Custom hooks with generics & inference
Write reusable custom hooks with for values, results, and errors; leverage inference and constraints for safe APIs.
Custom hooks with generics & inference is a free TypeScript Academy lesson on CoddyKit. This is lesson 1 of 2. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the TypeScript Academy learning path, and your progress syncs across the web and the CoddyKit app. The TypeScript Academy course includes 2 lessons in total.
Intro
Goal: Build generic hooks that feel native: a typed localStorage hook, an async loader, and a controlled/uncontrolled helper. You will rely on inference so callers rarely annotate types.
- Return tuples for React-like ergonomics
- Use constraints when needed
- Keep no any
useLocalStorage<T>
Return a [value, set] tuple like useState. Keep T consistent across value and setter.
import { useEffect, useState } from "react"
export function useLocalStorage<T>(key: string, initial: T): [T, (v: T | ((prev: T) => T)) => void] {
const [value, setValue] = useState<T>(() => {
try {
const raw = localStorage.getItem(key)
return raw ? (JSON.parse(raw) as T) : initial
} catch {
return initial
}
})
useEffect(() => {
try { localStorage.setItem(key, JSON.stringify(value)) } catch {}
}, [key, value])
const set = (v: T | ((prev: T) => T)) => {
setValue(prev => (typeof v === "function" ? (v as (p: T) => T)(prev) : v))
}
return [value, set]
}
// Usage
// const [user, setUser] = useLocalStorage("user", { id: 1, name: "Ada" })useAsync<T>
Return an object with value, error, loading. The type of value is inferred from the promise returned by fn.
import { useEffect, useState } from "react"
export function useAsync<T>(fn: () => Promise<T>, deps: unknown[] = []) {
const [value, setValue] = useState<T | null>(null)
const [error, setError] = useState<unknown>(null)
const [loading, setLoading] = useState<boolean>(false)
useEffect(() => {
let cancelled = false
setLoading(true)
fn().then(
(v) => { if (!cancelled) { setValue(v); setError(null) } },
(e) => { if (!cancelled) { setError(e); setValue(null) } }
).finally(() => { if (!cancelled) setLoading(false) })
return () => { cancelled = true }
}, deps)
return { value, error, loading }
}
// Usage
// const { value, error, loading } = useAsync(() => fetch("/api").then(r => r.json()))useControlled<T>
Support both controlled and uncontrolled patterns with one helper. The tuple keeps T aligned.
import { useState } from "react"
type ControlledProps<T> = {
value?: T
defaultValue: T
onChange?: (v: T) => void
}
export function useControlled<T>({ value, defaultValue, onChange }: ControlledProps<T>): [T, (v: T) => void] {
const [inner, setInner] = useState<T>(defaultValue)
const isControlled = value !== undefined
const current = isControlled ? (value as T) : inner
const set = (v: T) => { isControlled ? onChange?.(v) : setInner(v) }
return [current, set]
}
// Usage
// const [v, setV] = useControlled({ value, defaultValue: "", onChange: setValue })Inference & constraints
Prefer inference from arguments; add extends constraints only when your hook needs specific fields.
// Inference works from the initial value
const [settings, setSettings] = useLocalStorage("settings", { theme: "dark", pageSize: 20 })
// T = { theme: string; pageSize: number }
// Constrain T when needed
interface WithId { id: string }
function useById<T extends WithId>(items: T[], id: string) {
return items.find(x => x.id === id) || null
}
// Usage
const user = useById([{ id: "u1", name: "Ada" }], "u1")Tips
Tips:
- Return tuples for React feel and consistent typing.
- Never use
anyin public hook signatures. - Accept updaters
(prev) => nextfor ergonomic state transitions.
Hook typing check
Quick check: Which signature best fits a generic localStorage hook?
Recap
Recap: You built generic hooks that rely on inference, use constraints only when necessary, and expose tuple/object results that feel like native React APIs.
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Frequently Asked Questions
Is the “Custom hooks with generics & inference” lesson free?
Yes — the full text of “Custom hooks with generics & inference” is free to read here on the web. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the TypeScript Academy course, upgrade to CoddyKit PRO. The TypeScript Academy course includes 2 lessons in total.
What will I learn in “Custom hooks with generics & inference”?
Write reusable custom hooks with for values, results, and errors; leverage inference and constraints for safe APIs. You practise TypeScript Academy with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.
Do I need any experience to start TypeScript Academy?
No prior experience is required. TypeScript Academy on CoddyKit is structured for beginners through advanced learners, so you can start here or from the beginning and move at your own pace. This is lesson 1 of 2.
How long does the “Custom hooks with generics & inference” lesson take?
Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.
Can I write and run code in this TypeScript Academy lesson?
Yes. Every TypeScript Academy lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.
All lessons in this course
- Custom hooks with generics & inference
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