When I first started reading about TypeScript, I had one main concern: how am I going to make this work with the weird mix of modular code and old-school JS libraries in my existing codebase?

The features of the language itself are very well covered. I found various great introductions (and now there’s the awesome official Handbook), but they all seemed to gloss over certain fundamental questions I had about the physical organisation of code. So I’m going to go through the basics here and try to answer those questions as I go.

Modularity in the Browser

Let’s get the the controversial opinionated bit out of the way. (Spoiler: turns out my opinions on this are irrelevant to TS!)

How should you physically transport your JS into your user’s browser? There are those who suggest you should asynchronously load individual module files on the fly. I am not one of them. Stitch your files together into one big chunk, minify it, let the web server gzip it, let the browser cache it. This means it gets onto the user’s machine in a single request, typically once, like any other binary resource.

The exception would be during the development process: the edit-refresh-debug cycle. Clearly it shouldn’t be minified here. Nor should it be cached by the browser (load the latest version, ya varmint!) And ideally it shouldn’t be one big file, though that’s not as much of an issue as it was a few years ago (even as late as version 9, IE used to crash if you tried to debug large files, and Chrome would get confused about breakpoints).

But I’ve found it pretty straightforward to put a conditional flag in my applications, a ?DEBUG mode, which controls how it serves up the source. In production it’s the fast, small version. In ?DEBUG , it’s the convenient version (separate files).

In neither situation does it need to be anything other than CommonJS. For about four years now I’ve been using CommonJS-style require / exports as my module API in the browser, and it’s the smoothest, best-of-all-worlds experience I could wish for.

So what’s the point of AMD? Apparently “… debugging multiple files that are concatenated into one file [has] practical weaknesses. Those weaknesses may be addressed in browser tooling some day…” In my house they were addressed in the browser in about 2011.

But anyway… deep breath, calms down… it turns out that TypeScript doesn’t care how you do this. It turns us all into Lilliputians arguing over which way up a boiled egg must be eaten.

The kinds of file in TypeScript

In TS, modules and physical files are not necessarily the same thing. If you want to work that way, you can. You can mix and match. So however you ended up with your codebase, TS can probably handle it.

If a TS file just contains things like:

var x = 5; function f() { return x; }

Then the compiler will output the same thing (exactly the same thing, in that example). You can start to make it modular (in a sense) without splitting into multiple files:

module MyStuff { var x = 5; export function f() { return x; } } var y = MyStuff.f();

That makes an (or extends an existing) object called MyStuff with one property, f , because I prefixed it with export . Modules can nest. So just as in JavaScript there’s one big global namespace that your source contributes properties to, but you can achieve modularity by using objects to contain related things.

You can at this point roll your own pattern: write lots of separate files in the above style, each file being responsible for wrapping its code in a named module, then pass them to the TS compiler and stitch the result into one file.

Now try using export at the top level in your file:

var x = 5; export function f() { return x; }

The compiler will complain that you haven’t told it what module system you want to use. You tell it with the flag --module commonjs (or --module amd if you’re crazy). Now it works and does exactly what you’d expect as a user of your chosen module system.

But what does this mean in terms of the static type system of TS and so on? It means that this particular file no longer contributes any properties to the global namespace. By just using the export prefix at the top level, you converted it into what TS calls an external module.

In order to make use of it from another module, you need to require it:

import myModule = require("super-modules/my-module");

(Subsequent versions of TS will add more flexible ways to write this, based on ES6.)

Nagging question that can’t be glossed over: What happens to the string "super-modules/my-module" ? How is it interpreted? In the output JS it’s easy: it is just kept exactly as it is. So your module system better understand it. But the compiler also wants to find a TS file at compile time, to provide type information for the myModule variable.

Suppose the importing module is saved at the location:

somewhere/awesome-code/not-so-much/domestic/toaster.ts

The compiler will try these paths, in this order, until one exists:

somewhere/awesome-code/not-so-much/domestic/super-modules/my-module.ts

somewhere/awesome-code/not-so-much/super-modules/my-module.ts

somewhere/awesome-code/super-modules/my-module.ts

somewhere/super-modules/my-module.ts

i.e. it searches up the tree until it runs out of parent directories. (It will also accept a file with the extension .d.ts , or it can be “tricked” into not searching at all, but we’ll get to that later).

This is a little different to node’s take on CommonJS, where you’d only get that behaviour if your import path started with ./ – otherwise it inserts node_modules in the middle. But this doesn’t matter, as we’ll see.

One advantage of external modules over the first pattern we tried is that it avoids name clashes. Every module decides what name it will use to “mount” modules into its own namespace. Also note that by importing an external module in this way, your module also becomes external. Nothing you declare globally will actually end up as properties of the global object (e.g. window ) any more.

So we have two kinds of file: external modules, and what I’m going to call plain files. The latter just pollute the global namespace with whatever you define in them. The compiler classifies all files as plain files unless they make use of import or export at the top level.

How do you call JavaScript from TypeScript?

No need to explain why this is an important question, I guess. The first thing to note is that widely-used JS libraries are packaged in various ways, many of them having longer histories than any popular JS module systems.

What if you’re dealing with something like jQuery and in your own JS you’ve been blithely assuming that $ exists globally? What you’re wishing for is that someone would rewrite jQuery as a plain TS file that says something like:

function $(selector: any) { // Um... }

No use of export , see? It’s a little trickier than that in reality because $ is not just a function; it has properties of its own. Don’t worry – TS has ways to declare that.

Of course, no one can be bothered to rewrite jQuery in TS and fortunately they don’t have to. TypeScript supports ambient declarations, which are prefixed with the keyword declare like this:

declare var x: number; declare function f(): number;

These tell the compiler that somehow arrangements will be made such that the global namespace has properties x and f with those particular shapes. Just trust that they’ll be there, Mr Compiler, and don’t ask any questions. In fact the compiler won’t generate any output code for ambient declarations. (If you’re familiar with the old world of C , think header files, prototypes and extern ).

Note that I don’t initialise x or provide a body for f , which would not be allowed; as a result the compiler cannot infer their types. To make the declarations be worth a damn, I specify the type number where necessary.

Finally, you can make sure that a file contains only ambient declarations by naming it with the extension .d.ts . That way, you can tell at a glance whether a file emits code. Your linking process (whatever it is) never needs to know about these declaration files. (Again, by analogy to C , these are header files, except the compiler bans them from defining anything. They can only declare.)

(In case you’re panicking at this point, it isn’t necessary to write your own declarations for jQuery, or for many other libraries (whether in the browser or Node). See DefinitelyTyped for tons of already-written ones.)

What if third party code does use a module system such as CommonJS? For example, if you’re using TS in Node and you want to say:

import path = require("path");

You have a couple of options. The first, and least popular as far as I can tell, is to have a file called path.d.ts that you put somewhere so it can be found by the compiler’s searching algorithm. Inside that file you’d have declarations such as:

export declare function join(...path: string[]): string;

The other option is that you have a file called path.d.ts that you put anywhere you like, as long as you give it to the TS compiler to read. In terms of modules it will be a plain file, not an external module. So it can declare anything you want. But somewhere in it, you write a peculiar module declaration:

declare module "path" { export function join(...path: string[]): string; }

Note how the module name is given as a quoted string. This tells the compiler: if anyone tries to import "path" , use this module as the imported type structure. It effectively overrides the searching algorithm. This is by far the most popular approach.

Reference comments

In some TS code you’ll see comments at the top of the file like this:

///<reference path="something/blah.d.ts" />

This simply tells the compiler to add that file (specified relative to the containing directory of the current file) to the set of files it is compiling. It’s like a crummy substitute for project files. In some near-future version of TS the compiler will look for a tsconfig.json in the current directory, which will act as a true project file (the superb TypeStrong plugin for the Atom editor already reads and writes the proposed format).

In Visual Studio projects, just adding a .ts file to a project is sufficient to get the compiler to read it. The only reason nowadays to use reference comments is to impose an order in which declarations are read by the compiler, as TypeScript’s approach to overloading depends on the order in which declarations appear.

DefinitelyTyped and tsd

If you install node and then (with appropriate permissions) say:

npm install -g tsd

You’ll get a command-line tool that will find, and optionally download, type definition files for you. e.g.

tsd query knockout

Or if you actually want to download it:

tsd query knockout --action install

This will just write a single file at typings/knockout/knockout.d.ts relative to the current directory. You can also add the option --save :

tsd query knockout --action install --save

That will make it save a file called tsd.json recording the precise versions of what you’ve downloaded. They’re all coming from the same github repository, so they are versioned by changeset.

Migrating

I uhmm-ed and ahhh-ed for a while trying to decide what approach to take with my existing JS code. Should I write type declarations and only write brand new code in TS? Should I convert the most “actively developed” existing JS into TS?

The apparent dilemma stems from the way that .d.ts files let you describe a module without rewriting it, and “rewriting” sounds risky.

But it turned out, in my experience, that this is a false dilemma. The “rewriting” necessary to make a JS file into a TS file is

Not that risky, as most of the actually code flow is completely unmodified. You’re mostly just declaring interfaces, and adding types to the variable names wherever they’re introduced. Phenomenally, indescribably worth the effort. By putting the types right in the code, the TS compiler helps you ensure that everything is consistent. Contrast this with the external .d.ts which the compiler has to trust is an accurate description. A .d.ts is like a promise from a politician.

In the end, I decided that the maximum benefit would come from rewriting two kinds of existing JS:

Anything where we have a lot of churn.

Anything quite fundamental that lots of other modules depend on, even if it’s not churning all that much.

You may come to a different conclusion, but this is working out great for me so far. Now when someone on the team has to write something new, they do it in TS and they have plenty of existing code in TS to act as their ecosystem.

I think that’s everything. What have I missed?