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427/**
* Tests for Intersection Observer concept page
* Source: /docs/beyond/concepts/intersection-observer.mdx
*
* Note: Intersection Observer is a browser API that cannot be fully tested
* without a real browser environment. These tests verify the concepts and
* patterns described in the documentation using mocks and simulated behavior.
*/
import { describe, it, expect, vi, beforeEach, afterEach } from 'vitest'
describe('Intersection Observer Concepts', () => {
describe('IntersectionObserverEntry Properties', () => {
// MDX lines ~105-130: IntersectionObserverEntry Properties table
it('should understand entry property types', () => {
// Simulating the shape of IntersectionObserverEntry
const mockEntry = {
target: { id: 'test-element' }, // Element being observed
isIntersecting: true, // boolean
intersectionRatio: 0.5, // number (0.0 to 1.0)
boundingClientRect: { top: 100, left: 0, width: 200, height: 100 },
intersectionRect: { top: 100, left: 0, width: 200, height: 50 },
rootBounds: { top: 0, left: 0, width: 800, height: 600 },
time: 1234567890.123 // DOMHighResTimeStamp
}
expect(typeof mockEntry.target).toBe('object')
expect(typeof mockEntry.isIntersecting).toBe('boolean')
expect(typeof mockEntry.intersectionRatio).toBe('number')
expect(mockEntry.intersectionRatio).toBeGreaterThanOrEqual(0)
expect(mockEntry.intersectionRatio).toBeLessThanOrEqual(1)
expect(typeof mockEntry.time).toBe('number')
})
// MDX lines ~125-135: intersectionRatio calculation
it('should calculate visibility percentage from intersectionRatio', () => {
const intersectionRatio = 0.75
const visibilityPercent = Math.round(intersectionRatio * 100) + '%'
expect(visibilityPercent).toBe('75%')
})
})
describe('Observer Options Validation', () => {
// MDX lines ~140-165: The root Option
it('should understand root option defaults', () => {
const defaultOptions = {
root: null, // viewport by default
rootMargin: '0px', // no margin by default
threshold: 0 // any pixel visible by default
}
expect(defaultOptions.root).toBeNull()
expect(defaultOptions.rootMargin).toBe('0px')
expect(defaultOptions.threshold).toBe(0)
})
// MDX lines ~180-215: The rootMargin Option
it('should understand rootMargin string format', () => {
// rootMargin follows CSS margin format
const validMargins = [
'100px', // All sides
'100px 0px', // top/bottom, left/right
'100px 0px 100px 0px', // top, right, bottom, left
'-50px', // Negative margins shrink
'0px 0px -50%', // Percentages work
'200px 0px 200px 0px' // Large buffer for preloading
]
validMargins.forEach(margin => {
expect(typeof margin).toBe('string')
})
})
// MDX lines ~220-260: The threshold Option
it('should accept single threshold values', () => {
const singleThresholds = [0, 0.5, 1.0]
singleThresholds.forEach(threshold => {
expect(threshold).toBeGreaterThanOrEqual(0)
expect(threshold).toBeLessThanOrEqual(1)
})
})
it('should accept array of threshold values', () => {
const thresholdArray = [0, 0.25, 0.5, 0.75, 1.0]
expect(Array.isArray(thresholdArray)).toBe(true)
thresholdArray.forEach(threshold => {
expect(threshold).toBeGreaterThanOrEqual(0)
expect(threshold).toBeLessThanOrEqual(1)
})
})
})
describe('Lazy Loading Pattern Logic', () => {
// MDX lines ~315-365: Implementing Lazy Loading Images
it('should swap data-src to src pattern', () => {
const mockImage = {
src: 'placeholder.svg',
dataset: { src: 'real-image.jpg', srcset: 'image-1x.jpg 1x, image-2x.jpg 2x' },
classList: {
removed: [],
added: [],
remove(cls) { this.removed.push(cls) },
add(cls) { this.added.push(cls) }
}
}
// Simulate lazy loading logic
function lazyLoadImage(img) {
img.src = img.dataset.src
if (img.dataset.srcset) {
img.srcset = img.dataset.srcset
}
img.classList.remove('lazy')
img.classList.add('loaded')
}
lazyLoadImage(mockImage)
expect(mockImage.src).toBe('real-image.jpg')
expect(mockImage.srcset).toBe('image-1x.jpg 1x, image-2x.jpg 2x')
expect(mockImage.classList.removed).toContain('lazy')
expect(mockImage.classList.added).toContain('loaded')
})
})
describe('Infinite Scroll Pattern Logic', () => {
// MDX lines ~385-440: Building Infinite Scroll
it('should track loading state to prevent duplicate requests', async () => {
let isLoading = false
let page = 1
const fetchCalls = []
async function loadMoreContent() {
if (isLoading) return // Prevent duplicate calls
isLoading = true
fetchCalls.push(page)
page++
// Simulate async fetch
await new Promise(resolve => setTimeout(resolve, 10))
isLoading = false
}
// Simulate rapid scroll events (should only trigger one load)
await Promise.all([
loadMoreContent(),
loadMoreContent(),
loadMoreContent()
])
// Only one fetch should have happened due to isLoading guard
expect(fetchCalls.length).toBe(1)
expect(page).toBe(2)
})
it('should stop observing when no more content', () => {
let observing = true
const posts = [] // Empty array = no more content
function handleIntersection() {
if (posts.length === 0) {
observing = false // Stop observing
return
}
}
handleIntersection()
expect(observing).toBe(false)
})
})
describe('Scroll Animation Pattern Logic', () => {
// MDX lines ~455-500: Scroll-Triggered Animations
it('should create reusable animation observer factory', () => {
function createScrollAnimator(options = {}) {
const {
threshold = 0.2,
rootMargin = '0px',
animateOnce = true,
animatedClass = 'animated'
} = options
return { threshold, rootMargin, animateOnce, animatedClass }
}
const defaultAnimator = createScrollAnimator()
expect(defaultAnimator.threshold).toBe(0.2)
expect(defaultAnimator.rootMargin).toBe('0px')
expect(defaultAnimator.animateOnce).toBe(true)
expect(defaultAnimator.animatedClass).toBe('animated')
const customAnimator = createScrollAnimator({
threshold: 0.5,
animateOnce: false
})
expect(customAnimator.threshold).toBe(0.5)
expect(customAnimator.animateOnce).toBe(false)
})
})
describe('Cleanup Patterns', () => {
// MDX lines ~605-650: The #1 Intersection Observer Mistake
it('should demonstrate cleanup function pattern', () => {
let disconnected = false
function setupObserver() {
const mockObserver = {
observe: vi.fn(),
unobserve: vi.fn(),
disconnect: () => { disconnected = true }
}
// Return cleanup function
return () => mockObserver.disconnect()
}
const cleanup = setupObserver()
expect(disconnected).toBe(false)
cleanup() // Simulate component unmount
expect(disconnected).toBe(true)
})
it('should unobserve after lazy loading', () => {
const unobservedElements = []
const mockObserver = {
unobserve: (element) => unobservedElements.push(element)
}
// Simulate lazy load callback
function handleIntersection(entry, observer) {
if (entry.isIntersecting) {
// Load image...
observer.unobserve(entry.target)
}
}
const mockEntry = { isIntersecting: true, target: { id: 'img1' } }
handleIntersection(mockEntry, mockObserver)
expect(unobservedElements).toContainEqual({ id: 'img1' })
})
})
describe('Visibility Detection Logic', () => {
// MDX lines ~30-60: isIntersecting vs intersectionRatio
it('should use isIntersecting for simple visibility checks', () => {
const entries = [
{ isIntersecting: true, intersectionRatio: 0.5 },
{ isIntersecting: false, intersectionRatio: 0 },
{ isIntersecting: true, intersectionRatio: 1.0 }
]
const visibleEntries = entries.filter(e => e.isIntersecting)
expect(visibleEntries.length).toBe(2)
})
it('should use intersectionRatio for progressive visibility tracking', () => {
const entry = { intersectionRatio: 0.75 }
// Ad viewability: count impression when 50%+ visible
const isViewable = entry.intersectionRatio >= 0.5
expect(isViewable).toBe(true)
// Progress tracking
const percentVisible = Math.round(entry.intersectionRatio * 100)
expect(percentVisible).toBe(75)
})
})
describe('Threshold Behavior', () => {
// MDX lines ~220-260: threshold option behavior
it('should understand threshold 0 triggers on any visibility', () => {
// threshold: 0 means callback fires when target touches root boundary
const callback = vi.fn()
// Simulate entries at threshold 0
const entryJustVisible = { intersectionRatio: 0.01, isIntersecting: true }
// Even 1% visible should trigger with threshold: 0
if (entryJustVisible.isIntersecting) {
callback(entryJustVisible)
}
expect(callback).toHaveBeenCalled()
})
it('should understand threshold 1.0 requires full visibility', () => {
// threshold: 1.0 means callback fires only when 100% visible
const threshold = 1.0
const partiallyVisible = { intersectionRatio: 0.9 }
const fullyVisible = { intersectionRatio: 1.0 }
expect(partiallyVisible.intersectionRatio >= threshold).toBe(false)
expect(fullyVisible.intersectionRatio >= threshold).toBe(true)
})
it('should warn about threshold 1.0 with tall elements', () => {
// If element is taller than viewport, it can never be 100% visible
const elementHeight = 1200 // pixels
const viewportHeight = 800 // pixels
const canBeFullyVisible = elementHeight <= viewportHeight
expect(canBeFullyVisible).toBe(false)
// In this case, use a lower threshold or check intersectionRatio
const practicalThreshold = 0.8
expect(practicalThreshold).toBeLessThan(1.0)
})
})
describe('Common Mistakes Prevention', () => {
// MDX lines ~680-750: Common Mistakes section
it('Mistake 2: should use single observer for multiple elements', () => {
const observers = []
const elements = ['el1', 'el2', 'el3', 'el4', 'el5']
// BAD: Creating new observer for each element
function badPattern() {
elements.forEach(el => {
observers.push({ target: el })
})
return observers.length
}
// GOOD: One observer watching many elements
function goodPattern() {
const singleObserver = { targets: [] }
elements.forEach(el => {
singleObserver.targets.push(el)
})
return 1 // Just one observer
}
expect(badPattern()).toBe(5) // 5 "observers"
expect(goodPattern()).toBe(1) // 1 observer
})
it('Mistake 3: should check isIntersecting in callback', () => {
// Callback fires immediately on observe() call
const entriesReceived = []
function handleEntries(entries) {
entries.forEach(entry => {
// WRONG: Assuming callback only fires when visible
// loadImage(entry.target) // Would load all images immediately!
// RIGHT: Check isIntersecting first
if (entry.isIntersecting) {
entriesReceived.push(entry)
}
})
}
// Initial callback with non-intersecting elements
const initialEntries = [
{ isIntersecting: false, target: 'img1' },
{ isIntersecting: false, target: 'img2' },
{ isIntersecting: true, target: 'img3' }
]
handleEntries(initialEntries)
expect(entriesReceived.length).toBe(1) // Only img3
})
})
describe('Feature Detection', () => {
// MDX lines ~775-790: Browser Support
it('should demonstrate feature detection pattern', () => {
// Simulate browser without IntersectionObserver
const windowWithoutIO = {}
const windowWithIO = { IntersectionObserver: function() {} }
function hasIntersectionObserver(win) {
return 'IntersectionObserver' in win
}
expect(hasIntersectionObserver(windowWithoutIO)).toBe(false)
expect(hasIntersectionObserver(windowWithIO)).toBe(true)
})
})
describe('Key Takeaways Validation', () => {
// MDX lines ~800-830: Key Takeaways
it('Takeaway 1: Observer is more performant than scroll events', () => {
// Scroll events fire 60+ times per second
// IntersectionObserver only fires when visibility changes
const scrollEventFrequency = 60 // per second
const observerCallsPerVisibilityChange = 1
expect(scrollEventFrequency).toBeGreaterThan(observerCallsPerVisibilityChange)
})
it('Takeaway 4: One observer can watch many elements', () => {
const observedElements = []
const mockObserver = {
observe: (el) => observedElements.push(el)
}
// Can observe multiple elements with single observer
const elements = ['el1', 'el2', 'el3']
elements.forEach(el => mockObserver.observe(el))
expect(observedElements.length).toBe(3)
})
it('Takeaway 6: rootMargin enables preloading', () => {
// Positive margin = detect before visible
const preloadMargin = '100px'
// Element at position 700px from top of viewport (800px height)
// With rootMargin: 100px, detection happens at 800 + 100 = 900px
const viewportHeight = 800
const marginValue = 100
const effectiveDetectionArea = viewportHeight + marginValue
expect(effectiveDetectionArea).toBe(900)
})
})
})