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useCallback,
useEffect,
useLayoutEffect,
useMemo,
useRef,
useState,
} from "react";
import type {
UseIntersectionObserverOptions,
UseIntersectionObserverReturn,
IntersectionEntry,
OnChangeCallback,
} from "./types";
import {
isIntersectionObserverSupported,
toIntersectionEntry,
createInitialEntry,
createNoopRef,
normalizeThreshold,
} from "./utils";
/**
* useLayoutEffect logs a warning when invoked during server rendering.
* Fall back to useEffect on the server so the latest-callback ref can still
* be kept up to date without noise in SSR environments.
*/
const useIsomorphicLayoutEffect =
typeof window !== "undefined" ? useLayoutEffect : useEffect;
/**
* A React hook for observing element visibility using the Intersection Observer API.
*
* Features:
* - Efficient viewport/container visibility detection
* - Threshold-based intersection callbacks
* - TriggerOnce support for lazy loading patterns
* - Dynamic enable/disable support
* - SSR compatible with graceful degradation
* - TypeScript support with full type inference
*
* @param options - Configuration options for the observer
* @returns Object containing entry data, inView boolean, and ref callback
*
* @example
* ```tsx
* // Basic usage - check if element is visible
* function Component() {
* const { ref, inView } = useIntersectionObserver();
* return (
* <div ref={ref}>
* {inView ? 'Visible!' : 'Not visible'}
* </div>
* );
* }
* ```
*
* @example
* ```tsx
* // Lazy load image when it enters viewport
* function LazyImage({ src, alt }: { src: string; alt: string }) {
* const { ref, inView } = useIntersectionObserver({
* triggerOnce: true,
* threshold: 0.1,
* });
*
* return (
* <div ref={ref}>
* {inView ? (
* <img src={src} alt={alt} />
* ) : (
* <div className="placeholder" />
* )}
* </div>
* );
* }
* ```
*
* @example
* ```tsx
* // Infinite scroll with sentinel element
* function InfiniteList() {
* const { ref, inView } = useIntersectionObserver({
* threshold: 1.0,
* rootMargin: '100px',
* });
*
* useEffect(() => {
* if (inView) {
* loadMoreItems();
* }
* }, [inView]);
*
* return (
* <div>
* {items.map(item => <Item key={item.id} {...item} />)}
* <div ref={ref} />
* </div>
* );
* }
* ```
*
* @example
* ```tsx
* // Track scroll progress with multiple thresholds
* function ProgressTracker() {
* const { ref, entry } = useIntersectionObserver({
* threshold: [0, 0.25, 0.5, 0.75, 1.0],
* onChange: (entry, inView) => {
* console.log('Progress:', Math.round(entry.intersectionRatio * 100), '%');
* },
* });
*
* return <div ref={ref}>Long content...</div>;
* }
* ```
*
* @example
* ```tsx
* // Custom scroll container as root
* function ScrollContainer() {
* const containerRef = useRef<HTMLDivElement>(null);
* const { ref, inView } = useIntersectionObserver({
* root: containerRef.current,
* rootMargin: '0px',
* });
*
* return (
* <div ref={containerRef} style={{ overflow: 'auto', height: 400 }}>
* <div style={{ height: 1000 }}>
* <div ref={ref}>{inView ? 'In container view' : 'Outside'}</div>
* </div>
* </div>
* );
* }
* ```
*/
export function useIntersectionObserver(
options: UseIntersectionObserverOptions = {}
): UseIntersectionObserverReturn {
const {
threshold = 0,
root = null,
rootMargin = "0px",
triggerOnce = false,
enabled = true,
initialIsIntersecting = false,
onChange,
delay = 0,
} = options;
// ============ SSR Check ============
const isSupported = isIntersectionObserverSupported();
// ============ Refs for internal state (no re-renders) ============
const observerRef = useRef<IntersectionObserver | null>(null);
const hasTriggeredRef = useRef<boolean>(false);
const delayTimeoutRef = useRef<ReturnType<typeof setTimeout> | null>(null);
// Store previous entry values for comparison (to avoid unnecessary re-renders)
const prevEntryRef = useRef<{
isIntersecting: boolean;
intersectionRatio: number;
} | null>(null);
// Store the latest onChange callback in a ref so identity changes don't
// recreate the observer. Assigned post-commit (never during render) to stay
// concurrent/StrictMode safe.
const onChangeRef = useRef<OnChangeCallback | undefined>(onChange);
useIsomorphicLayoutEffect(() => {
onChangeRef.current = onChange;
});
// Track the observed node as state so the observer effect re-runs when the
// ref callback receives a new element — no manual forceUpdate required.
const [node, setNode] = useState<Element | null>(null);
// ============ State (triggers re-renders) ============
const [entry, setEntry] = useState<IntersectionEntry | null>(() =>
initialIsIntersecting ? createInitialEntry(true, null) : null
);
// Compute inView from entry state, coercing at the boundary to a strict boolean.
const inView = entry ? entry.isIntersecting === true : initialIsIntersecting;
// Stabilize the threshold so an inline array (e.g. threshold={[0, 0.5, 1]})
// does not tear down and recreate the native observer on every render.
const thresholdKey = useMemo(
() => JSON.stringify(normalizeThreshold(threshold)),
[threshold]
);
// ============ Observer Callback ============
const handleIntersection = useCallback(
(entries: IntersectionObserverEntry[]) => {
entries.forEach((nativeEntry) => {
const intersectionEntry = toIntersectionEntry(nativeEntry);
// Check if meaningful values have changed (ignore time which always changes)
const prevEntry = prevEntryRef.current;
const hasChanged =
!prevEntry ||
prevEntry.isIntersecting !== nativeEntry.isIntersecting ||
prevEntry.intersectionRatio !== nativeEntry.intersectionRatio;
// Update previous entry ref
prevEntryRef.current = {
isIntersecting: nativeEntry.isIntersecting,
intersectionRatio: nativeEntry.intersectionRatio,
};
// Only update state if meaningful values changed
if (hasChanged) {
setEntry(intersectionEntry);
}
// Call onChange callback if provided (always call, even if state didn't change)
if (onChangeRef.current) {
onChangeRef.current(intersectionEntry, nativeEntry.isIntersecting);
}
// Handle triggerOnce - unobserve after first intersection
if (triggerOnce && nativeEntry.isIntersecting === true) {
hasTriggeredRef.current = true;
// Unobserve the target
Eif (observerRef.current && nativeEntry.target) {
observerRef.current.unobserve(nativeEntry.target);
}
}
});
},
[triggerOnce]
);
// ============ Ref Callback ============
const ref = useCallback((element: Element | null) => {
setNode(element);
}, []);
// ============ Effect: Manage Observer Lifecycle ============
useEffect(() => {
// SSR guard
Iif (!isSupported) {
return;
}
// Clean up existing observer
Iif (observerRef.current) {
observerRef.current.disconnect();
observerRef.current = null;
}
// Don't create observer if disabled
if (!enabled) {
return;
}
// Don't create if triggerOnce already triggered
Iif (triggerOnce && hasTriggeredRef.current) {
return;
}
// Don't create if no target
if (!node) {
return;
}
const thresholds = JSON.parse(thresholdKey) as number[];
// Create and observe function
const createAndObserve = () => {
observerRef.current = new IntersectionObserver(handleIntersection, {
threshold: thresholds,
root,
rootMargin,
});
observerRef.current.observe(node);
};
// Handle delay
if (delay > 0) {
delayTimeoutRef.current = setTimeout(() => {
createAndObserve();
delayTimeoutRef.current = null;
}, delay);
return () => {
if (delayTimeoutRef.current) {
clearTimeout(delayTimeoutRef.current);
delayTimeoutRef.current = null;
}
if (observerRef.current) {
observerRef.current.disconnect();
observerRef.current = null;
}
};
}
// Create observer immediately
createAndObserve();
// Cleanup
return () => {
Eif (observerRef.current) {
observerRef.current.disconnect();
observerRef.current = null;
}
};
}, [
enabled,
triggerOnce,
delay,
thresholdKey,
root,
rootMargin,
handleIntersection,
isSupported,
node,
]);
// ============ Effect: Reset hasTriggered when triggerOnce is disabled ============
useEffect(() => {
if (!triggerOnce) {
hasTriggeredRef.current = false;
}
}, [triggerOnce]);
// ============ SSR Return ============
Iif (!isSupported) {
return {
entry: initialIsIntersecting ? createInitialEntry(true, null) : null,
inView: initialIsIntersecting,
ref: createNoopRef(),
};
}
return {
entry,
inView,
ref,
};
}
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