JavaScript Core & Event Loop

Equality Operators

Question	Answer
What is the difference between == and ===?	== compares values with type coercion. === compares both value and type (no coercion).

Data Types

Question	Answer
What are the data types in JavaScript?	Primitive: string, number, bigint, boolean, null, undefined, symbol. Non-Primitive: objects, arrays, functions.
Which are primitive vs non-primitive?	Primitive → immutable, stored by value. Non-Primitive → reference types, stored by reference.

Variables

Question	Answer
Difference between var, let, and const?	var: function-scoped, hoisted, can redeclare. let: block-scoped, cannot redeclare in same scope. const: block-scoped, must be initialized, cannot reassign.

Null vs Undefined

Question	Answer
Difference between null and undefined?	null: intentional empty value. undefined: variable declared but not assigned.

Functions

Question	Answer
Difference between normal function and arrow function regarding this?	Normal function: this depends on how it’s called. Arrow function: this is lexically inherited from outer scope.

Other Core Concepts

Question	Answer
What is hoisting?	Variables (var) and functions are moved to the top of their scope before execution.
What is a closure?	A function that retains access to variables from its lexical scope even after that scope has finished.
Key ES6 updates?	let, const, arrow functions, template literals, destructuring, modules, promises, classes, spread/rest, Map, Set, etc.
How to clone an array?	arr.slice(), [...arr], or Array.from(arr).
How to loop on objects?	for...in → keys for...of → values (arrays/iterables only) Object.keys(), Object.values(), Object.entries().
Difference between map and forEach?	forEach: executes callback, returns undefined. map: transforms array, returns new array.

---

Event Loop Example

Code	Expected Output
js<br>console.log("1 - Start");<br><br>setTimeout(() => { console.log("2 - setTimeout"); }, 0);<br><br>Promise.resolve()<br> .then(() => { console.log("3 - Promise then 1"); })<br> .then(() => { console.log("4 - Promise then 2"); });<br><br>console.log("5 - End");<br>	<br>1 - Start<br>5 - End<br>3 - Promise then 1<br>4 - Promise then 2<br>2 - setTimeout<br>

---

Senior-Level JavaScript

Question	Answer
What happens when typing google.com in browser?	DNS resolution → TCP/TLS handshake → HTTP request → server response → browser parses HTML → builds DOM/CSSOM → executes JS → renders page.
How to remove duplicates from an array?	const unique = [...new Set(arr)];
Difference between call, apply, and bind?	call: invoke with this + args (comma-separated). apply: invoke with this + args (array). bind: returns new function with bound this.
What is Tree Shaking?	Build optimization that removes unused code (works best with ES modules).
What are Web Vitals?	Performance metrics: LCP (Largest Contentful Paint), FID (First Input Delay), CLS (Cumulative Layout Shift). They help measure real-world user experience.

---

Angular

Question	Answer
What is Angular?	A TypeScript-based framework for building web apps.
What are Angular’s building blocks?	Components, Templates, Modules, Services, Dependency Injection, Directives, Pipes, Routing.
Lifecycle hooks in components?	ngOnInit, ngOnChanges, ngDoCheck, ngAfterViewInit, ngAfterContentInit, ngOnDestroy, etc.
What are directives?	Structural: *ngIf, *ngFor. Attribute: [ngClass], [ngStyle].
What are pipes?	Pure pipes: depend only on input. Impure pipes: re-run frequently, may cause performance issues.
Difference between Promise and Observable?	Promise → single async value, eager, not cancellable. Observable → multiple values over time, lazy, cancellable, supports RxJS operators.
How does change detection work?	Runs after async events (e.g., DOM events, XHR, setTimeout). Re-checks bindings and updates DOM.
What are Signals?	Fine-grained reactivity primitive (Angular 16+). Better control than global change detection.
Difference between Signals and previous CD strategy?	Signals trigger reactivity only where used, unlike zone-based CD that checks the whole tree.
What does encapsulation property in component decorator do?	Controls CSS scoping: Emulated (default), None (global), ShadowDom (native shadow DOM).
What is Dependency Injection?	A pattern where Angular provides services to components instead of components instantiating them manually.
Difference between service and normal class?	Service: @Injectable, managed by Angular DI. Normal class: not injectable.
Service provided in root vs module?	Root: singleton across app. Module: new instance for that module’s injector.
What is an interceptor?	Middleware for HTTP requests (e.g., add auth token, logging, error handling).
What is a guard?	Used in routing to control access (e.g., CanActivate, CanDeactivate).
What is a resolver?	Preloads data before navigating to a route.
Types of compilation in Angular?	JIT (compile in browser) and AOT (compile at build time).
Authentication vs Authorization?	Authentication → identity (who you are). Authorization → permissions (what you can access).

---

Performance Optimization in Angular

Question	Answer
How to improve Angular app performance?	1. Use OnPush change detection. 2. Use trackBy with *ngFor. 3. Lazy load modules. 4. Use PreloadingStrategy for critical modules. 5. Use Signals for efficient CD. 6. Bundle optimizations (Tree Shaking, Build Optimizer). 7. Lazy load images/files. 8. Use Web Workers for heavy tasks. 9. Cache data in services/state management. 10. Use Angular Universal (SSR) for faster first paint.

---

Behavioral

Question	Answer
How would you rate your level of seniority (junior, mid-level, senior)?	Candidate’s self-assessment here.
What experiences support that rating?	Candidate describes examples from career (projects, leadership, challenges solved).