React

By the end of this lesson you'll be able to type a React component's props, its useState and useRef , and its event handlers — so your editor catches mistakes as you type instead of your users finding them at runtime.

Part of the free TypeScript course at LearnCodingFast — hands-on lessons with examples you run in your browser, plus practice exercises and a quick quiz.

What You'll Learn

💡 Real-World Analogy

A component's props interface is the label on a shipping box : "Contains: 1 string (name), 1 number (age), optional VIP sticker." Anyone sending you a box must match that label, or the post office rejects it at the counter — before it ships. Plain React lets any box through and you discover the missing parts after delivery (at runtime). TypeScript checks the label at the counter, so wrong or missing props never reach your users.

1. Typing Component Props

Every React component receives a single props object. To type it, you describe its shape once with an interface , then annotate the parameter with that interface. From then on TypeScript enforces required props, rejects wrong types, and autocompletes prop names for you. A ? after a prop name makes it optional. Read this worked example carefully.

You can describe props with either an interface or a type alias — for prop shapes they're nearly identical. A common convention: use interface for object shapes you might extend, and type for unions and one-offs. Notice the "primary" | "danger" union — that prop can only be one of those two strings.

🎯 Your Turn: finish the props interface

Fill in each ___ below. This snippet is read-only — work it out in your head (or type it into a real .tsx file) and check against the // ✅ Expected comments.

2. Typing useState & useRef

Both hooks are generic — they take a type inside angle brackets, like useState<number>() . Most of the time you don't need to write it: useState(0) is inferred as a number from its initial value. You add an explicit <T> when the initial value can't describe the real type — an empty array, or a value that's null now but will hold an object later.

The never[] trap

Write const [items, setItems] = useState([]); and TypeScript infers the type as never[] — "an array that can hold nothing." The moment you call setItems(["a"]) you get "Argument of type string is not assignable to parameter of type never."

The fix is to tell it what the array holds: useState<string[]>([]) . This is the single most common state-typing mistake.

🎯 Your Turn: type three pieces of state

Replace each ___ with the correct generic. Check yourself against the // ✅ Expected lines.

3. Typing Events & Children

React gives each event a specific type, and you tell it which HTML element fired it using a generic. Type a change handler as React.ChangeEvent<HTMLInputElement> and e.target.value is correctly known to be a string. A click handler is React.MouseEvent<HTMLButtonElement> . To accept nested content (anything between your component's tags), type the children prop as React.ReactNode — the catch-all for "anything renderable."

4. Run It: the immutable state-update pattern

The TSX above can't run in this editor, but the update pattern you'll use with a typed setState<Todo[]> is plain JavaScript. The golden rule: never mutate the existing state — build and return a brand-new value . This runnable demo simulates setTodos with a function so you can press Run and watch it work.

🔎 Deep Dive: React.FC — and why many avoid it

You'll meet React.FC (short for "Function Component") in older code and tutorials. It works, but most teams have moved away from it in favour of typing the props directly on a plain function.

Common Errors (and the fix)

• Untyped props — "Parameter 'props' implicitly has an 'any' type" : you wrote function Greeting(props) { with no type. Add an interface and annotate it: function Greeting(props: GreetingProps) .
• Wrong event type — "Property 'value' does not exist on type EventTarget" : you typed a change handler as a generic Event . Use React.ChangeEvent<HTMLInputElement> so e.target.value exists.
• useState inferred as never — "Argument of type 'string' is not assignable to parameter of type 'never'" : you started state at [] . Add the generic: useState<string[]>([]) .
• Reading nullable state — "Object is possibly 'null'" : state typed User | null must be guarded. Wrap access in if (user) { ... } or use optional chaining user?.name .
• Ref used too early — "Object is possibly 'null'" on inputRef.current : it's null until React attaches the element. Access it with inputRef.current?.focus() .

📋 Quick Reference

What you're typing

Code

Component props

function C(props: Props) { ... }

Optional prop

isVip?: boolean;

State (inferred)

useState(0)

State (array)

useState<string[]>([])

State (nullable)

useState<User | null>(null)

DOM ref

useRef<HTMLInputElement>(null)

Change event

React.ChangeEvent<HTMLInputElement>

Click event

React.MouseEvent<HTMLButtonElement>

Children

children: React.ReactNode

Frequently Asked Questions

For props they're almost interchangeable. A common convention: interface for object/prop shapes (it can be extended later), and type for unions, intersections, and one-offs. Pick one style and stay consistent within a project.

Q: Do I always have to write the generic on useState ?

No — TypeScript infers it from the initial value, so useState(0) is a number automatically. Add the explicit <T> only when the initial value can't describe the real type: empty arrays ( useState<string[]>([]) ) and nullable values ( useState<User | null>(null) ).

Because the event wasn't typed to a specific element. A plain Event has no value . Type the handler React.ChangeEvent<HTMLInputElement> and TypeScript knows e.target is an input, so .value appears.

Not wrong — just out of fashion. It used to silently add a children prop and makes generic components awkward. The React docs now show plain functions with directly-typed props, so prefer function C(props: Props) unless your codebase already standardises on FC .

Mini-Challenge: a typed Counter

No blanks this time — just a brief and an outline. Build it in a real react-ts project, then use the self-check at the bottom to confirm your types are right.

Pro Tips

• 💡 Let inference do the work: only add an explicit <T> when the initial value can't describe the type.
• 💡 Hover, don't memorise: in VS Code, hover over onChange on any JSX element to see the exact event type it expects.
• 💡 Type the children prop as React.ReactNode for any wrapper component — it accepts text, JSX, numbers, and arrays.
• 💡 Prefer plain typed functions over React.FC for new components.

🎉 Lesson Complete

• ✅ Describe props with an interface (or type ) and annotate the props parameter
• ✅ useState usually infers; add <T> for empty arrays and nullable values
• ✅ useRef<HTMLInputElement>(null) for DOM, useRef<number>(0) for mutable values
• ✅ Event handlers carry their element type: React.ChangeEvent<HTMLInputElement> , React.MouseEvent
• ✅ children: React.ReactNode accepts any renderable content
• ✅ Prefer plain typed functions over React.FC
• ✅ Next lesson: TypeScript Best Practices for production codebases

Practice quiz

Continue this course

• Previous: Advanced Types
• Next: TypeScript Best Practices