Converting to TypeScript

Written by Liam McLennan

My most recent side project is a web based curl - wurl - intended to be a convenient interface for experimenting with HTTP APIs.

Wurl is a useful tool that I created to satisfy a need - but it is also a project that I use to experiment with development techniques and technologies.

In the browser we are limited to JavaScript, a dynamic language not easily integrated with whatever server-side technology is in use (in this case Asp.Net MVC and F#). To help prevent runtime JavaScript bugs I recently converted wurl from JavaScript to TypeScript, a statically typed superset of JavaScript. The purpose of this post is to document that experience.

Where to Start?

It is not entirely obvious where to start a JavaScript -> TypeScript migration. I found that the best way is to start with bits of JavaScript that are small and have few dependencies.

Converting a JavaScript Module to TypeScript

Wurl has a JavaScript module used to contain its models. The Template model represents an HTTP request and how it is transformed.

define('models', [], function() {

    var Template = dbc.makeConstructor({
        id: [{validator:'type',args:['string']}],
        label: [{validator:'type',args:['string']}],
        request: [{validator:'type',args:['string']}],
        transformer: [{validator:'type',args:['string']}],
        view: [{validator:'type',args:['string']}]
    Template.url = '/template';

    return {
        Template: Template

Step 1 was to rename the file with a .ts extension. Visual Studio immediately displayed a compilation error because the type of the define function is not known. define is a function that comes from the smodules (small modules) JavaScript module library. To be able to use smodules, a JavaScript library, from TypeScript I need a type definitions file that defines all the types exposed by smodules. To create a type definition for smodules I create a new file with the same name as the smodules library file, but with the extension .d.ts. In that file I add a type declaration for the define function.

declare function define(name:string, dependencies: string[], moduleFactory: any): void;

This tells TypeScript that define is a function, with arguments of type string, string array and any. Define does not return anything. I also add a declaration for the other function exposed by smodules:

declare function require(name: string) : any;

The issue now is that TypeScript does not know where to look for the type definitions, so I add a reference to the models module:

/// <reference path="packages\02-smodules.d.ts" />

Now when I try to compile I get no errors about smodules, instead the compiler complains about the call to dbc.makeConstructor. dbc is a library providing runtime type assertions. Here it is being used to define a runtime ‘type’ check. This compilation error is fixed according to the same strategy: create a .d.ts file for dbc containing:

declare module dbc {
    function makeConstructor(spec: any): any;

    function check(o: any, spec: any, message?: string): void;

Using Well-Known JavaScript libraries

wurl makes use of many JavaScript libraries including:

* underscore
* react
* jquery
* q
* toastr
* codemirror

To be able to use these libraries from TypeScript I need type definitions for them all. Fortunately, there is a github repository containing many type definition files and they are available as nuget packages.


Wurl has perhaps 1,000 lines of JavaScript, so it is a small project. It took me about two hours to convert it to TypeScript, without having any idea what I was doing. It is easier to start a project with TypeScript from the beginning, however doing a JavaScript -> TypeScript conversion is perfectly feasible, even for larger projects.