# TypeScript in the Wild

When I previously used JavaScript, the grand debate was which JS utility library to use: jQuery vs. mootools vs. YUI vs. dojo. While I’m glad this tradition of grand debates has continued (Angular vs. React vs. Vue), the community has also quickly coalesced around the biggest improvement to JavaScript in the last decade: TypeScript.

In this post, I want to talk about my experience migrating an existing JavaScript codebase to TypeScript. But first, a quick digression on why I love TypeScript, and how TypeScript is the culmination of a number of great ideas in PL.

The first great idea is JavaScript as a compilation target. We took baby steps with transpiled languages in the late aughts with CoffeeScript (which still looked mostly like Javascript), but the state of the art has progressed such that in 2020 nearly any language (Python, Java, C++, Ruby) can be compiled to JavaScript, and with near-native levels of performance to boot. Even pure JavaScript developers will commonly use transpilers to work around uneven ES language support across browsers.

The second great idea is gradual typing. Previous attempts to bring types to JavaScript usually required rewriting everything in a new language, which meant a huge initial porting effort as well as losing access to the JavaScript library ecosystem. TypeScript took a different tack and was designed as a strictly compatible superset of JavaScript. While this limits what TypeScript can do as a language, it means you can easily convert an existing JavaScript codebase to TypeScript, incrementally gaining value as you continue to add type annotations.

The third great idea is developer tooling. The TypeScript support in VS Code and other IDEs is excellent, thanks to the TypeScript language server. The type annotations at DefinitelyTyped are frequently updated and cover all the popular packages I’ve used. The official TypeScript documentation is approachable and up-to-date. It’s clear that Microsoft wants TS to be useful for real-life working developers, and that effort has paid dividends in the form of a vibrant developer community.

Given how much obviously better TypeScript is than JavaScript, perhaps its rapid ascent is unsurprising. I think it’s an incredible innovation not just for JavaScript, but for programming languages in general.

That said, TypeScript is a very complicated and powerful language, and you have to understand its limitations to use it correctly. Also, no matter the language, sometimes bad code is just bad code.

Let’s begin.

## Implicit anys abound

TypeScript has a special any type that allows a variable to be assigned any value. Since this circumvents the point of a type system, use of any is strongly discouraged. The task of converting an existing JS codebase to TS essentially boils down to replacing all the any types with real types.

This can be daunting since every unannotated variable starts off, implicitly, as an any. This is the blessing and the curse of a gradually typed language like TypeScript.

When we ran an initial type check with noImplicitAny enabled, there were more than ten thousand of these errors in our <100K line codebase. Granted, a lot of these were easy to fix, but we weren’t going to be able to drive this to zero any time soon. Meanwhile, without enforcement of noImplicitAny in our PR checks, more anys were sneaking into the codebase with every new commit.

The solution we came up with was a codemod that created a new type $FIXME = any and added an explicit$FIXMEannotation wherever there was an implicit any. This was good for two reasons: it let us turn on noImplicitAny enforcement for new commits, and also made it clear if the any was added automatically by the codemod or by a person who had looked at it and wasn’t sure of the type.

With noImplicitAny enabled, we still had to actually add types. We encouraged developers to add proper types when working in an area of the code, but it wasn’t yielding good results. A lot of core functions were still taking and returning any types, so all we’d achieved was making our implicit anys in the rest of the code explicit. We needed a better approach.

The more systematic method was to add types going bottom up in the dependency graph. This started with installing TypeScript annotations for all third-party packages (thank you, DefinitelyTyped!). Then, adding types for our most common Mongoose models, since almost every meaningful operation involved querying the database. Next, model helpers and shared utility code, and after that, the most important pieces of business logic. Eventually, we had enough type coverage that we could reasonably expect that new code would not contain additional any types.

This was a huge milestone, and at this point we were reaping a lot of benefit from TypeScript.

## Just because you can, doesn’t mean you should

Another set of problems we quickly ran into were ambiguous objects with unclear types. Since it’s better to have a too-wide type than an incorrect type, we’d leave behind an explicit any and a TODO to indicate a complicated situation we weren’t sure about.

When digging into some of these complicated cases, I found some truly crazy code. As a hypothetical example, let’s suppose you’re working at a food truck with the following menu:

• Hot dog, which can be topped with sauerkraut or peppers, and sauced with relish, ketchup, or mustard
• Single or double patty hamburger, which can be topped with cheese, lettuce, tomato, or onion, and sauced with ketchup or mayonnaise.
• French fries, which can come with ketchup, mustard, or mayonnaise.

Starting with your homegrown ordering system written in JavaScript, an initial attempt at typing the Order API request object might look like this:

type Sauce = 'relish' | 'ketchup' | 'mustard' | 'mayonnaise';
type Meat = 'hotdog' | 'hamburger';
type Topping = 'cheese' | 'lettuce' | 'tomato' | 'onion';

interface Order {
type: 'hotdog' | 'hamburger' | 'fries';
customer: string;
amountCents: number;
params: {
meat?: Meat | Meat[];
sauerkraut?: boolean;
peppers?: boolean;
toppings?: Topping[];
isDoubleCheeseBurger?: boolean;
sauce?: Sauce | Sauce[];
extraFries?: boolean;
extraFriesParams?: {
sauerkraut?: boolean;
peppers?: boolean;
sauce?: Sauce | Sauce[];
}
}
}

Looking at it, this type is difficult to use. It still allows a lot of invalid combinations of items, the toppings are modeled differently for hot dogs and hamburgers, meat can be a single item, an array, or undefined, sauce is different for standalone fries vs. extra fries, and isDoubleCheeseBurger could be set even if it’s not a double patty burger and cheese isn’t a topping.

Let’s apply TypeScript best practices and model this as a discriminated union instead:

type BaseOrder = {
customer: string;
amountCents: number;
}

type FriesSauce = 'ketchup' | 'mustard' | 'mayonnaise';
type ExtraFries = {
params: {
extraFries?: boolean;
extraFriesParams?: {
sauerkraut?: boolean;
peppers?: boolean;
sauce?: FriesSauce[];
}
}
}

type HotdogSauce = 'relish' | 'ketchup' | 'mustard';
type HotdogOrder = BaseOrder & ExtraFries & {
type: 'hotdog';
params: {
meat?: 'hotdog'
sauerkraut?: boolean;
peppers?: boolean;
sauce?: HotdogSauce | HotdogSauce[];
}
}

type HamburgerSauce = 'ketchup' | 'mayonnaise';
type HamburgerTopping = 'cheese' | 'lettuce' | 'tomato' | 'onion';

type HamburgerOrder = BaseOrder & ExtraFries & {
type: 'hamburger';
params: {
meat?: 'hamburger';
toppings?: HamburgerTopping[];
isDoubleCheeseBurger?: false;
sauce?: HamburgerSauce | HamburgerSauce[];
}
}

type DoubleHamburgerOrder = BaseOrder & ExtraFries & {
type: 'hamburger';
params: {
meat?: ['hamburger' | 'hamburger'];
toppings?: HamburgerTopping[];
isDoubleCheeseBurger: boolean;
sauce?: HamburgerSauce | HamburgerSauce[];
}
}

type FriesOrder = BaseOrder & {
type: 'fries';
params: {
sauce?: FriesSauce | FriesSauce[];
}
}

type Order = HotdogOrder | HamburgerOrder | DoubleHamburgerOrder | FriesOrder;

This is a lot better! We’re able to ensure the right combinations of meat, sauce, and toppings for each order, that isDoubleCheeseBurger is only set for double patty hamburgers, and we’ve prevented invalid orders like fries with extra fries.

However, there are still some major issues:

• The inconsistent topping styles between hot dogs, hamburgers, and double cheese burgers.
• We were unable to unify the types for Fries and ExtraFries. Also maybe instead of ExtraFries, an order should contain multiple items?
• All the Order params are optional, which means we annoyingly need to check and handle nulls on access.

At this point, we’ve gone about as far as we can within just the type system. There’s some tech debt to clean up in the business logic before we can further improve these definitions.

Also, note that even this innocuous set of types might surface undocumented behavior in our business logic! Perhaps the chef lets her friends order special off-menu items like hot dogs with cheese. We’ll need to track down these special cases and reconcile our types with the code.

## You still need a data model

In the food truck example, we were actually fortunate in that we had a preexisting spec (the menu) which mostly described how the system should work. Sometimes, you aren’t even that lucky: you’ll be shown a MongoDB collection with records from different schema versions and plenty of mixed types, or a bunch of hand-coded backend routes without parameter validation.

TypeScript’s type checker is only useful insofar as the type accurately describes the data. If you’re dealing with untrusted input and can’t be sure about the object’s structure, you need to validate it first.

As an example, maybe in the past we used to serve gyros at our food truck. We’ve since removed it from our menu and deleted all the gyro related code, but there are still old Order records in our DB with type: gyro.

What happens when we write a quick script to analyze our order history?

const orders = await OrderModel.find({}) as Order[];

const sums: Record<Order["type"], number> = {
hotdog: 0,
hamburger: 0,
fries: 0,
}

for (const order of orders) {
sums[order.type] += 1
}
console.log(sums); // Will see a 'gyro' property with invalid value 'NaN'!

TypeScript only runs at compile time, so the type system can’t catch this error. Unless we validate input at runtime, we can run into trouble.

So, use database-side schema validation (supported by MySQL, PostgreSQL, MongoDB, etc), and use an RPC framework that helps with modeling and validation (OpenAPI, GRPC, etc). These techniques synergize wonderfully with TypeScript since you can often generate TS types from these schemas, resulting in a chain of strong typing from the client to the server to the DB and all the way back out again.

## Type assertions are any by another name

TypeScript lets you assert (or cast) the type of a variable using a type assertion. As a beginning TypeScript programmer, you will quickly encounter situations where you need to use type assertions, and they are discussed at length in the official docs and other online resources:

• Since TypeScript types do not exist at runtime, use type guards to determine an object’s type based on its structure and assert that it is that type.
• The ! non-null assertion operator narrows a union type by removing null|undefined.
• When you know something the compiler doesn’t, e.g. you’ve enabled schema validation on your database and know all the records are well-formed.

The big mistake is using a type assertion when you aren’t 100% sure about the type. Type guards are fine since they validate the object before asserting the type. I have a less favorable view of the ! operator. It doesn’t validate, is used in the same context as the very common and useful ? optional chaining operator, but is really just a type assertion. If you’re sure that the value is not null, ask yourself, could I fix this instead where the value was originally assigned? Or just do a null check (using an assertion signature):

function assertDefined<T>(obj: T): asserts obj is NonNullable<T> {
if (obj === undefined || obj === null) {
throw new Error('Must not be a nullable value');
}
}

We don’t need to discuss the issues with raw type assertions. They’re a code smell, and should be minimized and isolated whenever possible.

As an aside, I also wish the TypeScript designers had just called it “cast” instead of “type assertion”, since in C-like languages (including JavaScript!) assert is a runtime error check, not a compile-time unchecked type conversion.

## Accidentally widening inferred types

Type inference is another powerful language feature that requires some care. The lack of type inference was historically a major critique of Java, since you would have to “tell it twice” when declaring and defining a variable:

HashMap<String, HashMap<String, String>> mapping = new HashMap<String, HashMap<String, String>>();

Why do I need to say mapping is a HashMap<String, HashMap<String, String>> when it’s being assigned a value that’s a HashMap<String, HashMap<String, String>>? Java 7 gave us a half-measure with the diamond operator, but Java 10 finally gave us the var keyword which makes this a lot snappier:

var mapping = new HashMap<String, HashMap<String, String>>();

This works great! The type of this variable is unambiguous for both the compiler and the reader since it’s being assigned immediately.

Typescript extends this even further and will automatically infer the types of function return values. This is nice since it means you can immediately get some typechecking when starting with unannotated JavaScript:

function isEven(n) { // Typescript infers the return type is boolean
return n % 2 === 0;
}

This is better than getting an implicit any return type! However, I think this practice should be discouraged when going full Typescript. Imagine our function is later modified as follows:

function isEven(n) { // Return type is inferred as boolean|null
if (typeof n !== 'number') {
return null;
}
return n % 2 === 0;
}

Due to type inference, we accidentally widened the return value from boolean to boolean|null. This will start throwing type errors near all the isEven callsites when callers try to use the new wider return value. We can still debug this, but it’d be easier if the compiler flagged the incorrect null return value instead:

function isEven(n): boolean {
if (typeof n !== 'number') {
return null; // Type 'null' is not assignable to type 'boolean'.
}
return n % 2 === 0;
}

Function signatures are a contract between the caller and the callee. By being explicit about the function’s intended behavior, it makes it clear whether a change in behavior is accidental or deliberate.

## Roundup of other pitfalls

There are a few other miscellaneous things I’ve run into and want to briefly cover:

• Objects can have extra properties beyond what’s specified in the type. This can bite you during serialization since simply calling JSON.stringify will serialize these extra properties too. Pick out only the properties you want with lodash first.
• Although Record<string, string> looks very similar to a Map<String, String> from Java, you almost always want to use a Partial<Record<string, string>>. This makes it clear that undefined will be returned for keys that aren’t present (which is what will happen, whether Typescript thinks so or not). Even better is using a union of string literals as keys, like in our order analysis script.
• Beware the subtle differences when iterating the keys of string and numeric enums. Numeric enums contain both a forward and reverse mapping, while string enums just have the forward mapping.
• I could go on about the numerous warts carried over from JavaScript, but that’s deserving of a separate post.

## Conclusion

This was a peek into my experience converting a decently large codebase from JavaScript to TypeScript. There’s a definite learning curve to TypeScript’s gradual, structural, algebraic type system, but it’s an incredibly powerful productivity improvement and I’m glad that Microsoft has invested so much in making it a success.

My thanks go out to great resources like the official TypeScript docs, the TypeScript Playground, and the Effective TypeScript book from O’Reilly which has even more in-depth advice like this about using TypeScript.

I’m always interested to hear about other’s experience with TypeScript. What do you like and not like about the language? Are there any stories you’d share in a post like this one? Let me know in the comments!

Thanks to my editors Calvin Huang and Steven Salka, who went on this TypeScript journey with me and taught me a lot along the way!