Safety with types: Best practices

Types test your code 🔬

Type safety collapses runtime bugs to compile-time guarantees.

There are some simple best practices that will guarantee safety in your apps long-term. Here are the 3 strategies I use the most 👇

Brands

You are working on a blogging app, so naturally you can edit a post.

What's the signature of the editPost function?

Here is the wrong (not ideal) answer:

const editPost = (postId: string, post: Post) => {
  /// ...
}

We are not safe here. Is postId really any string?

The general assumption is that you get a Post from an API, and the user can edit it. Therefore, postId must come from the API, it's not just string 🤔

Coming back to safety means using branded types (from effect) ✨

It works like this:

Define a Schema for a branded type (Schema.brand)
Decoded from the schema when fetching the Post
Use postId from the API to make the edit request

import { Schema } from "effect"

const PostId = Schema.String.pipe(Schema.brand("PostId"));
type PostId = typeof PostId.Type;

const Post = Schema.struct({
  postId: PostId,
});

// Get `Post` from an API
const getPost: Effect<Post> = getPostFromApi.pipe(
  // Decode schema for valid `Post`
  Effect.flatMap(Schema.decode(Post)),
);

const editPost = (
  postId: PostId,
  post: Omit<typeof Post.Type, "postId">
) => {
  /// ...
}

Now it's impossible to call editPost unless you have a valid PostId. And the only place where you get a valid PostId is after calling getPost.

Result: you guaranteed that editing a post is safe in all cases (all!) 💯

Note: You are safe as long as PostId is verified. Any "exception" (e.g. using as unknown as PostId) when creating branded types tosses you out of safety 😬

Literal types

Safety requires strictness 👊

Being strict with types means allowing only the minimal set of values to cover your use case 🔬

Back to the Post example.

First, it's more safe an maintainable to avoid something like isPublished: boolean, and instead opt for a more general status field.

Second, don't type status: string. Use a string literal instead:

import { Schema } from "effect"

const Post = Schema.struct({
  postId: PostId,
  status: Schema.Literal("published", "draft"),
});

Major advantages:

You can extend status with other states (e.g. in-review)
You avoid impossible states on a type-level
You can pattern match on all literals

Bonus tip: you can type a Record key using literals. You will be required to provide values for all keys.

I use this all the times for labels 📧

type Language = "en" | "it" | "jp";
type LanguageLabel: Record<Language, string> = {
  en: "English",
  it: "Italian",
  jp: "Japanese"
};

NonEmptyArray

Arrays are insidious, specifically this: arr[0] 👀

David K 🎹

@DavidKPiano

·Follow

Tip: instead of checking the array length to see if it's non-empty, you can check the first item when all the items are expected to be truthy. ❌ if (arr.length > 0) ✅ if (arr[0]) It's shorter, cleaner, and plays nicer with TypeScript's `noUncheckedIndexedAccess` setting.

interface Item {
name: string;
}

function processArray1(arr: Item[]) {
if (arr.length > 0) {
// Still need type narrowing or ! to avoid TS error
arr[0].name;
}
}

function processArray2(arr: Item[]) {
if (arr[0]) {
// Cleaner, type-safe with noUncheckedIndexedAccess
arr[0].name;
}
}

1:54 PM · Jun 10, 2025

824

Read 97 replies

First, never forget to add noUncheckedIndexedAccess to your tsconfig.json.

noUncheckedIndexedAccess adds | undefined to the typeof of arr[0] 🙌

Second, verify a value exists every time you access any index in an array. With effect you can make arr[0] safe using NonEmptyArray.

Sandro Maglione

@SandroMaglione

·Follow

There is only 1 real way of checking for non-empty array 🙌

David K 🎹

@DavidKPiano

10:48 AM · Jun 11, 2025

Read more on X

Even without effect, type safe arrays are a simple trick:

type NonEmptyArray = {
  [0]: string;
} & string[];

// Property '[0]' is missing in type '[]' but required in type '{ 0: string; }'
const notValid: NonEmptyArray = [];

const valid: NonEmptyArray = ["valid"];

And guess what? You can even make something like ArrayWithAtLeastTwoElements:

type ArrayWithAtLeastTwoElements = {
  [0]: string;
  [1]: string;
} & string[];

// Type '[]' is not assignable to type '{ 0: string; 1: string; }'
const notValid: ArrayWithAtLeastTwoElements = [];

// Property '[1]' is missing in type '[string]' but required in type '{ 0: string; 1: string; }'
const stillInvalid: ArrayWithAtLeastTwoElements = ["valid"];

const valid: ArrayWithAtLeastTwoElements = ["valid", "also-valid"];

Three weeks streak of effect on your X feed. Everyone is jumping in:

Matt Pocock

@mattpocockuk

·Follow

Effect Costs: - Basically feels like learning a new language - Unfamiliar syntax - So many API's to learn - Pay the onboarding cost initially, and keep paying it as new devs come in Benefits: - A good codebase that actually stays good

11:34 AM · Jun 22, 2025

931

Read 69 replies

If you are jumping in right now as well, join the Discord channel:

Matt Pocock

@mattpocockuk

·Follow

Effect community is good btw

Matt Pocock

@mattpocockuk

54 views/3 accurate and helpful responses is an insane ratio. Nice community you got there

8:26 AM · Jun 19, 2025