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JavaScript

705 problems50 sections📅 2026 curriculum

Learn JavaScript by typing real, idiomatic code.

Type your way through the JavaScript you actually write: closures, destructuring, async/await, and the array methods (map, filter, reduce) that shape everyday code. Each snippet is real, idiomatic JavaScript, and an AI tutor explains every line — why a promise resolves the way it does, when to reach for a spread — while you build the muscle memory to type it fast.

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Run JavaScript on your machine

1. Install the toolchain

brew install node # or: sudo apt install nodejs

2. Save your code as hello.js, then compile & run

node hello.js

Official install docs ↗

progress by section

1Fundamentals0/12start →

Hello, JavaScript let and const Functions if / else if / else Numbers Template literals Ternary operator Arrow functions Default parameters for and for...of switch Destructuring

2Strings In Depth0/18start →

Change case Concatenate Repeat Character with at()startsWith and endsWith Strings are immutable Join an array Parse a number Search text Slice a substring Split into parts Template literals Trim whitespace Pad Replace Iterate code points Code points Match with a regex

3Values & References0/18start →

Primitives copy by value Arrays share a reference const binds, not contents Reference identity Shallow copy an object Objects share a reference Primitive arguments are safe Shallow copy with spread Compare by contents === vs Object.is Object.freeze Functions mutate objects Rebinding is local Return new vs mutate Deep copy fill shares one reference Object identity as a key Shallow copies share nesting

4Slices & Arrays0/18Unlock →

Arrays Reverse Access with at()concat and join Shallow copy fill Find and test Head and tail Negative indices Slice a range flat Sort Sort by a key splice inserts splice removes toSpliced with()Chunk into groups

5Data structures0/9Unlock →

Array methods Object as a record Array as a stack Map word count Array as a queue Set dedup Spread and rest Typed arrays WeakMap

6Iterators0/20Unlock →

7Traits0/20Unlock →

8Generics0/20Unlock →

9Lifetimes0/20Unlock →

10Closures0/20Unlock →

11Pattern Matching0/20Unlock →

12Error Handling0/20Unlock →

13Option & Result0/20Unlock →

14Conversions0/15Unlock →

Boolean()Number()String()Template coercion Array.from iterables Array.from a string codePointAt String.fromCodePoint JSON.parse JSON.stringify parseFloat parseInt with radix toString(radix)Unary plus Spread into characters

15Collection Conversions0/18Unlock →

Array to Set Pairs to a Map Set to sorted array Dedupe keeping order Array.flat Object.fromEntries Map keys and values Map to Object Object.entries Object to Map flatMap Frequency with a Map Invert an object Zip two arrays into a Map Object.groupBy Group with reduce Map.groupBy Set algebra

16Math0/14Unlock →

Math constants Float functions Integer arithmetic min, max, clamp Rounding Logarithms & exp Trigonometry Factorial & BigInt GCD and LCM hypot & atan2 Primality test Sign-safe modulo Min/max of an array Mean, median, stdev

17Encoding0/22Unlock →

18Random0/20Unlock →

19Data I/O0/18Unlock →

Parse CSV into rows JSON stringify and parse Tab-separated values Build a CSV string Select one column CSV into a number matrix Split off the header Whitespace-separated matrix Read a data file Skip blanks and comments Print an aligned table Write a matrix as text Append a row key=value config to a Map key=value config to object Stream lines with readline Round-trip write then read Drop unparseable rows

20Linear Algebra0/21Unlock →

21Text Processing0/16Unlock →

Trim whitespace Word count Count occurrences Strip all whitespace Replace text Reverse word order Count vowels Zero-pad a number Capitalize the first letter Character frequency Longest word Number the lines Pad and align Title case Truncate with an ellipsis Levenshtein distance

22Date & Time0/12Unlock →

ISO-8601 string Epoch milliseconds now Day of the week Time a block of work To and from epoch ms Year, month, day Seconds to HH:MM:SS Parse an ISO string Add days to a date Difference in days Format in a time zone Parse a clock string

23Memory0/12Unlock →

Swap two values References vs copies Replace and keep the old Object.is vs ===Deep copy with structuredClone Take, leaving empty Memory held by closures Shallow vs deep copy ArrayBuffer and typed arrays WeakMap as a cache Garbage collection WeakRef

24Modules0/20Unlock →

25Macros0/21Unlock →

26Testing0/20Unlock →

27Algorithms0/8Unlock →

Greatest common divisor Binary search Partition by predicate Palindrome (two pointers)Fibonacci with memoization Merge two sorted arrays Fisher-Yates shuffle Two sum

28Sorting Algorithms0/12Unlock →

Array.sort with a comparator Check if sorted Bubble sort Insertion sort Selection sort Counting sort Sort stability Merge sort Shell sort Heapsort Quicksort Radix sort

29Expression Parsing0/6Unlock →

Evaluate postfix (RPN)Tokenize an expression Evaluate infix Recursive-descent calculator Right-associative power Shunting-yard

30Idioms0/9Unlock →

Destructuring Spread Template literals Methods over loops Optional chain + nullish Short-circuiting Shorthand + defaults Ternary chains Arrows and ?? vs ||

31OOP & Traits0/8Unlock →

Class and methods Composition over inheritance Getters and setters extends and super instanceof and toString Private fields Static members Polymorphism via override

32Trait Objects0/14Unlock →

Array sharing a method Accept anything with the method Return one of several types Array of functions Closure carrying data Object with two roles Recover the concrete type Runtime type check Shared default behavior Shared object reference Sum over mixed objects Lazy vs eager sequence Handler registry Symbol-based protocol

33Design Patterns0/14Unlock →

Singleton Adapter Builder Factory Method Lazy Initialization Strategy Decorator Dependency Injection Observer Prototype Command State Object Pool Actor

34Async0/11Unlock →

async and await Sleep as a promise Promise.all Start now, await later Error propagation Draining a queue An async generator Concurrent vs sequential race, any, and allSettled A timeout with AbortSignal A custom async iterator

35File I/O0/6Unlock →

Read a file Write a file Append to a file Write then read back Async file I/O Read lines with readline

36Filesystem0/18Unlock →

Does a path exist?Copy a file Build a path Create nested directories List a directory Rename or move Change the extension Files vs directories File metadata Path components Remove files and trees Sorted listing Write to the temp dir Filter by extension Last modified time Resolve to a real path Total size of a tree Walk the tree

37Networking0/16Unlock →

Parse a URL Split host and port Status codes Parse a request line Route by path URLSearchParams Build a raw HTTP response Bind an ephemeral port Read request headers Cancel a request with AbortController Serve and consume JSON An HTTP server on an ephemeral port POST a request body A TCP echo server and client Read newline-delimited data A UDP datagram exchange

38Concurrency0/15Unlock →

async and await Promise.all Sleeping without blocking Promise.allSettled Promise.any Promise.race Microtasks vs timers Concurrent vs sequential An async task queue A short-lived worker thread Atomics operations A mutex from a promise chain A concurrency limiter Passing data to a worker Atomics on a SharedArrayBuffer

39Unsafe & FFI0/14Unlock →

Bytes to and from hex Typed arrays ArrayBuffer and a view Buffer reads Buffer writes structuredClone Byte manipulation SharedArrayBuffer A shared subarray Atomics on shared memory DataView endianness Bits to a float Reinterpret the same bytes Transferring a buffer

40Commands0/14Unlock →

Skip node and the script Command-line arguments Current directory Env var with a default Read an environment variable Iterate all env vars Set and remove a variable Exit with a code Path to this module Pass env to a child Run a command Spawn and wait Command exit status Read all of stdin

41Array0/10Unlock →

Contains Duplicate Two Sum Best Time to Buy and Sell Stock Maximum Subarray Container With Most Water Find Minimum in Rotated Sorted Array Product of Array Except Self Maximum Product Subarray Search in Rotated Sorted Array 3Sum

42Binary0/5Unlock →

Missing Number Number of 1 Bits Counting Bits Reverse Bits Sum of Two Integers

43String0/10Unlock →

Valid Anagram Valid Palindrome Valid Parentheses Encode and Decode Strings Group Anagrams Longest Substring Without Repeating Characters Longest Palindromic Substring Longest Repeating Character Replacement Palindromic Substrings Minimum Window Substring

44Matrix0/4Unlock →

Rotate Image Set Matrix Zeroes Spiral Matrix Word Search

45Interval0/5Unlock →

Meeting Rooms Merge Intervals Insert Interval Non-overlapping Intervals Meeting Rooms II

46Linked List0/6Unlock →

Reverse Linked List Linked List Cycle Merge Two Sorted Lists Remove Nth Node From End Merge K Sorted Lists Reorder List

47Tree0/14Unlock →

Maximum Depth of Binary Tree Invert Binary Tree Same Tree Lowest Common Ancestor of a BST Binary Tree Level Order Traversal Subtree of Another Tree Kth Smallest Element in a BST Implement Trie (Prefix Tree)Validate Binary Search Tree Add and Search Word Construct Binary Tree from Preorder and Inorder Binary Tree Maximum Path Sum Serialize and Deserialize Binary Tree Word Search II

48Graph0/8Unlock →

Number of Connected Components Number of Islands Clone Graph Course Schedule Longest Consecutive Sequence Graph Valid Tree Alien Dictionary Pacific Atlantic Water Flow

49Heap0/3Unlock →

Kth Largest Element in an Array Top K Frequent Elements Find Median from Data Stream

50Dynamic Programming0/11Unlock →

Climbing Stairs House Robber Unique Paths Coin Change House Robber II Jump Game Combination Sum IV Decode Ways Longest Common Subsequence Longest Increasing Subsequence Word Break