Category Blueprint

JavaScript Interview Library

Q: What are template literals?

**Core concept (TL;DR).** Template literals are strings delimited by **backticks** that add three things over quoted strings: ${...} **interpolation** of any expression, real **multi-line** strings, and freedom from escaping the other quote characters. They're the default way to build strings in modern JavaScript. <svg viewBox="0

Q: Tricky JavaScript Scenario #41: Object.groupBy

**Intuition.** Understanding Object.groupBy is key for optimizing SDE applications. V8 relies on standard properties to evaluate native object property grouping. <text x="14" y="22" fill="currentColor" font-si

Q: Tricky JavaScript Scenario #36: Page Visibility API interval freezing

**Intuition.** Understanding Page Visibility API interval freezing is key for optimizing SDE applications. V8 relies on standard properties to evaluate optimizing frames when tab is backgrounded. <text x="14"

Q: What is the difference between var, let, and const?

**Core Concept (TL;DR)** Modern JavaScript distinguishes variables by block scoping, reassignment locks, and hoisting/initialization semantics. `var` is function-scoped and hoisted with initialization, while `let` and `const` are block-scoped and remain uninitialized (in the Temporal Dead Zone) until declared. <svg viewBox="0 0 520 180" style=

Q: Tricky JavaScript Scenario #46: Array.prototype.with

**Intuition.** Understanding Array.prototype.with is key for optimizing SDE applications. V8 relies on standard properties to evaluate replacing single element without mutation. <text x="14" y="22" fill="curre

Q: Tricky JavaScript Scenario #40: strict mode this bindings binding rules

**Intuition.** Understanding strict mode this bindings binding rules is key for optimizing SDE applications. V8 relies on standard properties to evaluate sloppy vs strict context parsing limits. <text x="14" y

Q: Tricky JavaScript Scenario #43: Array.prototype.toSorted

**Intuition.** Understanding Array.prototype.toSorted is key for optimizing SDE applications. V8 relies on standard properties to evaluate non-mutating sorting operations. <text x="14" y="22" fill="currentColo

Q: What is the difference between for...of and for...in?

**Core concept (TL;DR).** for…in iterates the **enumerable property keys** of an object (including inherited ones) — use it for plain objects. for…of iterates the **values** of an iterable (arrays, strings, Map , Set ). Rule of thumb: in for object **keys**, of for iterable **values** — and don't use for…in on arrays. <div style="margin:1.25rem auto;max-width:560px;border:1px solid rgba(245,158,11,0.25);bor

Q: What does Array.filter do?

**Core concept (TL;DR).** filter returns a **new array** containing only the elements for which the callback returns a truthy value. It never mutates the original, and the result is the same length or shorter — the idiomatic way to "keep the items that match a condition." <svg viewBox="0 0 520 156" style="width:100%;display:block"

Q: What is hoisting in JavaScript?

**Core Concept (TL;DR)** Hoisting is a behavior in JavaScript where the engine allocates memory for variable and function declarations during the compilation phase of the execution context, before any code is executed. This makes declarations accessible in their scope before the line where they are defined. <svg viewBox="0 0 800 520" style="width:

Closures, async programming & the engine core

Questions196

Views119.8k

Upvotes8.4k

Filter

Question Catalog

(196 visible)

JavaScript Interview Library

Question Catalog

What are template literals?

Tricky JavaScript Scenario #41: Object.groupBy

Tricky JavaScript Scenario #36: Page Visibility API interval freezing

What is the difference between var, let, and const?

Tricky JavaScript Scenario #46: Array.prototype.with

Tricky JavaScript Scenario #40: strict mode this bindings binding rules

Tricky JavaScript Scenario #43: Array.prototype.toSorted

What is the difference between for...of and for...in?

What does Array.filter do?

What is hoisting in JavaScript?

Tricky JavaScript Scenario #44: Array.prototype.toReversed

What are static methods and properties?

Tricky JavaScript Scenario #63: Passive event listener scroll enhancements

Tricky JavaScript Scenario #76: Array.prototype.with

Tricky JavaScript Scenario #73: Array.prototype.toSorted

What are optional chaining and nullish coalescing?

Tricky JavaScript Scenario #100: strict mode this bindings binding rules

What is the difference between includes and indexOf?

Describe the concept of 'scope' in JavaScript.

Tricky JavaScript Scenario #70: strict mode this bindings binding rules

Tricky JavaScript Scenario #11: Object.groupBy

Tricky JavaScript Scenario #66: Page Visibility API interval freezing

What is the difference between preventDefault and stopPropagation?

What are `let`, `const`, and `var`? What are their differences?

What is the difference between null and undefined?

What is the difference between find and filter?

What is globalThis?

Tricky JavaScript Scenario #93: Passive event listener scroll enhancements

What is the difference between Object.is, ==, and ===?

What is short-circuit evaluation?

Tricky JavaScript Scenario #71: Object.groupBy

Tricky JavaScript Scenario #35: High Resolution Time API precision

What are the quirks of the typeof operator?

What is event bubbling and capturing?

What do Array.some and Array.every do?

What is the difference between synchronous and asynchronous code?

Tricky JavaScript Scenario #65: High Resolution Time API precision

What is the difference between localStorage, sessionStorage, and cookies?

Tricky JavaScript Scenario #74: Array.prototype.toReversed

Tricky JavaScript Scenario #14: Array.prototype.toReversed

Tricky JavaScript Scenario #33: Passive event listener scroll enhancements

Tricky JavaScript Scenario #95: High Resolution Time API precision

Tricky JavaScript Scenario #96: Page Visibility API interval freezing

How does instanceof work?

Tricky JavaScript Scenario #16: Array.prototype.with

What is the difference between map and forEach?

What is destructuring?

What do Object.keys, values, and entries return?

What is JSON and why is it commonly used?

What is the difference between == and ===?

Tricky JavaScript Scenario #13: Array.prototype.toSorted

How does `try...catch` work in JavaScript?

Debounce vs throttle — implement debounce

How do you implement infinite currying in JavaScript?

What is the difference between Object.freeze, seal, and preventExtensions?

Tricky JavaScript Scenario #90: BroadcastChannel multi-tab updates

How do you convert a callback-based function to a Promise?

Explain 'this' keyword in JavaScript.

How does the fetch API work?

When should you use Map and Set over objects and arrays?

What is memoization?

How does WeakMap allow garbage collection of keys?

Tricky JavaScript Scenario #78: Proxy traps custom sets

Implement a robust Event Emitter class.

Tricky JavaScript Scenario #34: navigator.sendBeacon final telemetry

Tricky JavaScript Scenario #31: History API route state handling

Tricky JavaScript Scenario #68: ResizeObserver sizing alterations tracking

Tricky JavaScript Scenario #18: Proxy traps custom sets

What does Object.create do?

How do you correctly check for NaN?

What are modules in JavaScript?

What is lexical scoping and the scope chain?

How do you implement a deepFreeze function recursively?

What is a Promise and what are its states?

Explain event delegation.

What is an IIFE?

How does Array.reduce work?

What is event delegation?