Jquery Interview Questions 2025 Advanced Interview Questions & Answers
40 questions available
Mid
Answer
jQuery remains in legacy systems because it:
Normalizes browser quirks pre-ES6
Bundles DOM + AJAX utility
Exists in millions of old enterprise apps
Requires zero build tooling
Lightweight for simple pages.
Modern frameworks haven’t fully replaced it in maintenance projects.
Mid
Answer
bind() ? direct event binding, deprecated
delegate() ? event delegation to parent
on() ? unified method replacing both; supports:
delegation
namespaces
dynamic elements
chaining.
Mid
Answer
Uses bubbling phase:
Handler bound to ancestor
Event bubbles upward
jQuery checks event.target + selector match
Executes handler only for matched descendants.
Efficient for dynamic DOM.
Mid
Answer
Sizzle:
Parses CSS selectors
Tokenizes selector groups
Uses optimized DOM lookup algorithms
Falls back to querySelectorAll
Highly optimized for older browsers.
Mid
Answer
.html() inserts raw HTML and executes inline scripts.
Allows malicious injections.
Secure alternatives:
.text()
DOMPurify
Server-side sanitization.
Mid
Answer
.attr() ? HTML attributes (initial)
.prop() ? DOM properties (current)
checked ? property
HTML value attribute ? attribute.
Mid
Answer
.css() forces layout/reflow.
Multiple .css() calls cause layout thrashing.
Optimize via:
.addClass()
Batching style updates.
Mid
Answer
Deferred:
resolve(), reject(), then(), done(), fail().
Not fully standard until jQuery 3.x.
Used for async logic before native Promises.
Mid
Answer
$.fn is an alias of jQuery prototype.
Used to create custom plugins:
$.fn.myPlugin = function() {}
Mid
Answer
Uses:
Array.prototype.slice.call(obj)
Checks length
Converts NodeLists, arguments objects, etc.
Mid
Answer
Because:
Bound at document root
Bad performance
Hard to control scope
Replaced by .on() with delegation.
Mid
Answer
serialize() ? URL encoded string
serializeArray() ? array of { name, value } objects.
Mid
Answer
jQuery queues:
fx queue for animations
Custom queues via .queue()
FIFO task execution.
Mid
Answer
Every method returns the same jQuery object.
Enables:
$("#box").addClass("a").fadeIn().slideDown();
Mid
Answer
jQuery optimizes via:
Selector caching
Minimized DOM traversals
Native API delegation
Sizzle for complex selectors.
Mid
Answer
jQuery.ready() fires when DOM is parsed and includes old IE fallbacks.
Native DOMContentLoaded lacks full legacy browser support.
jQuery normalized behavior across inconsistent browsers.
Mid
Answer
width() ? content
innerWidth() ? content + padding
outerWidth() ? content + padding + border (+ margin optional).
Mid
Answer
.animate() uses JS timers and CPU-based tweening.
Cannot offload to GPU.
Heavy DOM ? slow animations.
CSS transitions are faster.
Mid
Answer
.clone() ? clones element only
.clone(true) ? clones element + event handlers.
Mid
Answer
jQuery stores data in an internal cache keyed by element ID.
Prevents DOM pollution.
.removeData() frees memory.
Mid
Answer
Because jQuery mutates DOM manually.
React/Angular rely on Virtual DOM or templating.
Direct DOM manipulation breaks component lifecycle and rendering.
Mid
Answer
.empty() ? removes children
.remove() ? removes node + events
.detach() ? removes node but keeps events/data for reuse.
Mid
Answer
Prefilters modify AJAX options before sending.
Transports implement low-level AJAX mechanics.
Used for mocking, overriding defaults, or custom transports.
Mid
Answer
.promise() ? waits for queue/animation completion
$.when() ? coordinates multiple Deferreds across async flows.
Mid
Answer
.toggleClass():
Avoids redundant DOM reads
Supports boolean toggling
Handles complex class sets.
Mid
Answer
.each() returns jQuery object, passes (index, element), supports early termination via return false.
Native loops do not wrap or chain.
Mid
Answer
jQuery returns only element nodes.
.next() skips text nodes by design.
Native DOM needed to include text nodes.
Mid
Answer
Event namespaces allow selective unbinding and organized event structures:
$("#btn").on("click.save", handler);
Mid
Answer
.offset() ? relative to document
.position() ? relative to offset parent.
Mid
Answer
Because wrapper enables:
Chaining
Custom methods
Bulk DOM operations
Utility functions.
Mid
Answer
jQuery.ajax() supports:
Automatic timeout
JSONP
beforeSend hooks
Global AJAX events
Automatic content-type handling
Fetch lacks many of these legacy features.
Mid
Answer
.filter() ? applies JS predicate to matched set.
:has() ? DOM structural selector.
.filter() is procedural; :has() is declarative.
Mid
Answer
jQuery normalizes event object:
event.target
coordinates
button values
Browser inconsistencies are unified.
Native DOM events vary across browsers.
Mid
Answer
.parseHTML() builds DOM instantly and can execute script tags.
Huge XSS attack vector.
Use DOMPurify or server-side sanitization.
Mid
Answer
.find() ? deep traversal
.children() ? direct descendants only.
Mid
Answer
.wrap():
Creates a wrapper
Moves element inside it
Triggers DOM reflow
Used for grouping or styling structure.
Mid
Answer
.show() ? toggles CSS display
.fadeIn() ? animates opacity using jQuery’s fx engine
.fadeIn() incurs CPU animation cost.
Mid
Answer
jQuery wrapper stores array-like DOM elements.
.get(0) extracts the raw DOM node for native API access.
Mid
Answer
Deferred existed before ES6 Promises.
Needed backward compatibility.
Provides .done(), .fail(), .progress().
Native Promises lack progress notifications.
Mid
Answer
Used when $ conflicts with other libraries like Prototype or MooTools.
var jq = jQuery.noConflict();
Prevents namespace collision in legacy codebases.