Mike Saburenkov

TypeScript. Bad Parts

I've been writing TypeScript for more than a year now and gone through well-known five stages of grief so it's time to share my experience. It all started as a compilation of tips & tricks I've discovered so far. Then it went to a more meta level of what types bring to JS at all. And as I was writing two pieces asynchronously I began to find myself noticing ‘bad’ parts here and there. At some point, there were enough of them for a separate discussion. So to keep those original ideas free of them, here they are, unordered.


Everything is pointing out it could be considerably faster. I should admit it's slow but stable so if authors had to choose between those, they did the right choice, but hey, it's v3 already, maybe it's time to address that.

Cryptic error messages

They are. When you rely much on type inferring, it's even worse. Basically, they are generic stack traces which most of the times I see in a hint window in VSCode. They're not alone, some other languages are also known for it, but that's not an excuse.

Types vs. interfaces

As of TS v3, I don't see any difference in them, practically speaking. Yes, interfaces can be extended and merged but differences are so subtle so it adds more confusion in the current state of things than benefits.

Const enums

If you prepend enum declaration with const the compiler inlines it without creating an object. I can't see why I have to make that choice, the compiler has all the knowledge (actually even more than me) to decide whether it makes sense or not.

Index signatures

In JS you can put anything as a key in object and it will be converted to a string. TS decided to improve that a bit and allows { [key: number]: boolean }. Basically, it points that even though key is to be coerced to a string, let's check that we pass only numbers here. But then we get these:

type A = { [k: number]: any };
const a2: A = { 2: true }; // alright
const a1: A = { string: true }; // error, fine
const a3: A = { "2": true }; // no error?

type B = { [k: string]: any };
const b1: B = { string: true }; // alright
const b3: B = { "2": true }; // no error, fine
const b2: B = { 2: true }; // no error?

And Object.keys() is not a generic, because it can't be as it always returns strings. The coercion was irreversible, nobody can convert strings back to original types for you. You're alone.

Object.keys(); // {} => string[]

And the last, not the least, is that you can only use primitives there, not your own types:

type Id = number;
type Item = { id: Id };
type Index = { [k: Id]: Item }; // nope, number please

Type Inference

That's nice. You feel good when you can drop obvious cases. It feels like language has done its homework. But then it starts taking defaults when there are multiple options and now you should be aware of them.

const x = [2, true];
// (number | boolean)[], not [number, boolean]

And when you return different shapes of objects from a function (say, redux reducer), inference creates monsters.

function foo(x: boolean, arr: []) {
  if (x) {
    return { a: true, "3": 2 };
  } else {
    return { b: 2, 3: arr.length };
// Inferred: {'a': boolean; '3': number; 'b'?: undefined;}
//           | {'bar': number; 3: 0; 'a'?: undefined}
// Meant to be: {a?: boolean; '3': number; b?: number;}

Possible solution might be to add strict check similar to disabling implicit ‘any’ to avoid this monsters guessing and fail to infer when it is ambigious.

Global scope

If you write TS for a browser, thus adding dom in compilerOptions/lib, lots of useful types become available in the global scope. And you don't need to import them explicitly. You know what it means. And they aren't grouped into a namespace. Yup. Some are easy to expect as XMLHTTPRequest but others have more abstract names: AlignSetting, Transport. You will notice errors if you would try to define them in your code:

type Transport = ...;
// duplicate identifier, was already declared in dom.d.ts

But you might miss the error if you just forget to define it and thus built-in will be used. And even worse will happen if you clash with built-in interfaces, as they will be merged:

interface Account {
  foo: "bar";
let x: Account = { foo: "bar" };
// missing displayName, id, rpDisplayName from type 'Account'.

Official documentation

It's lacking behind. TS v3.3 is the latest now. BigInt has been added in v3.2 but official documentation hasn't been updated.


JS started to feel heavy when they added object destructuring. Then defaults and renaming came. Only type annotations were missing. Oh, it could be nested. I know it can be easily avoided by code style rules but still sometimes I find myself hitting the timeout in trying to parse the function declaration.

Alright. The path for more positive things looks much clear now. If you have any comments, please reach out to me, I'd love to discuss that in more details. And stay tuned, originally planned articles will follow.


This article started a series on TypeScript. You can read them in any order as they aren't really connected apart from covering the same topic: