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All JavaScript interview questions

Data types in JS
Data types in JavaScript
== vs ===
Loose (==) vs strict (===) equality in JavaScript
Difference between primitives and non-primitives in JavaScript
Ways to declare a variable: var, let, const
Ways to declare a variable
What is the difference between null and undefined?
What is scope (variable visibility)?
Scope in JavaScript
Variable before declaration
Hoisting mechanism
Boxing and unboxing in JavaScript
Dynamic typing
The typeof operator in JavaScript
The typeof operator
Why typeof null is object
typeof null
NaN
Type Coercion
Explicit and implicit type conversion
Strict mode in JavaScript
Why [] == false returns true
[] == false ??
Template literals
Increment and decrement
Increment and decrement
The ! operator
The ! operator (logical NOT)
The in operator
The in operator
The delete operator
The delete operator
The instanceof operator?
The instanceof operator
The new operator
The void operator
The void operator
Ternary operator
The && operator
The && operator
Nullish coalescing operator
The nullish coalescing operator (??)
Truthy/falsy values
Accessing a non-existent object property
Nonexistent object property
String concatenation
What is JSON?
Working with JSON
Working with JSON: stringify and parse
Checking for an array
Checking whether a value is an array
Pass by value vs pass by reference
By reference or by value
Primitives and objects in memory
Immutable and mutable data
Immutable/mutable data
Symbol in JavaScript
Symbol in JS
BigInt vs Int
BigInt vs Number
Checking whether a property exists in an object
Property presence in an object
Ways to create an object
Creating an object
Spread operator
Errors and error handling
Errors and their handling
Optional chaining operator
Optional chaining operator
Conditional constructs
Conditional statements
break in switch
Types of loops in JS
Types of loops in JS
The continue keyword
The continue keyword
The break keyword
The break keyword
The for of loop
The for in loop
The label statement in JavaScript
The label operator
The throw statement in JavaScript
Global variables
Why is eval dangerous?
Why eval is dangerous
Arrow functions vs regular functions
Function declarations
Difference between FD and FE
Anonymous functions
hoisting
Arguments by value
Closures
Function context
The arguments keyword
...rest parameters
Default parameters
spread when calling a function
When is it better not to use arrow functions?
IIFE
Function Constructor
Function.prototype.call
Function.prototype.apply
call vs apply
Function.prototype.bind?
toString() on a function
Function factory
wrapper function
Decorator function
Generators
function* in JS
Yield in generators
Generator return value
Yield inside () => {}
Generators and iterators
this in JavaScript
Losing context
Benefits of closures
Mistake with var in closures and loop
Scope of a function
Scope with var | let | const
use strict
Global variables
Callback function
Callback in another function
Parameters in a callback
Async/sync callback
setTimeout(fn, 0)
Recursion
Base case in recursion
What problems does recursion solve
Tail recursion
Deep recursion
Higher-order function
Pure function
Side effects
Stateless function
Immutability in functions
Currying
Partial application
Currying vs partial application
Function composition
Implementing compose
Errors inside functions
Object in JS
Ways to declare an object
Object vs array
Key-value
New property in an object
Deleting a property from an object
Checking whether a property exists in an object
Object.keys()
Object.entries()
Copying an object
Shallow vs deep object copying
How do you make a deep copy of an object?
Why can't you copy an object with the = operator?
Copying through Object.assign
structuredClone
Object literal
Bracket notation
Missing property
Property shorthand ({ x, y })
Object methods after creation
Readonly properties
What does Object.defineProperty() do?
What does Object.assign() do?
What does Object.create() do?
What does Object.freeze() do?
What does Object.seal() do?
What does Object.preventExtensions() do?
What is a prototype?
Prototype chain
How does __proto__ differ from prototype?
Pure object without prototype
Property lookup in the prototype chain
instanceof and prototypes
this inside an object method
toString() on an object
What does the toJSON() method do?
What is a "property descriptor"?
The writable, enumerable, configurable flags?
What does Reflect.defineProperty() do?
How does Reflect differ from Object?
What does the Proxy object do?
What is a trap (traps)?
Filter in an array
forEach in an array
find() in an array
reduce() in an array
some() in an array
every() in an array
sort() in an array
flatMap() vs map()
How to work with an array
Array index
typeof []
Array check
Different data types in an array
Changing length manually
delete arr[0]
Accessing a nonexistent index
"dense" and "sparse" array
How to get the last element of an array?
What does push() do?
What does pop() do?
What does shift() do?
What does unshift() do?
push() vs concat()
What does splice() do?
What does reverse() do?
Sorting an array of numbers
What does join() do?
Shallow copy of an array
What does Array.from() do?
What does Array.of() do?
What is an array-like object?
What does copyWithin() do?
What does toReversed() do?
What does toSorted() do?
What does toSpliced() do?
for...in VS for...of in an array
The ... spread operator
Destructuring with skipped elements
What is a two-dimensional array?
How to iterate a two-dimensional array?
Flat array from a multidimensional array
What is WeakSet?
What is Map?
map VS object
WeakMap VS Map
async/await for asynchronous requests
What is asynchrony in JavaScript?
Why is JS considered single-threaded?
Synchronous VS asynchronous code
Multithreading in JS?
Blocking operation
What is the event loop?
What is the call stack (call stack)?
What is the task queue?
What are microtasks (microtasks)?
Micro vs Macro tasks
Infinite loop
Types of asynchronous operations
What does setTimeout() do and return?
What does setInterval() do?
setTimeout(fn, 0)
Timer execution guarantee
Nested timers
What does queueMicrotask() do?
What does requestAnimationFrame() do?
What does requestIdleCallback() do?
What is a callback function?
What is "callback hell"?
What is a Promise?
When is a promise considered "fulfilled"?
Promise states
How does resolve differ from reject?
then, catch, finally
How does promise chaining work?
What does Promise.all() do?
What does Promise.race() do?
What does Promise.any() do?
How to cancel a promise?
What do AbortController and AbortSignal do?
What does Promise.withResolvers() do?
How does a "promise microtask" work?
What does an async function return?
What does await do?
What happens if you don't use await?
Blocking the current function with await
await and arrow functions
Errors in async
What does the Web API do in the browser?
What is the "microtask queue"?
How does WebSocket work?
How do streams (ReadableStream, WritableStream) work?
What does a Web Worker do?
What does postMessage() do between threads?
Why doesn't a Web Worker have access to the DOM?
What does SharedWorker do?
What does a Service Worker do?
What is the DOM tree?
What is document?
What is window?
How does window differ from document?
What does navigator do?
What does the history object do?
Selectors for working with DOM elements
How do you create an element in the DOM?
How to remove an element from the DOM?
Security and innerHTML
Attributes vs properties of a DOM element
What are data-* attributes?
What is an event in the DOM?
What do addEventListener() and removeEventListener() do?
How does onclick differ from addEventListener()?
What does event.target do?
How does event.target differ from event.currentTarget?
What does event.preventDefault() do?
Event delegation (Event Delegation)
Event bubbling
Event capturing
What is "event delegation"?
await in an event handler
An anonymous function as a handler
What does form.reset() do?
How to disable page scrolling?
What does IntersectionObserver do?
How does MutationObserver work?
What does window.location do?
What does localStorage do?
What does sessionStorage do?
localStorage VS sessionStorage
How does DOMContentLoaded differ from load?
What does the beforeunload event do?
What does the unload event do?
What does defer do in <script>?
What does async do in <script>?
What is the Canvas API?
What is Shadow DOM?
Shadow DOM vs DOM
What is indexedDB?
What is a class constructor?
What is a class in JavaScript?
Class vs constructor function
How does a class differ from a constructor function?
Is it possible not to define a constructor?
Can you omit the constructor
How to add a method to a class
How do you add a method to a class?
Class method VS function in an object property
What does this do inside a class?
Arrow methods vs regular methods
How do arrow methods differ from regular ones?
Class as an expression const A = class {}
A class as an expression const A = class {}
The extends keyword
The super keyword
What does the super() keyword do?
Multiple constructors in a class
Multiple constructors in a class
return inside a constructor
What does return do inside a constructor?
Default values for properties
The static keyword in classes
What does the static keyword do?
How to call a static method
How do you call a static method?
Private properties
Private class properties
Private class fields outside a class
Can private properties be inherited?
Inheriting private class fields
Getters and setters in a class
Getters and setters
How are classes related to prototypes?
Classes and prototypes
Checking for a class instance
Checking a class instance
What is an error in JavaScript?
Syntax error VS runtime error
What is an exception?
What does the throw operator do?
What types of errors exist in JavaScript?
How do you create a custom error type?
What does the try...catch construct do?
When does the catch block run?
What does the finally block do?
Does finally run if try has a return?
How do you create an error manually?
What does the call stack (stack trace) contain?
What does instanceof Error do?
How to extend Error with custom properties?
What does catch() do in promises?
Errors in async functions
What is debugging?
Browser debugging methods
What does the debugger statement do?
ES6 and why it is the most important update
How do let and const differ from var?
What are arrow functions?
What is destructuring?
Default values in destructuring
What does the ... operator do?
Template literals
What does a "tagged template literal" do?
Classes in JS
import/export
What is "tree shaking"?
What did ES6 bring for asynchrony?
What is a promise?
What is Map?
What does Set do?
What is Symbol?
What is an iterator?
Proxy, traps
What does Reflect do?
What is BigInt?
What does structuredClone() do?
Metrics for evaluating performance
Bottlenecks in JS code
Single-threaded JS
What does "non-blocking code" mean?
What is memoization?
How do you implement function memoization?
Input data size
What is a "bottleneck" in algorithms?
What is a memory leak in JS?
What does the garbage collector (GC) do?
What is "tree shaking"?
Timers and event listeners
How to profile memory usage?
What are "reflow" and "repaint"?
Batching DOM changes
Why is virtual DOM used?
Array methods vs loops
Why not to block the event loop?
What does a Web Worker do?
Minimizing HTTP requests
What is "lazy loading"?
What is "code splitting"?
What does "tree shaking" do?
Code minification
How does "defer" differ from "async" in <script>?
Scripts in head
How to measure JS code performance?
How do WeakMap and WeakSet help the garbage collector?
What does "debounce" do?
What does "throttle" do?
What does Core Web Vitals measure?
Execution context
What is an execution context made of?
Scope chain (chain of scopes)
Lexical scope in JavaScript
Global object
Loop with var and setTimeout
Event loop in detail
What does AbortController do?
What is the garbage collector (Garbage Collector)? Explain in detail how it is structured
How do you avoid memory leaks?
What is a property descriptor?
What does Proxy do?
What does Reflect do?
Why is Reflect needed together with Proxy?
What is an iterator?
What is a generator (function*)?
How does "tree shaking" work?
What is "top-level await"?
Dynamic import
Why is eval() dangerous?
What does WeakRef do?
What does SharedArrayBuffer do?
What is the Temporal API?
Ways to declare functions
Destructuring assignment in JavaScript
Function declaration vs function expression
Spread and REST operators in JavaScript: differences and examples
What is a polyfill?
Passing arguments by value or by reference
Closures
Function context
The arguments object
IIFE, immediately invoked function expression
Function.prototype.apply
call vs apply
Function.prototype.bind
Explicitly setting this in JavaScript
Factory function
Wrapper function
Generator functions
The function* keyword
The yield operator in generators
What a generator call returns
Yield inside an arrow function
The this keyword
Losing the this context
What closures are used for
The var and closures pitfall in a loop
Scope of var, let and const
The use strict directive
Callback function
Callback hell
Passing a callback to another function
Parameters in a callback
Synchronous and asynchronous callback
setTimeout(fn, 0)
Tasks solved with recursion
Replacing recursion with a loop
Deep recursion
Higher-order function
Pure function
Side effects
Stateless function
Immutability in functions
Currying
Implementing compose
Errors inside functions
Differences between arrow Function, Function declaration and Function expression
Ways to declare an object
Key-value pairs
Adding a property to an object
Call, apply and bind methods in JavaScript
Removing a property from an object
What are higher-order functions in JavaScript (hof)
Checking whether a property exists in an object
The Object.keys() method
Pure functions and side effects in JavaScript
The Object.values() method
The Object.entries() method
Deep copy of an object
Why the = operator does not copy an object
Copying with Object.assign
structuredClone
Missing property vs undefined value
Property shorthand
Getters and setters in an object
Adding methods after an object is created
Read-only properties
Object.create()
Object.freeze() in JavaScript
Object.seal() in JavaScript
Object.preventExtensions() in JavaScript
What is stored in __proto__
Clean object without a prototype
Property lookup in the prototype chain
The toJSON() method in JavaScript
The writable, enumerable and configurable flags
Reflect.defineProperty()
Proxy traps
Array map method
scope in JavaScript: types and working principles
Array filter method
Array find method
Lexical environment in JavaScript
Array reduce() method
Array some() method
Array every() method
Array sort() method
Creating an array
typeof for an array
Checking whether a value is an array
Mixed data types in one array
Changing an array length manually
The delete operator on an array
delete vs splice()
Accessing a non-existent array index
Dense and sparse arrays
The push() method
The pop() method
The unshift() method
The splice() method
What the reverse() method does
Sorting an array of numbers
What the join() method does
What the split() method does
Shallow copy of an array
The Array.from() method
The copyWithin() method
The toSorted() method
The toSpliced() method
for...in vs for...of on an array
Two-dimensional array
Iterating a two-dimensional array
Flattening a nested array
WeakSet in JavaScript
Map vs object
WeakMap vs Map
Asynchrony in JavaScript
Difference between in operator and hasOwnProperty() method in JavaScript
Object.freeze(), object.seal() and object.assign() in JavaScript
Why JavaScript is considered single threaded
What the term non-blocking means
How to copy an Object in JavaScript?
Synchronous vs asynchronous code
OOP in JavaScript (Object-oriented programming)
Prototypes and prototypal inheritance in JavaScript
Blocking operation
What is the event loop
Static methods in JavaScript
Task queue in JavaScript
Microtasks in JavaScript
Microtasks vs macrotasks
Infinite loop
What setTimeout() does and returns
setTimeout(fn, 0)
Timer accuracy guarantee
Nested timers
The queueMicrotask() function
The requestAnimationFrame() function
The requestIdleCallback() function
Callback function
What is callback hell
Promise
Difference between resolve and reject
Promise chaining
Promise.all() in JavaScript
Promise.allSettled() in JavaScript
Promise.race() in JavaScript
Promise.any() in JavaScript
Promise.withResolvers()
The promise microtask
The async keyword
What await does
What happens if you omit await
Why await pauses only the current function
await in arrow functions
Error handling in async functions
What Web API does in the browser
Macrotask queue
Streams: ReadableStream and WritableStream
Web Workers in JavaScript
postMessage() between threads
Why a Web Worker has no DOM access
SharedWorker in JavaScript
Service Worker
The DOM tree
Difference between DOM and HTML
Mutating and non-mutating Array methods in JavaScript
The document object
Set, Map, WeakSet and WeakMap in JavaScript
The window object
The difference between window and document
The navigator object
DOM selectors in JavaScript
Creating a DOM element
Security and innerHTML
addEventListener and removeEventListener
onclick vs addEventListener
The event.target property
The event.stopPropagation() method
The event.preventDefault() method
Event bubbling
Event delegation
await inside an event handler
Cancelling form submission
The form.reset() method
Blocking page scroll
IntersectionObserver
MutationObserver
window.location
localStorage
sessionStorage
The beforeunload event
The unload event
The defer attribute in script
The async attribute in script
Shadow DOM vs regular DOM
setTimeout and setInterval in JavaScript
What is promise chaining in JavaScript
Promise.all, promise.race, promise.allsettled, promise.any
What is the difference between async/await and chaining?
Event propagation in JavaScript and its phases
What happens when an error occurs
Difference between event.preventdefault() and event.stoppropagation()
Syntax error vs runtime error
What is an exception
What the throw statement does
Types of errors in JavaScript
How to create a custom error type
The try...catch statement
When the catch block runs
The finally block
A new error thrown inside catch
Throwing an error manually
What a stack trace contains
Extending Error with custom properties
Errors in async functions
Browser debugging techniques
The debugger statement
How can algorithm complexity be determined?
ES6 as the most important JS update
The difference between let, const and var
Destructuring
Spread and rest operator
Difference between spread and rest
Tagged template literals
Classes in JavaScript
JavaScript modules: import/export
What ES6 brought for async code
The Map collection in JavaScript
The Set collection in JavaScript
The Symbol type in JavaScript
Proxy and the get and set traps
The Reflect object
The structuredClone() function
Performance evaluation metrics
Bottlenecks in JS code
CPU-bound and I/O-bound tasks
Non-blocking code
Function memoization
Input size and performance
Memory leaks in JS
The garbage collector (GC) in JavaScript
Timers and event listeners
Profiling memory usage
Reflow and repaint
Batching DOM updates
Array methods vs loops
Blocking the event loop
Splitting a heavy task into chunks
Minimizing HTTP requests
Tree shaking
Code minification
defer vs async in a script tag
Scripts in head
Dynamic import()
Measuring JS code performance
Throttle in JavaScript
Core Web Vitals
What is idempotence?
Execution context components
What is a dead letter queue?
Lexical scope
A loop with var and setTimeout
Temporal dead zone (TDZ)
AbortController in JavaScript
Garbage collector (GC)
How to avoid memory leaks
Property descriptor
The Proxy object
The Reflect object
Iterator
Generators and yield
Async generator
Tree shaking
Top-level await
Dynamic import
Why eval() is dangerous
What WeakRef does
What SharedArrayBuffer does
Temporal API in JavaScript

Interview Questions/Frontend/JavaScript

JavaScript

The language the web runs on: types and coercion, closures, prototypes, the event loop, promises and async.

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01Language Basics96

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Expand section

Data types in JavaScriptJunior
Data types in JSJunior
Difference between primitives and non-primitives in JavaScriptJunior
Loose (==) vs strict (===) equality in JavaScriptJunior
== vs ===Junior
Ways to declare a variable: var, let, constJunior
Ways to declare a variableJunior
What is the difference between null and undefined?Junior
Scope in JavaScriptJunior
What is scope (variable visibility)?Junior
Variable before declarationJunior
Hoisting mechanismJunior
Boxing and unboxing in JavaScriptMiddle
Dynamic typingJunior
The typeof operator in JavaScriptMiddle
The typeof operatorMiddle
Why typeof null is objectJunior
typeof nullJunior
NaNMiddle
Type CoercionJunior
Strict mode in JavaScriptMiddle
Explicit and implicit type conversionJunior
Why [] == false returns trueJunior
[] == false ??Junior
Template literalsJunior
Increment and decrementJunior
Increment and decrementJunior
The ! operator (logical NOT)Junior
The ! operatorJunior
The in operatorJunior
The in operatorJunior
The delete operatorJunior
The delete operatorJunior
The instanceof operatorJunior
The instanceof operator?Junior
The new operatorJunior
The void operatorJunior
The void operatorJunior
Ternary operatorJunior
The && operatorJunior
The && operatorJunior
The nullish coalescing operator (??)Junior
Nullish coalescing operatorJunior
Truthy/falsy valuesJunior
Accessing a non-existent object propertyMiddle
Nonexistent object propertyMiddle
String concatenationJunior
What is JSON?Junior
Working with JSON: stringify and parseJunior
Working with JSONJunior
Checking whether a value is an arrayJunior
Checking for an arrayJunior
By reference or by valueJunior
Pass by value vs pass by referenceJunior
Primitives and objects in memoryJunior
Immutable and mutable dataMiddle
Immutable/mutable dataMiddle
Symbol in JavaScriptMiddle
Symbol in JSMiddle
BigInt vs IntJunior
BigInt vs NumberJunior
Property presence in an objectJunior
Checking whether a property exists in an objectJunior
Creating an objectMiddle
Ways to create an objectMiddle
Spread operatorJunior
Errors and error handlingJunior
Errors and their handlingJunior
Optional chaining operatorJunior
Optional chaining operatorJunior
Conditional constructsJunior
Conditional statementsJunior
break in switchMiddle
Types of loops in JSJunior
Types of loops in JSJunior
The continue keywordJunior
The continue keywordJunior
The break keywordJunior
The break keywordJunior
The for of loopJunior
The for in loopJunior
The label statement in JavaScriptMiddle
The label operatorMiddle
The throw statement in JavaScriptJunior
Global variablesJunior
Why is eval dangerous?Junior
Why eval is dangerousJunior

02Functions65

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Expand section

Arrow functions vs regular functionsMiddle
Function declarationsJunior
Difference between FD and FEJunior
Anonymous functionsJunior
hoistingJunior
Arguments by valueJunior
ClosuresMiddle
Function contextJunior
The arguments keywordJunior
...rest parametersJunior
Default parametersJunior
spread when calling a functionJunior
When is it better not to use arrow functions?Junior
IIFEJunior
Function ConstructorJunior
Function.prototype.callJunior
Function.prototype.applyJunior
call vs applyJunior
Function.prototype.bind?Middle
toString() on a functionJunior
Function factoryJunior
wrapper functionMiddle
Decorator functionJunior
GeneratorsMiddle
function* in JSMiddle
Yield in generatorsJunior
Generator return valueMiddle
Yield inside () => {}Middle
Generators and iteratorsMiddle
this in JavaScriptJunior
Losing contextJunior
Benefits of closuresJunior
Mistake with var in closures and loopJunior
Scope of a functionJunior
Scope with var | let | constJunior
use strictJunior
Global variablesJunior
Callback functionJunior
Callback in another functionJunior
Parameters in a callbackJunior
Async/sync callbackMiddle
setTimeout(fn, 0)Junior
RecursionJunior
Base case in recursionJunior
What problems does recursion solveJunior
Tail recursionJunior
Deep recursionMiddle
Higher-order functionJunior
Pure functionJunior
Side effectsJunior
Stateless functionJunior
Immutability in functionsJunior
CurryingMiddle
Partial applicationJunior
Currying vs partial applicationMiddle
Function compositionJunior
Implementing composeJunior
Errors inside functionsJunior

03Objects and prototypes46

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Object in JSMiddle
Ways to declare an objectJunior
Object vs arrayJunior
Key-valueJunior
New property in an objectJunior
Deleting a property from an objectJunior
Checking whether a property exists in an objectJunior
Object.keys()Junior
Object.entries()Junior
Copying an objectJunior
Shallow vs deep object copyingJunior
How do you make a deep copy of an object?Junior
Why can't you copy an object with the = operator?Junior
Copying through Object.assignJunior
structuredCloneJunior
Object literalJunior
Bracket notationJunior
Missing propertyJunior
Property shorthand ({ x, y })Junior
Object methods after creationJunior
Readonly propertiesJunior
What does Object.defineProperty() do?Junior
What does Object.assign() do?Junior
What does Object.create() do?Junior
What does Object.freeze() do?Junior
What does Object.seal() do?Junior
What does Object.preventExtensions() do?Middle
What is a prototype?Junior
Prototype chainJunior
How does __proto__ differ from prototype?Junior
Pure object without prototypeJunior
Property lookup in the prototype chainJunior
instanceof and prototypesJunior
this inside an object methodJunior
toString() on an objectMiddle
What does the toJSON() method do?Junior
What is a "property descriptor"?Middle
The writable, enumerable, configurable flags?Middle
What does Reflect.defineProperty() do?Middle
How does Reflect differ from Object?Middle
What does the Proxy object do?Middle
What is a trap (traps)?Senior

04Arrays and data structures51

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Expand section

Filter in an arrayMiddle
forEach in an arrayJunior
find() in an arrayJunior
reduce() in an arrayJunior
some() in an arrayJunior
every() in an arrayJunior
sort() in an arrayJunior
flatMap() vs map()Junior
How to work with an arrayJunior
Array indexJunior
typeof []Junior
Array checkJunior
Different data types in an arrayJunior
Changing length manuallyJunior
delete arr[0]Junior
Accessing a nonexistent indexJunior
"dense" and "sparse" arrayJunior
How to get the last element of an array?Junior
What does push() do?Junior
What does pop() do?Junior
What does shift() do?Junior
What does unshift() do?Junior
push() vs concat()Junior
What does splice() do?Junior
What does reverse() do?Middle
Sorting an array of numbersJunior
What does join() do?Junior
Shallow copy of an arrayJunior
What does Array.from() do?Junior
What does Array.of() do?Junior
What is an array-like object?Junior
What does copyWithin() do?Middle
What does toReversed() do?Middle
What does toSorted() do?Junior
What does toSpliced() do?Junior
for...in VS for...of in an arrayJunior
The ... spread operatorJunior
Destructuring with skipped elementsJunior
What is a two-dimensional array?Junior
How to iterate a two-dimensional array?Junior
Flat array from a multidimensional arrayJunior
What is WeakSet?Junior
What is Map?Junior
map VS objectJunior
WeakMap VS MapJunior

05Asynchrony and the Event Loop57

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Expand section

async/await for asynchronous requestsJunior
What is asynchrony in JavaScript?Junior
Why is JS considered single-threaded?Junior
Synchronous VS asynchronous codeJunior
Multithreading in JS?Middle
Blocking operationJunior
What is the event loop?Junior
What is the call stack (call stack)?Middle
What is the task queue?Junior
What are microtasks (microtasks)?Junior
Micro vs Macro tasksJunior
Infinite loopJunior
Types of asynchronous operationsJunior
What does setTimeout() do and return?Junior
What does setInterval() do?Junior
setTimeout(fn, 0)Junior
Timer execution guaranteeJunior
Nested timersJunior
What does queueMicrotask() do?Junior
What does requestAnimationFrame() do?Junior
What does requestIdleCallback() do?Middle
What is a callback function?Junior
What is "callback hell"?Junior
What is a Promise?Junior
When is a promise considered "fulfilled"?Junior
Promise statesJunior
How does resolve differ from reject?Middle
then, catch, finallyJunior
How does promise chaining work?Junior
What does Promise.all() do?Junior
What does Promise.race() do?Junior
What does Promise.any() do?Junior
How to cancel a promise?Junior
What do AbortController and AbortSignal do?Junior
What does Promise.withResolvers() do?Middle
How does a "promise microtask" work?Junior
What does an async function return?Junior
What does await do?Junior
What happens if you don't use await?Junior
Blocking the current function with awaitJunior
await and arrow functionsJunior
Errors in asyncJunior
What does the Web API do in the browser?Junior
What is the "microtask queue"?Junior
How does WebSocket work?Middle
How do streams (ReadableStream, WritableStream) work?Senior
What does a Web Worker do?Senior
What does postMessage() do between threads?Senior
Why doesn't a Web Worker have access to the DOM?Senior
What does SharedWorker do?Senior
What does a Service Worker do?Senior

06DOM and the browser46

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What is the DOM tree?Junior
What is document?Junior
What is window?Junior
How does window differ from document?Junior
What does navigator do?Junior
What does the history object do?Junior
Selectors for working with DOM elementsJunior
How do you create an element in the DOM?Junior
How to remove an element from the DOM?Junior
Security and innerHTMLJunior
Attributes vs properties of a DOM elementJunior
What are data-* attributes?Junior
What is an event in the DOM?Junior
What do addEventListener() and removeEventListener() do?Middle
How does onclick differ from addEventListener()?Junior
What does event.target do?Junior
How does event.target differ from event.currentTarget?Junior
What does event.preventDefault() do?Junior
Event delegation (Event Delegation)Junior
Event bubblingJunior
Event capturingJunior
What is "event delegation"?Junior
await in an event handlerJunior
An anonymous function as a handlerJunior
What does form.reset() do?Junior
How to disable page scrolling?Junior
What does IntersectionObserver do?Middle
How does MutationObserver work?Middle
What does window.location do?Junior
What does localStorage do?Junior
What does sessionStorage do?Junior
localStorage VS sessionStorageJunior
How does DOMContentLoaded differ from load?Middle
What does the beforeunload event do?Junior
What does the unload event do?Junior
What does defer do in <script>?Junior
What does async do in <script>?Junior
What is the Canvas API?Middle
What is Shadow DOM?Middle
Shadow DOM vs DOMJunior
What is indexedDB?Middle

07Classes42

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What is a class constructor?Junior
What is a class in JavaScript?Junior
Class vs constructor functionJunior
How does a class differ from a constructor function?Junior
Is it possible not to define a constructor?Junior
Can you omit the constructorJunior
How do you add a method to a class?Junior
How to add a method to a classJunior
Class method VS function in an object propertyJunior
What does this do inside a class?Junior
How do arrow methods differ from regular ones?Junior
Arrow methods vs regular methodsJunior
Class as an expression const A = class {}Junior
A class as an expression const A = class {}Junior
The extends keywordJunior
The super keywordJunior
What does the super() keyword do?Junior
Multiple constructors in a classJunior
Multiple constructors in a classJunior
What does return do inside a constructor?Junior
return inside a constructorJunior
Default values for propertiesJunior
What does the static keyword do?Junior
The static keyword in classesJunior
How to call a static methodJunior
How do you call a static method?Junior
Private propertiesJunior
Private class propertiesJunior
Private class fields outside a classJunior
Inheriting private class fieldsJunior
Can private properties be inherited?Junior
Getters and setters in a classJunior
Getters and settersJunior
Classes and prototypesJunior
How are classes related to prototypes?Junior
Checking a class instanceJunior
Checking for a class instanceJunior

08Errors and debugging21

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What is an error in JavaScript?Junior
Syntax error VS runtime errorJunior
What is an exception?Junior
What does the throw operator do?Junior
What types of errors exist in JavaScript?Junior
How do you create a custom error type?Junior
What does the try...catch construct do?Junior
When does the catch block run?Junior
What does the finally block do?Junior
Does finally run if try has a return?Junior
How do you create an error manually?Junior
What does the call stack (stack trace) contain?Junior
What does instanceof Error do?Junior
How to extend Error with custom properties?Junior
What does catch() do in promises?Junior
Errors in async functionsMiddle
What is debugging?Junior
Browser debugging methodsJunior
What does the debugger statement do?Junior

09Modern JS ES6+24

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Expand section

ES6 and why it is the most important updateJunior
How do let and const differ from var?Junior
What are arrow functions?Junior
What is destructuring?Junior
Default values in destructuringJunior
What does the ... operator do?Junior
Template literalsJunior
What does a "tagged template literal" do?Junior
Classes in JSJunior
import/exportJunior
What is "tree shaking"?Junior
What did ES6 bring for asynchrony?Junior
What is a promise?Junior
What is Map?Junior
What does Set do?Junior
What is Symbol?Junior
What is an iterator?Middle
Proxy, trapsMiddle
What does Reflect do?Middle
What is BigInt?Junior
What does structuredClone() do?Junior

10Performance in JS35

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Metrics for evaluating performanceJunior
Bottlenecks in JS codeJunior
Single-threaded JSJunior
What does "non-blocking code" mean?Junior
What is memoization?Junior
How do you implement function memoization?Junior
Input data sizeJunior
What is a "bottleneck" in algorithms?Junior
What is a memory leak in JS?Junior
What does the garbage collector (GC) do?Junior
What is "tree shaking"?Junior
Timers and event listenersJunior
How to profile memory usage?Junior
What are "reflow" and "repaint"?Middle
Batching DOM changesJunior
Why is virtual DOM used?Middle
Array methods vs loopsJunior
Why not to block the event loop?Junior
What does a Web Worker do?Middle
Minimizing HTTP requestsJunior
What is "lazy loading"?Junior
What is "code splitting"?Junior
What does "tree shaking" do?Junior
Code minificationJunior
How does "defer" differ from "async" in <script>?Junior
Scripts in headJunior
How to measure JS code performance?Junior
How do WeakMap and WeakSet help the garbage collector?Middle
What does "debounce" do?Junior
What does "throttle" do?Junior
What does Core Web Vitals measure?Junior

11Advanced topics25

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Execution contextSenior
What is an execution context made of?Senior
Scope chain (chain of scopes)Senior
Lexical scope in JavaScriptMiddle
Global objectJunior
Loop with var and setTimeoutJunior
Event loop in detailMiddle
What does AbortController do?Junior
What is the garbage collector (Garbage Collector)? Explain in detail how it is structuredMiddle
How do you avoid memory leaks?Junior
What is a property descriptor?Middle
What does Proxy do?Middle
What does Reflect do?Middle
Why is Reflect needed together with Proxy?Middle
What is an iterator?Junior
What is a generator (function*)?Middle
How does "tree shaking" work?Middle
What is "top-level await"?Junior
Dynamic importJunior
Why is eval() dangerous?Junior
What does WeakRef do?Senior
What does SharedArrayBuffer do?Senior
What is the Temporal API?Middle

12Other questions310

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Destructuring assignment in JavaScriptJunior
Ways to declare functionsJunior
Spread and REST operators in JavaScript: differences and examplesJunior
Function declaration vs function expressionJunior
What is a polyfill?Junior
Passing arguments by value or by referenceJunior
ClosuresMiddle
Function contextJunior
The arguments objectJunior
IIFE, immediately invoked function expressionJunior
Function.prototype.applyJunior
call vs applyJunior
Function.prototype.bindMiddle
Explicitly setting this in JavaScriptJunior
Factory functionJunior
Wrapper functionMiddle
Generator functionsMiddle
The function* keywordMiddle
The yield operator in generatorsJunior
What a generator call returnsMiddle
Yield inside an arrow functionMiddle
The this keywordJunior
Losing the this contextJunior
What closures are used forJunior
The var and closures pitfall in a loopJunior
Scope of var, let and constJunior
The use strict directiveJunior
Callback functionJunior
Callback hellJunior
Passing a callback to another functionJunior
Parameters in a callbackJunior
Synchronous and asynchronous callbackMiddle
setTimeout(fn, 0)Junior
Tasks solved with recursionJunior
Replacing recursion with a loopJunior
Deep recursionMiddle
Higher-order functionJunior
Pure functionJunior
Side effectsJunior
Stateless functionJunior
Immutability in functionsJunior
CurryingMiddle
Implementing composeJunior
Errors inside functionsJunior
Differences between arrow Function, Function declaration and Function expressionMiddle
Ways to declare an objectJunior
Key-value pairsJunior
Adding a property to an objectJunior
Call, apply and bind methods in JavaScriptMiddle
Removing a property from an objectJunior
What are higher-order functions in JavaScript (hof)Middle
Checking whether a property exists in an objectJunior
The Object.keys() methodJunior
Pure functions and side effects in JavaScriptMiddle
The Object.values() methodJunior
The Object.entries() methodJunior
Deep copy of an objectJunior
Why the = operator does not copy an objectJunior
Copying with Object.assignJunior
structuredCloneJunior
Missing property vs undefined valueJunior
Property shorthandJunior
Getters and setters in an objectJunior
Adding methods after an object is createdJunior
Read-only propertiesJunior
Object.create()Junior
Object.freeze() in JavaScriptJunior
Object.seal() in JavaScriptJunior
Object.preventExtensions() in JavaScriptMiddle
What is stored in __proto__Junior
Clean object without a prototypeJunior
Property lookup in the prototype chainJunior
The toJSON() method in JavaScriptJunior
The writable, enumerable and configurable flagsMiddle
Reflect.defineProperty()Middle
Proxy trapsSenior
scope in JavaScript: types and working principlesMiddle
Array map methodJunior
Array filter methodMiddle
Array find methodJunior
Lexical environment in JavaScriptMiddle
Array reduce() methodJunior
Array some() methodJunior
Array every() methodJunior
Array sort() methodJunior
Creating an arrayJunior
typeof for an arrayJunior
Checking whether a value is an arrayJunior
Mixed data types in one arrayJunior
Changing an array length manuallyJunior
The delete operator on an arrayJunior
delete vs splice()Junior
Accessing a non-existent array indexJunior
Dense and sparse arraysJunior
The push() methodJunior
The pop() methodJunior
The unshift() methodJunior
The splice() methodJunior
What the reverse() method doesMiddle
Sorting an array of numbersJunior
What the join() method doesJunior
What the split() method doesJunior
Shallow copy of an arrayJunior
The Array.from() methodJunior
The copyWithin() methodMiddle
The toSorted() methodJunior
The toSpliced() methodJunior
for...in vs for...of on an arrayJunior
Two-dimensional arrayJunior
Iterating a two-dimensional arrayJunior
Flattening a nested arrayJunior
WeakSet in JavaScriptJunior
Map vs objectJunior
WeakMap vs MapJunior
Difference between in operator and hasOwnProperty() method in JavaScriptMiddle
Asynchrony in JavaScriptJunior
Object.freeze(), object.seal() and object.assign() in JavaScriptMiddle
Why JavaScript is considered single threadedJunior
How to copy an Object in JavaScript?Middle
What the term non-blocking meansJunior
Synchronous vs asynchronous codeJunior
OOP in JavaScript (Object-oriented programming)Middle
Prototypes and prototypal inheritance in JavaScriptMiddle
Blocking operationJunior
Static methods in JavaScriptMiddle
What is the event loopJunior
Task queue in JavaScriptJunior
Microtasks in JavaScriptJunior
Microtasks vs macrotasksJunior
Infinite loopJunior
What setTimeout() does and returnsJunior
setTimeout(fn, 0)Junior
Timer accuracy guaranteeJunior
Nested timersJunior
The queueMicrotask() functionJunior
The requestAnimationFrame() functionJunior
The requestIdleCallback() functionMiddle
Callback functionJunior
What is callback hellJunior
PromiseJunior
Difference between resolve and rejectMiddle
Promise chainingJunior
Promise.all() in JavaScriptJunior
Promise.allSettled() in JavaScriptJunior
Promise.race() in JavaScriptJunior
Promise.any() in JavaScriptJunior
Promise.withResolvers()Middle
The promise microtaskJunior
The async keywordJunior
What await doesJunior
What happens if you omit awaitJunior
Why await pauses only the current functionJunior
await in arrow functionsJunior
Error handling in async functionsJunior
What Web API does in the browserJunior
Macrotask queueJunior
Streams: ReadableStream and WritableStreamSenior
Web Workers in JavaScriptSenior
postMessage() between threadsSenior
Why a Web Worker has no DOM accessSenior
SharedWorker in JavaScriptSenior
Service WorkerSenior
The DOM treeJunior
Mutating and non-mutating Array methods in JavaScriptJunior
Difference between DOM and HTMLJunior
Set, Map, WeakSet and WeakMap in JavaScriptMiddle
The document objectJunior
The window objectJunior
The difference between window and documentJunior
The navigator objectJunior
DOM selectors in JavaScriptJunior
Creating a DOM elementJunior
Security and innerHTMLJunior
addEventListener and removeEventListenerMiddle
onclick vs addEventListenerJunior
The event.target propertyJunior
The event.stopPropagation() methodJunior
The event.preventDefault() methodJunior
Event bubblingJunior
Event delegationJunior
await inside an event handlerJunior
Cancelling form submissionJunior
The form.reset() methodJunior
Blocking page scrollJunior
IntersectionObserverMiddle
MutationObserverMiddle
window.locationJunior
localStorageJunior
sessionStorageJunior
The beforeunload eventJunior
The unload eventJunior
The defer attribute in scriptJunior
The async attribute in scriptJunior
Shadow DOM vs regular DOMJunior
setTimeout and setInterval in JavaScriptJunior
What is promise chaining in JavaScriptMiddle
Promise.all, promise.race, promise.allsettled, promise.anyMiddle
What is the difference between async/await and chaining?Junior
Event propagation in JavaScript and its phasesMiddle
What happens when an error occursJunior
Difference between event.preventdefault() and event.stoppropagation()Middle
Syntax error vs runtime errorJunior
What is an exceptionJunior
What the throw statement doesJunior
Types of errors in JavaScriptJunior
How to create a custom error typeJunior
The try...catch statementJunior
When the catch block runsJunior
The finally blockJunior
A new error thrown inside catchJunior
Throwing an error manuallyJunior
What a stack trace containsJunior
Extending Error with custom propertiesJunior
Errors in async functionsMiddle
Browser debugging techniquesJunior
The debugger statementJunior
How can algorithm complexity be determined?Junior
ES6 as the most important JS updateJunior
The difference between let, const and varJunior
DestructuringJunior
Spread and rest operatorJunior
Difference between spread and restJunior
Tagged template literalsJunior
Classes in JavaScriptJunior
JavaScript modules: import/exportJunior
What ES6 brought for async codeJunior
The Map collection in JavaScriptJunior
The Set collection in JavaScriptJunior
The Symbol type in JavaScriptJunior
Proxy and the get and set trapsMiddle
The Reflect objectMiddle
The structuredClone() functionJunior
Performance evaluation metricsJunior
Bottlenecks in JS codeJunior
CPU-bound and I/O-bound tasksMiddle
Non-blocking codeJunior
Function memoizationJunior
Input size and performanceJunior
Memory leaks in JSJunior
The garbage collector (GC) in JavaScriptJunior
Timers and event listenersJunior
Profiling memory usageJunior
Reflow and repaintMiddle
Batching DOM updatesJunior
Array methods vs loopsJunior
Blocking the event loopJunior
Splitting a heavy task into chunksJunior
Minimizing HTTP requestsJunior
Tree shakingJunior
Code minificationJunior
defer vs async in a script tagJunior
Scripts in headJunior
Dynamic import()Junior
Measuring JS code performanceJunior
Throttle in JavaScriptJunior
Core Web VitalsJunior
What is idempotence?Senior
What is a dead letter queue?Senior
Execution context componentsSenior
Lexical scopeMiddle
A loop with var and setTimeoutJunior
Temporal dead zone (TDZ)Middle
AbortController in JavaScriptJunior
Garbage collector (GC)Middle
How to avoid memory leaksJunior
Property descriptorMiddle
The Proxy objectMiddle
The Reflect objectMiddle
IteratorJunior
Generators and yieldMiddle
Async generatorMiddle
Tree shakingMiddle
Top-level awaitJunior
Dynamic importJunior
Why eval() is dangerousJunior
What WeakRef doesSenior
What SharedArrayBuffer doesSenior
Temporal API in JavaScriptMiddle

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