C · Reuse · intermediate · Week 3 · Chapter 6 of 11

Generics

Generics are type parameters — reusable functions and types that work across shapes while preserving type relationships. <T> on functions, interfaces, and classes. Constraints with extends. Avoid generic abuse — if T appears once, skip it.

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Generic functions

function first<T>(arr: T[]): T | undefined returns element type matching input. Caller picks T via argument.

Example

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

const n = first([1, 2, 3])     // number | undefined
const s = first(["a", "b"])   // string | undefined

Do this now

Write first, last, and findById<T>(items: T[], id: number, key: keyof T).

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Chapter learning outcomes

Generic functions
Generic interfaces
Constraints (extends)
Default type params
Generic pitfalls

Chapter videos

Fireship

TypeScript in 100 seconds

Types without the enterprise fog.

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Clear these before you leave

first/last/findById generics work
ApiResponse<T> wraps fetch results
sortByKey with keyof constraint
Removed unnecessary generics

Side quest

Generic cache

Create Cache<T> class with get(key: string): T | undefined, set(key: string, value: T). Type-safe for any T.