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536/**
* @vitest-environment jsdom
*/
import { describe, it, expect, vi, beforeEach, afterEach } from 'vitest'
/**
* Tests for Event Delegation concept page
* Source: docs/beyond/concepts/event-delegation.mdx
*/
describe('Event Delegation', () => {
let container
beforeEach(() => {
// Set up a fresh DOM container for each test
container = document.createElement('div')
container.id = 'test-container'
document.body.appendChild(container)
})
afterEach(() => {
// Clean up after each test
document.body.removeChild(container)
container = null
})
describe('event.target vs event.currentTarget', () => {
// Source: docs/beyond/concepts/event-delegation.mdx:78-85
it('should demonstrate target vs currentTarget difference', () => {
// Set up nested structure
container.innerHTML = `
<ul id="menu">
<li>
<button id="test-button">Click</button>
</li>
</ul>
`
const menu = document.getElementById('menu')
const button = document.getElementById('test-button')
let capturedTarget = null
let capturedCurrentTarget = null
menu.addEventListener('click', (event) => {
capturedTarget = event.target
capturedCurrentTarget = event.currentTarget
})
// Simulate click on the button
button.click()
// target is the button (what was clicked)
expect(capturedTarget).toBe(button)
expect(capturedTarget.tagName).toBe('BUTTON')
// currentTarget is the ul (where listener is attached)
expect(capturedCurrentTarget).toBe(menu)
expect(capturedCurrentTarget.tagName).toBe('UL')
})
// Source: docs/beyond/concepts/event-delegation.mdx:87-102
it('should show target changes based on click location while currentTarget stays constant', () => {
container.innerHTML = `
<div id="outer">
<p id="para">
<span id="inner">Click me</span>
</p>
</div>
`
const outer = document.getElementById('outer')
const span = document.getElementById('inner')
const para = document.getElementById('para')
const targets = []
const currentTargets = []
outer.addEventListener('click', (event) => {
targets.push(event.target)
currentTargets.push(event.currentTarget)
})
// Click the span
span.click()
expect(targets[0]).toBe(span)
expect(currentTargets[0]).toBe(outer)
// Click the paragraph
para.click()
expect(targets[1]).toBe(para)
expect(currentTargets[1]).toBe(outer) // Always outer
})
})
describe('Element.matches() for filtering', () => {
// Source: docs/beyond/concepts/event-delegation.mdx:104-130
it('should filter events using matches()', () => {
container.innerHTML = `
<div class="container">
<button class="btn">Regular</button>
<button class="btn delete-btn">Delete</button>
<button class="btn primary" disabled>Disabled Primary</button>
<button class="btn primary">Enabled Primary</button>
<span data-action="test">Not a button</span>
</div>
`
const containerEl = container.querySelector('.container')
const results = []
containerEl.addEventListener('click', (event) => {
if (event.target.matches('button')) {
results.push('button')
}
if (event.target.matches('.delete-btn')) {
results.push('delete')
}
if (event.target.matches('[data-action]')) {
results.push('action: ' + event.target.dataset.action)
}
if (event.target.matches('button.primary:not(:disabled)')) {
results.push('enabled-primary')
}
})
// Click delete button
container.querySelector('.delete-btn').click()
expect(results).toContain('button')
expect(results).toContain('delete')
// Reset and click span with data-action
results.length = 0
container.querySelector('[data-action]').click()
expect(results).toContain('action: test')
expect(results).not.toContain('button')
// Reset and click enabled primary button
results.length = 0
container.querySelector('button.primary:not(:disabled)').click()
expect(results).toContain('button')
expect(results).toContain('enabled-primary')
})
})
describe('Element.closest() for nested elements', () => {
// Source: docs/beyond/concepts/event-delegation.mdx:132-162
it('should find ancestor elements using closest()', () => {
container.innerHTML = `
<div class="container">
<button class="action-btn">
<i class="icon" id="icon-element">icon</i>
<span id="span-element">Delete</span>
</button>
</div>
`
const containerEl = container.querySelector('.container')
const icon = document.getElementById('icon-element')
const span = document.getElementById('span-element')
const button = container.querySelector('.action-btn')
let foundButton = null
containerEl.addEventListener('click', (event) => {
// Use closest to find the button, regardless of what was clicked
foundButton = event.target.closest('.action-btn')
})
// Click the icon inside the button
icon.click()
expect(foundButton).toBe(button)
// Click the span inside the button
foundButton = null
span.click()
expect(foundButton).toBe(button)
// Click the button itself
foundButton = null
button.click()
expect(foundButton).toBe(button)
})
it('should return null when no ancestor matches', () => {
container.innerHTML = `
<div class="container">
<span class="outside">Outside</span>
<button class="action-btn">Inside</button>
</div>
`
const containerEl = container.querySelector('.container')
const outside = container.querySelector('.outside')
let foundButton = null
containerEl.addEventListener('click', (event) => {
foundButton = event.target.closest('.action-btn')
})
outside.click()
expect(foundButton).toBeNull()
})
})
describe('Basic event delegation pattern', () => {
// Source: docs/beyond/concepts/event-delegation.mdx:188-215
it('should handle clicks on list items with delegation', () => {
container.innerHTML = `
<ul id="todo-list">
<li data-id="1">Buy groceries</li>
<li data-id="2">Walk the dog</li>
<li data-id="3">Finish report</li>
</ul>
`
const todoList = document.getElementById('todo-list')
const clickedIds = []
todoList.addEventListener('click', (event) => {
const item = event.target.closest('li')
if (item) {
clickedIds.push(item.dataset.id)
item.classList.toggle('completed')
}
})
// Click first item
container.querySelector('li[data-id="1"]').click()
expect(clickedIds).toContain('1')
expect(container.querySelector('li[data-id="1"]').classList.contains('completed')).toBe(true)
// Click second item
container.querySelector('li[data-id="2"]').click()
expect(clickedIds).toContain('2')
// Click first item again to toggle off
container.querySelector('li[data-id="1"]').click()
expect(container.querySelector('li[data-id="1"]').classList.contains('completed')).toBe(false)
})
})
describe('Handling dynamic elements', () => {
// Source: docs/beyond/concepts/event-delegation.mdx:221-262
it('should automatically handle dynamically added elements', () => {
container.innerHTML = `<ul id="todo-list"></ul>`
const todoList = document.getElementById('todo-list')
const clickedTexts = []
// Set up delegation BEFORE adding items
todoList.addEventListener('click', (event) => {
if (event.target.matches('li')) {
clickedTexts.push(event.target.textContent)
event.target.classList.toggle('completed')
}
})
// Add items dynamically
function addTodo(text) {
const li = document.createElement('li')
li.textContent = text
todoList.appendChild(li)
}
addTodo('First task')
addTodo('Second task')
addTodo('Third task')
// Click dynamically added items - they should work!
const items = todoList.querySelectorAll('li')
items[0].click()
items[1].click()
expect(clickedTexts).toEqual(['First task', 'Second task'])
expect(items[0].classList.contains('completed')).toBe(true)
expect(items[1].classList.contains('completed')).toBe(true)
expect(items[2].classList.contains('completed')).toBe(false)
})
})
describe('Action buttons with data-action pattern', () => {
// Source: docs/beyond/concepts/event-delegation.mdx:308-334
it('should dispatch to correct action based on data-action attribute', () => {
container.innerHTML = `
<div id="toolbar">
<button data-action="save">Save</button>
<button data-action="load">Load</button>
<button data-action="delete">Delete</button>
</div>
`
const executedActions = []
const actions = {
save() { executedActions.push('save') },
load() { executedActions.push('load') },
delete() { executedActions.push('delete') }
}
const toolbar = document.getElementById('toolbar')
toolbar.addEventListener('click', (event) => {
const action = event.target.dataset.action
if (action && actions[action]) {
actions[action]()
}
})
// Click save button
container.querySelector('[data-action="save"]').click()
expect(executedActions).toEqual(['save'])
// Click delete button
container.querySelector('[data-action="delete"]').click()
expect(executedActions).toEqual(['save', 'delete'])
// Click load button
container.querySelector('[data-action="load"]').click()
expect(executedActions).toEqual(['save', 'delete', 'load'])
})
})
describe('Tab interface pattern', () => {
// Source: docs/beyond/concepts/event-delegation.mdx:336-361
it('should switch tabs using delegation', () => {
container.innerHTML = `
<div class="tabs">
<button class="tab" data-tab="home">Home</button>
<button class="tab" data-tab="profile">Profile</button>
<button class="tab" data-tab="settings">Settings</button>
</div>
<div class="tab-content" id="home">Home content</div>
<div class="tab-content" id="profile" hidden>Profile content</div>
<div class="tab-content" id="settings" hidden>Settings content</div>
`
const tabsContainer = container.querySelector('.tabs')
tabsContainer.addEventListener('click', (event) => {
const tab = event.target.closest('.tab')
if (!tab) return
// Remove active class from all tabs
container.querySelectorAll('.tab').forEach(t => t.classList.remove('active'))
tab.classList.add('active')
// Hide all content, show selected
const tabId = tab.dataset.tab
container.querySelectorAll('.tab-content').forEach(content => {
content.hidden = content.id !== tabId
})
})
// Click profile tab
container.querySelector('[data-tab="profile"]').click()
expect(container.querySelector('[data-tab="profile"]').classList.contains('active')).toBe(true)
expect(container.querySelector('[data-tab="home"]').classList.contains('active')).toBe(false)
expect(document.getElementById('profile').hidden).toBe(false)
expect(document.getElementById('home').hidden).toBe(true)
expect(document.getElementById('settings').hidden).toBe(true)
// Click settings tab
container.querySelector('[data-tab="settings"]').click()
expect(container.querySelector('[data-tab="settings"]').classList.contains('active')).toBe(true)
expect(container.querySelector('[data-tab="profile"]').classList.contains('active')).toBe(false)
expect(document.getElementById('settings').hidden).toBe(false)
expect(document.getElementById('profile').hidden).toBe(true)
})
})
describe('Container boundary verification', () => {
// Source: docs/beyond/concepts/event-delegation.mdx:468-482
it('should verify element is within container using contains()', () => {
container.innerHTML = `
<table id="outer-table">
<tr>
<td>
Outer cell
<table id="inner-table">
<tr><td id="inner-cell">Inner cell</td></tr>
</table>
</td>
</tr>
<tr><td id="outer-cell">Real outer cell</td></tr>
</table>
`
const outerTable = document.getElementById('outer-table')
const innerCell = document.getElementById('inner-cell')
const outerCell = document.getElementById('outer-cell')
const handledCells = []
outerTable.addEventListener('click', (event) => {
const td = event.target.closest('td')
// Only handle cells that are direct children of outer table
// Using a more specific check
if (td && td.closest('table') === outerTable) {
handledCells.push(td.id || 'unnamed')
}
})
// Click inner cell - should NOT be handled (belongs to inner table)
innerCell.click()
expect(handledCells).not.toContain('inner-cell')
// Click outer cell - should be handled
outerCell.click()
expect(handledCells).toContain('outer-cell')
})
})
describe('focusin/focusout delegation (bubbling alternatives)', () => {
// Source: docs/beyond/concepts/event-delegation.mdx:444-461
it('should delegate focus events using focusin/focusout', () => {
container.innerHTML = `
<form class="form">
<input type="text" id="input1" />
<input type="email" id="input2" />
</form>
`
const form = container.querySelector('.form')
const input1 = document.getElementById('input1')
const input2 = document.getElementById('input2')
const focusedInputs = []
const blurredInputs = []
// focusin and focusout bubble, unlike focus and blur
form.addEventListener('focusin', (event) => {
if (event.target.matches('input')) {
event.target.classList.add('focused')
focusedInputs.push(event.target.id)
}
})
form.addEventListener('focusout', (event) => {
if (event.target.matches('input')) {
event.target.classList.remove('focused')
blurredInputs.push(event.target.id)
}
})
// Focus first input
input1.focus()
expect(focusedInputs).toContain('input1')
expect(input1.classList.contains('focused')).toBe(true)
// Focus second input (should blur first)
input2.focus()
expect(focusedInputs).toContain('input2')
expect(blurredInputs).toContain('input1')
expect(input2.classList.contains('focused')).toBe(true)
expect(input1.classList.contains('focused')).toBe(false)
})
})
describe('Common mistakes', () => {
// Source: docs/beyond/concepts/event-delegation.mdx:519-530
describe('Mistake 1: Not using closest() for nested elements', () => {
it('should demonstrate why matches() alone fails with nested elements', () => {
container.innerHTML = `
<div class="container">
<button class="btn">
<span class="icon">X</span>
<span class="text">Delete</span>
</button>
</div>
`
const containerEl = container.querySelector('.container')
const icon = container.querySelector('.icon')
let matchesFound = false
let closestFound = null
containerEl.addEventListener('click', (event) => {
// Wrong way - matches() returns false for child elements
matchesFound = event.target.matches('.btn')
// Right way - closest() finds the button ancestor
closestFound = event.target.closest('.btn')
})
// Click on the icon inside the button
icon.click()
// matches() fails because icon is not .btn
expect(matchesFound).toBe(false)
// closest() succeeds by traversing up to find .btn
expect(closestFound).not.toBeNull()
expect(closestFound.classList.contains('btn')).toBe(true)
})
})
})
describe('Performance comparison', () => {
// Source: docs/beyond/concepts/event-delegation.mdx:409-425
it('should handle many elements with single listener', () => {
// Create 100 items
container.innerHTML = '<ul class="list"></ul>'
const list = container.querySelector('.list')
for (let i = 0; i < 100; i++) {
const li = document.createElement('li')
li.className = 'item'
li.dataset.id = i.toString()
li.textContent = `Item ${i}`
list.appendChild(li)
}
const clickedItems = []
// Single delegated listener handles all items
list.addEventListener('click', (event) => {
if (event.target.matches('.item')) {
clickedItems.push(event.target.dataset.id)
}
})
// Click various items
list.querySelector('[data-id="0"]').click()
list.querySelector('[data-id="50"]').click()
list.querySelector('[data-id="99"]').click()
expect(clickedItems).toEqual(['0', '50', '99'])
})
})
})