TypeScript Generics

Generics let a function work for many types while keeping type safety. The syntax looks scary and the concept is simple.

CreatorsAI-Assisted Coding~27 min readAdvancedCoderBI2 · Representation & ReasoningBI4 · Natural InteractionPrint / PDF

Lesson map

What this lesson covers

45 min11 blocks4 concepts

Learning path

The main moves in order

1A Function, for Any Type
2generic
3constraint
4inference

Concept cluster

Terms to connect while reading

genericconstraintinferenceutility types

Sections2

Notes4

Code2

Terms1

Section 1

A Function, for Any Type

A generic is a placeholder that TypeScript fills in based on how you call the function. One `identity` works for strings, numbers, and users.

Generics carry the concrete type through the function. No `any`, no casting.

typescript

function identity<T>(x: T): T {
  return x;
}

const a = identity("hi");     // T = string
const b = identity(42);        // T = number
const c = identity({ id: 1 }); // T = { id: number }

function first<T>(arr: T[]): T | undefined {
  return arr[0];
}

const n = first([1, 2, 3]);    // number | undefined

Constraints narrow the allowed types

`extends` says: T can be any type as long as it has an id string. Lets you use i.id safely.

typescript

function byId<T extends { id: string }>(items: T[], id: string): T | undefined {
  return items.find((i) => i.id === id);
}

type User = { id: string; name: string };
const users: User[] = [{ id: "u_1", name: "Ada" }];

const u = byId(users, "u_1"); // typed as User | undefined
console.log(u?.name);

Check-in 1. Got it so far?

Key terms in this lesson

Check-in 2. Got it so far?

The big idea: generics carry types through reusable functions. Add constraints when you need properties, lean on built-in utility types, and stop when it stops helping.

End-of-lesson quiz

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