8 Steps to Migrating from JavaScript to TypeScript

June 04 2015

Migrating from JavaScript to TypeScript is no easy task. See how we managed to do it in 8 steps. Read More.

Recently, we’ve been moving our Browser RUM agent from JavaScript to TypeScript. Though it’s been a challenge, we enjoyed seeing how the change will benefit us and it’s been fun learning a new language in the process. Let me share a little of how we migrated to TypeScript, some of the difficulties that arose and how we tackled them.

Why TypeScript

Before moving to TypeScript, our Browser RUM agent had thousands lines of code, but was suppressed in to just two JavaScript files.

We felt obligated to refactor it before doing any real work, to make our life easier when adding additional features. Having experienced the pain of developing a large scale app in JavaScript, we decided to take a shot at their sibling languages that have better support for large-scale development.

After looking into languages such as TypeScript, CoffeeScript, and PureScript, we decided to go with TypeScript for a few reasons:

  1. Static Typing
  2. Module and Classes
  3. Superset of JavaScript, easier to learn for JavaScript developers
  4. Success story from our front-end team

8 Steps to Migrating to TypeScript

  1. Prepare Yourself

  1. Rename Files

We renamed all the js files to ts files and as TypeScript is just a superset of JavaScript, you can just start compiling your new ts files with the TypeScript compiler.

  1. Fix Compiling Errors

There were quite a few compiling errors due to the static type checking by the compiler. For instance, the compiler will complains about js code below:
Example One

var xdr = window.XDomainRequest;


// declare the specific property on our own
interface Window {
    XDomainRequest?: any;

Since “XDomainRequest” is an IE only property to send cross domain request, it’s not declared in the “lib.d.ts” file (it’s a file declaring the types of all the common JavaScript objects and APIs in the browser and it’s referenced by typescript compiler by default).
You will get “error TS2339: Property ‘XDomainRequest’ does not exist on type ‘Window’.”.
The solution is to extend the Window interface in “lib.d.ts” with an optional “XDomainRequest” property.

Example Two

function foo(a: number, b: number) {



// question mark the optional arg explicitly
function foo(a: number, b?: number) {

Optional function args need to be marked explicitly in typescript, or it gives “error TS2346: Supplied parameters do not match any signature of call target.”.
The solution is to explicitly use “?” to mark the parameter as optional.

Example Three

var myObj = {};
myObj.name = "myObj";


// use bracket to creat the new property
myObj['name'] = 'myObj';
// or define an interface for the myObj
interface MyObj {
    name?: string

var myObj: MyObj = {};
myObj.name = 'myObj';

When assign an empty object “{}” to a variable, typescript compiler infers the type of the variable to be empty object without any property.
So accessing “name” property gives “error TS2339: Property ‘name’ does not exist on type ‘{}'”.
The solution is to declare an interface with an optional “name” property for it.

It’s kind of fun to fix these errors and you learn about the language and how the compiler can help.

  1. Fix Test Cases

After successfully getting a JavaScript file from those ts files, we ran the tests against the new JavaScript files and fixed all the failures.

One example of the test failures caused by moving to TypeScript is the difference between these two ways of exporting a function:

export function foo() {}
export var foo = function() {}

Assuming your original JavaScript code is:

var A = {
    foo: function() {},
    bar: function() {foo();}

The test case shows:

var origFoo = A.foo;
var fooCalled = false;
A.foo = function(){fooCalled = true;};
A.foo = origFoo;

If the TypeScript rewrite for the JavaScript is:

module A {
    export function foo() {}
    export function bar() {foo();}

The test case will fail. Can you tell why?

If you look at the generated JavaScript code, you will be able to see why.

// generated from export function foo() {}
var A;
(function (A) {
    function foo() { }
    A.foo = foo;
    function bar() { foo(); }
    A.bar = bar;
})(A || (A = {}));

In the test case, when the A.foo is replaced, you are just replacing the “foo” property of A but not the foo function, the bar function still calls the same foo function.

export var foo = function(){}

can help here.


module A {
    export var foo = function () { };
    export var bar = function () { foo(); };


// generated from expot var foo = function() {}
var A;
(function (A) {
    A.foo = function () { };
    A.bar = function () { A.foo(); };
})(A || (A = {}));

Now we can replace the foo function called by A.bar.

  1. Refactor Code

TypeScript Modules and Classes help organize the code in a modularized and object-oriented way. Dependenies are referenced in the file header.

///<reference path=“moduleA.ts” />
///<reference path=“moduleB.ts” />
module ADRUM.moduleC.moduleD {

One thing I like when compiling a ts file is using the “–out” option to concatenate all the directly or indirectly referenced ts files, so I don’t need to use requirejs or browserify for the same purpose.

With TypeScript, we can define classes in a classical inheritance way rather than the prototypal inheritance way which is more familiar to Java and C++ programmers. However, you lose the flexibility JavaScript provides too.

For example, if you are seeking a way to hide a function in the class scope, save your time, it isn’t supported. The workaround is to define the function in the module and use it in the class.

TypeScript allows you to define modules and classes in an easy way and generates the idiomatic JavaScript for you. As a result, I feel like you may also have less opportunities to learn more advanced JavaScript knowledge than programming in pure JavaScript.

But just like moving from assembly to C/C++, by and large, it’s still a good thing.

We did not bother adding all the type information in the existing code, but we’ll need to do it when changing or adding code.

It is also worth moving the test cases to TypeScript, as the test cases could be auto updated when refactoring the code in the IDE.

  1. Fix Minification

Don’t be surprised if the minification is broken especially when you use Google Closure Compiler with advanced optimization.

Problem 1: Dead Code Mistakenly Removed

The advanced optimization has a “dead code removal” feature that removes the code which recognized as unused by the compiler.

Some early version closure compiler (i.e. version 20121212) mistakenly recognizes some code in TypeScript modules as unused and removes them. Fortunately, it’s been fixed in the latest version compiler.

Problem 2: Export Symbols in Modules

To tell the compiler not to rename the symbols in your code, you need to export the symbols by the quote notation. It means you need to export the API as shown below to allow the API name to stay constant even with the minified js file.

module A {
    export function fooAPI() { }
    A["fooAPI"] = fooAPI;

transpiled to:

var A;
(function (A) {
    function fooAPI() { }
    A.fooAPI = fooAPI;
    A["fooAPI"] = fooAPI;
})(A || (A = {}));

It’s a little bit tedious. Another option is to use the deprecated @expose annotation.

module A {
    * @expose
    export function fooAPI() { }

This looks like it will removed in future, and hopefully you might be able to use @export when it’s removed. (Refer to the discussion at @expose annotation causes JSC_UNSAFE_NAMESPACE warning.)

Problem 3: Export Symbols in Interfaces

If you define a BeaconJsonData interface to be passed to other libraries, you’ll want to keep the key names.

interface BeaconJsonData {
    url: string,
    metrics?: any

@expose does not help as the interface definition transpile to nothing.

interface BeaconData {
    * @expose
    url: string,
    * @expose
    metrics?: any

You can reserve the key names by quote notation:

var beaconData: BeaconData = {
    'url': "www.example.com",
    'metrics': {}

But what if you want to assign the optional key later?

var beaconData: BeaconData = {
    'url': "www.example.com"

// ‘metrics’ will not be renamed but you lose the type checking by ts compiler
// because you can create any new properties with quote notation
beaconData["metrics"] = {…};
beaconData["metricsTypo"] = {}; // no compiling error

// ‘metrics’ will be renamed but dot notation is protected by type checking
beaconData.metrics = {…};
beaconData.metricsTypo = {…}; // compiling error

What we did is to expose the key name as
/** @expose */ export var metrics;
in the interface file to prevent the closure compiler from renaming it.

  1. Auto-Generate Google Closure Compiler Externs Files

For the Closure Compiler, if your js code calls external js library’s APIs, you need to declare these APIs in an externs file to tell the compiler not to rename the symbols of these APIs. Refer to Do Not Use Externs Instead of Exports!

We used to manually create the externs files and any time we use a new API, we have to manually update its externs file. After using TypeScript, we found that TypeScript .d.ts and the externs file have the similar information.

They both contain the external API declarations — .d.ts files just have more typing information — so we can try to get rid of one of them..

The first idea came into my mind is to check if the TypeScript compiler supports minification. As the ts compiler understand the .d.ts file, it won’t need the externs files. Unfortunately, it doesn’t support it, so we have to stay with the Google Closure Compiler.

Then, we thought the right thing is to generate the externs files from the .d.ts files. Thanks to the open source ts compiler, we use it to parse the .d.ts files and convert them to externs file (see my solution at https://goo.gl/l0o6qX).

Now, each time we add a new external API declaration in our .d.ts file, the API symbols automatically appears in the externs file when build our project.

  1. Wrap the ts code in one function

Ts compiler generates code for modules like below:

// typescript
module A {
    export var a: number;

module A.B {
    export var b: number;

// transpiled to javascript
var A;
(function (A) {
})(A || (A = {}));

var A;
(function (A) {
    var B;
    (function (B) {
    })(B = A.B || (A.B = {}));
})(A || (A = {}));

For each module, there is a variable created and a function called. The function creates properties in the module variable for exported symbols. However, sometimes you want to stop execution for some conditions such as your libraries need to be defined or it has been disabled, you need to wrap all the js code in a function by yourself, like:

    if (global.ADRUM || global.ADRUM_DISABLED) {

    // typescript generated javascript goes here



Raphael Feng
Raphael Feng works in the development team of Real-User Monitoring at AppDynamics. He is developing the browser real-user monitoring product which is a cloud solution to help users to collect the data about end-user experience and analyze the corresponding business impacts in real-time to optimize the results. Prior to joining AppDynamics, he worked at Actuate and SAP on business intelligence products like cross-platform data visualizations and enterprise reporting.

Thank you! Your submission has been received!

Oops! Something went wrong while submitting the form