variables lettersRegexp and numberRegexp are implementation details, so they are left unexported and will not be visible to code outside the namespace. In the test code at the bottom of the file, we now need to qualify the names of the types when used outside the namespace, e.g. Validation.LettersOnlyValidator.

Splitting Across Files

Multi-file namespaces

Here, we’ll split our Validation namespace across many files. Even though the files are separate, they can each contribute to the same namespace and can be consumed as if they were all defined in one place. Because there are dependencies between files, we’ll add reference tags to tell the compiler about the relationships between the files. Our test code is otherwise unchanged.

Validation.ts

namespace Validation {
  export interface StringValidator {
    isAcceptable(s: string): boolean;
  }
}

LettersOnlyValidator.ts

ZipCodeValidator.ts

/// <reference path="Validation.ts" />
namespace Validation {
  const numberRegexp = /^[0-9]+$/;
  export class ZipCodeValidator implements StringValidator {
    isAcceptable(s: string) {
      return s.length === 5 && numberRegexp.test(s);
    }
  }
}

Test.ts

/// <reference path="Validation.ts" />
/// <reference path="LettersOnlyValidator.ts" />
/// <reference path="ZipCodeValidator.ts" />
// Some samples to try
let strings = ["Hello", "98052", "101"];
// Validators to use
let validators: { [s: string]: Validation.StringValidator } = {};
validators["ZIP code"] = new Validation.ZipCodeValidator();
validators["Letters only"] = new Validation.LettersOnlyValidator();
// Show whether each string passed each validator
for (let s of strings) {
  for (let name in validators) {
    console.log(
      `"${s}" - ${
        validators[name].isAcceptable(s) ? "matches" : "does not match"
      } ${name}`
    );
  }
}

Once there are multiple files involved, we’ll need to make sure all of the compiled code gets loaded. There are two ways of doing this.

First, we can use concatenated output using the --outFile flag to compile all of the input files into a single JavaScript output file:

The compiler will automatically order the output file based on the reference tags present in the files. You can also specify each file individually:

tsc --outFile sample.js Validation.ts LettersOnlyValidator.ts ZipCodeValidator.ts Test.ts

MyTestPage.html (excerpt)

<script src="Validation.js" type="text/javascript" />
<script src="LettersOnlyValidator.js" type="text/javascript" />
<script src="ZipCodeValidator.js" type="text/javascript" />
<script src="Test.js" type="text/javascript" />

Another way that you can simplify working with namespaces is to use to create shorter names for commonly-used objects. Not to be confused with the import x = require("name") syntax used to load modules, this syntax simply creates an alias for the specified symbol. You can use these sorts of imports (commonly referred to as aliases) for any kind of identifier, including objects created from module imports.

Notice that we don’t use the require keyword; instead we assign directly from the qualified name of the symbol we’re importing. This is similar to using var, but also works on the type and namespace meanings of the imported symbol. Importantly, for values, import is a distinct reference from the original symbol, so changes to an aliased var will not be reflected in the original variable.

Working with Other JavaScript Libraries

To describe the shape of libraries not written in TypeScript, we need to declare the API that the library exposes. Because most JavaScript libraries expose only a few top-level objects, namespaces are a good way to represent them.

We call declarations that don’t define an implementation “ambient”. Typically these are defined in .d.ts files. If you’re familiar with C/C++, you can think of these as .h files. Let’s look at a few examples.

Namespaces