Your first component in code
function Welcome() {
return <h1>Welcome</h1>;
}
<Welcome />;Read the example from data and control flow to the resulting UI. Keep the component boundary small.
Back to week 1
Topic
Your first component
Definition
A React component is a JavaScript function that returns React elements describing a UI region.
In simpler words
Write a function with a capitalized name, return JSX, then use it like a tag.
Function components, component composition, and capitalized component names.
After this you can
- Create and nest small components with a single UI responsibility.
- Explain the trade-off to a teammate using a small example.
- Name at least one common bug pattern for this topic.
Understand Your first component
Function components, component composition, and capitalized component names.
Start by identifying which value or browser behavior changes. Then describe the UI from that current input instead of editing the DOM as a separate source of truth.
Apply Your first component
Keep rendering as a calculation. Put user-triggered changes in event handlers, preserve UI memory in state, and reserve external synchronization for Effects or the server-state layer.
Name values by their UI meaning, test the loading and error path when data is remote, and avoid keeping two editable copies of the same value.
Ask before adding code: is this local UI memory, shared client state, or Nest-owned server state?
Where bugs hide
Definition
High-bug areas are places where a small API misuse looks correct but produces stale UI, duplicate work, or silent failures.
In simpler words
Each mistake below shows Wrong vs Right code — compare them side by side.
When something misbehaves, match the symptom to a pattern below before rewriting the feature.
Prefer fixing the ownership or update path over adding another Effect or sync step.
Mistake: Lowercase component tag
// Wrong
function welcome() { return <h1>Hi</h1>; }
<welcome />
// Right
function Welcome() { return <h1>Hi</h1>; }
<Welcome />Capital letter marks a React component.
Mistake: Side effects during render
// Wrong
function Page() {
fetch("/api/tickets"); // runs every render
return <List />;
}
// Right
function Page() {
// fetch in Query / event / Effect — not bare render
return <List />;
}Render must stay pure; start requests from handlers, Query, or Effects.
Mistake: God component
// Wrong
function App() {
// layout + fetch + form + table in one 800-line function
}
// Right
function App() {
return (
<>
<Header />
<TicketList />
<CreateForm />
</>
);
}Split by responsibility so each unit is testable.
Live playground
Your first component sandbox
Change one input at a time and predict the next render.
Hello, Ada
function Hello({ name }) {
return <h1>Hello, {name}</h1>;
}Keep in mind
- Keep the formal definition in mind; it explains which tool belongs where.
- Prefer one source of truth over synchronized copies of the same value.
- When behavior surprises you, trace: input → update → render → committed UI.
- Study the Wrong vs Right examples in “Where bugs hide” before you merge.
Test
Check your understanding
At least 10 questions — mix of concept, syntax, practical, and logic. Score ≥ 80% (enforced by the API) to save progress.
Checking your session…
10 questions · concept 3 · syntax 3 · practical 2 · logic 2