A curated collection of practical TypeScript patterns that improve safety, readability, maintainability, and developer experience.

Most of these are small individually. Together, they dramatically change how TypeScript code feels to work in.

Table of Contents

1. Prefer unknown Over any
2. Let Type Inference Do the Work
3. Prefer satisfies Over as
4. Derive Types From Values
5. Make Invalid States Impossible to Represent
6. Use Exhaustive Checks With never
7. Use as const for Constants
8. Use Type Predicates
9. Build Types From Existing Types
10. Validate External Data at Runtime
11. Avoid enum in Most Cases
12. Prefer Inferable Generics
13. Enable Strict Compiler Options
14. Learn Template Literal Types
15. Type Safety ≠ Runtime Safety

Prefer unknown Over any

A lot of type safety starts here.

unknown forces you to prove what a value is before using it. any skips the type system entirely, allowing unsafe operations to spread through your code.

function parse(data: unknown) {
  if (typeof data === "string") {
    return data.toUpperCase();
  }
}

Why it matters

• Forces validation before use
• Preserves type safety
• Prevents unsafe type leakage

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Let Type Inference Do the Work

The best TypeScript code often relies on inference instead of repeating information the compiler already knows.

Instead of:

const name: string = "Ada";

Over-annotation

• Widens types
• Hurts inference
• Creates maintenance overhead

Inference tends to scale better than annotation.

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Prefer satisfies Over as

One of the most important modern TypeScript features.

const routes = {
  home: "/",
  about: "/about",
} satisfies Record<string, string>;

Instead of:

const routes = {
  home: "/",
  about: "/about",
} as Record<string, string>;

satisfies checks that a value matches a type while preserving its inferred type.

Use satisfies when validating object shapes. Reserve as for cases where you're expressing information the compiler genuinely can't infer.

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Derive Types From Values Instead of Duplicating Them

One of the biggest TypeScript mindset shifts.

const roles = ["admin", "user", "guest"] as const;

type Role = (typeof roles)[number];

This creates a single source of truth. If the runtime values change, the type updates automatically, eliminating duplication and preventing the two from drifting apart.

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Make Invalid States Impossible to Represent

Good TypeScript models don't just describe data, they prevent impossible combinations from existing in the first place.

Discriminated unions are one of the most effective ways to model these constraints.

type State =
  | { status: "loading" }
  | { status: "success"; data: User }
  | { status: "error"; error: Error };

These models scale much better than loose optional property blobs because invalid states simply can't be represented.

Future refactors become safer because the compiler ensures every valid state is handled.

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Use Exhaustive Checks With never

Once you've modeled your states as a discriminated union, exhaustiveness checking ensures every case is handled.

default: {
  const exhaustive: never = state;
  return exhaustive;
}

Add a new state, and the compiler immediately points out every place that needs updating.

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Use as const for Configuration and Constants

Without as const:

const theme = {
  mode: "dark",
};

mode becomes string.

With as const:

const theme = {
  mode: "dark",
} as const;

Now it becomes 'dark'.

A small feature that dramatically improves inference for configuration objects and constants.

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Use Type Predicates for Reusable Narrowing

Connect runtime checks to compile-time intelligence.

function isUser(value: unknown): value is User {
  return typeof value === "object" && value !== null && "id" in value;
}

Then:

if (isUser(data)) {
  data.id;
}

This becomes especially useful around APIs and external input boundaries.

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Build New Types From Existing Types

Think in transformations instead of duplication.

type UserPreview = Pick<User, "id" | "name">;

Learn these utility types

• Pick
• Omit
• Partial
• Required
• Indexed access types

These utilities become much more valuable as applications grow.

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Validate External Data at Runtime

TypeScript does not validate API responses.

This is one of the most misunderstood parts of TypeScript.

const UserSchema = z.object({
  id: z.string(),
  name: z.string(),
});

Every API response, form submission, environment variable, JSON file, and user input is an untrusted boundary.

TypeScript can't validate external data; you need runtime validation for that.

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Avoid enum in Most Cases

Usually simpler:

const roles = ["admin", "user"] as const;

Than:

enum Role {
  Admin,
  User,
}

In most application code, literal unions are easier to refactor, serialize, and reason about than enums.

Enums still have valid use cases, but they're often unnecessary.

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Prefer Generics That Infer Automatically

Great TypeScript APIs rarely require manual generic arguments.

Less ideal:

Better:

Inference usually scales better than annotation-heavy APIs.

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Turn On the Strict Compiler Options

Many teams use TypeScript in "autocomplete mode."

Strict mode is where TypeScript really starts paying off.

{
  "strict": true,
  "useUnknownInCatchVariables": true,
  "noUncheckedIndexedAccess": true,
  "exactOptionalPropertyTypes": true
}

These flags dramatically improve correctness.

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Learn Template Literal Types

One of the most powerful modern TypeScript features.

type Route = `/api/${string}`;

Excellent for:

• Routes
• Event names
• CSS utilities
• Design systems
• Query keys

Once you start using them, they show up everywhere.

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"Type-Safe" Does Not Mean "Runtime Safe"

A perfect final tip because it reframes everything.

This compiles:

const user = (await response.json()) as User;

But it may still fail at runtime.

TypeScript improves correctness, but it isn't a runtime safety net.

• It does not validate external data
• It does not guarantee good architecture
• It does not eliminate runtime bugs

Use TypeScript to model your program well. Then validate anything that comes from the outside world.

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