Blind 75 in JavaScript: Built-ins, Gaps and Interview Idioms

JavaScript can solve every Blind 75 problem, but the list runs into the three gaps in its standard library more often than you might expect: no heap, no fast queue and no binary search helper. The good news is that only one Blind 75 problem really needs a heap for the expected complexity, and the others have clean workarounds. This page maps each gap to the problems it affects, gives JavaScript idioms for the ten groups in the original post, covers the key and type mistakes that produce wrong answers, and lays out a six-week order. For the list itself, see our Blind 75 guide; for the 150-problem version in the same language, NeetCode 150 in JavaScript.

Where JavaScript's gaps hit the Blind 75

Blind 75 has 19 easy, 49 medium and 7 hard problems by LeetCode's current labels, and six of them need LeetCode Premium. Counts were read from each list's own site data on November 12, 2026. Lists are edited from time to time, so numbers can drift.

Missing piece	Blind 75 problems affected	What to do
Heap	Merge K Sorted Lists, Top K Frequent Elements, Find Median from Data Stream, Meeting Rooms II (Premium)	Merge lists in pairs (O(N log k), no heap); bucket by frequency; sort start and end times separately. Only Find Median needs two real heaps.
Queue	Level Order Traversal, Clone Graph, Course Schedule, Number of Islands, Pacific Atlantic Water Flow, Alien Dictionary	Process one level array and build the next, or keep a head index. Never call `shift()` in a loop; it is O(n).
Binary search	Search in Rotated Sorted Array, Find Minimum in Rotated Sorted Array, the O(n log n) Longest Increasing Subsequence	Write a lower-bound loop by hand; there is no `bisect` or `lower_bound`.
Ordered map	None	Blind 75 never needs one; sorting the input is enough.
Fixed-width integers	Sum of Two Integers, Number of 1 Bits, Reverse Bits, Counting Bits	Bitwise operators work on 32-bit integers, which is exactly what these problems want.

Group by group in JavaScript

Array (10): Two Sum stores value to index in a Map. Contains Duplicate is new Set(nums).size !== nums.length. Maximum Product Subarray swaps with [max, min] = [min, max] when the number is negative.
Binary (5): Sum of Two Integers works as written, because & and << convert to 32-bit integers: while (b !== 0) { const carry = (a & b) << 1; a ^= b; b = carry; }. Counting Bits is ans[i] = ans[i >> 1] + (i & 1). LeetCode's current Reverse Bits keeps n even and below 2 to the power 31, so the result stays positive; on a full 32-bit version, finish with >>> 0.
String (10): Valid Palindrome moves two indexes past characters that fail /[a-z0-9]/i.test(ch). Group Anagrams keys by [...s].sort().join('') or by a 26-slot count joined with '#'. Encode and Decode Strings (Premium) writes `${s.length}#${s}` and finds each length with indexOf('#', i).
Matrix (4): Rotate Image transposes with [m[i][j], m[j][i]] = [m[j][i], m[i][j]], then calls row.reverse() on every row. Word Search marks a cell '#' and restores it.
Linked List (6): LeetCode defines ListNode; start merges from new ListNode(0). Merge K Sorted Lists merges list pairs with a doubling step size, which avoids a heap entirely.
Interval (5): sort with (a, b) => a[0] - b[0] and read the last merged interval with merged.at(-1).
Tree (14): Validate Binary Search Tree is simpler than in C++ or Java: start with -Infinity and Infinity as bounds. Serialize to an array of values and 'null' markers joined with commas; deserialize with an index into split(','). Trie children fit in a plain object or a Map.
Graph (8): build adjacency with Array.from({ length: n }, () => []). Number of Islands receives the strings '1' and '0', so compare with strings. Graph Valid Tree and Number of Connected Components (Premium) run union-find on an Int32Array of parents. Alien Dictionary keeps a Map from letter to a Set of letters.
Heap (3): on LeetCode, new MinPriorityQueue((node) => node.val) gives a heap of list nodes; enqueue adds and dequeue returns the node itself.
Dynamic programming (11): Coin Change starts from new Array(amount + 1).fill(Infinity) and returns -1 if the last cell is still Infinity. Word Break tests a dictionary word at position i with s.startsWith(w, i), which avoids slicing. Decode Ways needs only two rolling variables.

Mistakes that produce wrong answers

Object keys are strings: counting numbers in {} and returning Object.keys(count) for Top K Frequent Elements returns strings such as '1', which LeetCode marks wrong. Count in a Map, whose keys keep their type.
for...in over an array: it yields string indexes, so i + 1 becomes '01'. Use for (let i = 0; i < n; i++) or for...of.
includes inside a loop: it scans the whole array, which turns Longest Consecutive Sequence, required to run in O(n), into O(n squared). Put the numbers in a Set and use has.
Default sort on numbers: sorting start times for Meeting Rooms II with starts.sort() orders them as strings, so 10 comes before 9.
Version 5 heap code: many older posted solutions call .dequeue().element. That was version 5 of the datastructures-js priority queue; in version 6, dequeue returns the value itself, so .element is undefined.
Methods, not properties: the LeetCode queues use size() and isEmpty() as methods, unlike arrays' length; pq.size without brackets is a function and always truthy.

What LeetCode gives you, and what interviews may not

LeetCode runs JavaScript on Node.js 26.12.1 with lodash 4.17.21 included and the datastructures-js packages available, including priority-queue 6.3.5, heap 4.3.7, deque 1.0.8 and queue 4.3.0, as LeetCode's help page on language environments listed it on November 12, 2026. TypeScript runs as TypeScript 7.0.2 on the same Node.js version. The priority-queue package offers MinPriorityQueue and MaxPriorityQueue built from a value callback, and PriorityQueue built from a comparator such as (a, b) => a - b. Most interview editors have none of this, so for Find Median from Data Stream practise a hand-written binary heap with push, pop, sift-up and sift-down, or ask whether you may assume one and state its O(log n) operations. Newer methods such as at and toSorted may be missing on older runtimes, so ask which Node.js version the editor uses.

A six-week order

Week 1: Array (10) plus Valid Anagram, Group Anagrams, Valid Parentheses and Valid Palindrome, 14 problems with Map and Set.
Week 2: the other six String problems and Binary (5), 11 problems; windows and 32-bit tricks.
Week 3: Linked List (6), Matrix (4) and Interval (5), 15 problems.
Week 4: Tree (14), recursive and iterative, with the level-array BFS.
Week 5: write your own heap, then Graph (8), Top K Frequent Elements and Find Median from Data Stream, 10 problems.
Week 6: Dynamic programming (11), then redo failed problems in a plain editor without LeetCode's libraries.

For comparison, the same list in a language with real heaps and fixed 32-bit integers is in Blind 75 in Java.

Turning a list into interview skill

Any list only helps if the practice transfers to problems you have not seen. These habits make the difference:

Time-box every attempt: about 25 minutes for a medium before you read a solution.
Keep a log with one line per problem: the pattern, the key idea and the complexity.
Redo failed problems after three days, a week and three weeks, without looking at your old code.
Explain your approach out loud before and while you code, as you would to an interviewer.
In the last two weeks, switch to unseen problems without topic labels.

The patterns most lists are built on are explained in array and string patterns and tree, graph and recursion patterns.

Where CoPilot Interview fits

Finishing a list is not the same as passing an interview: interviewers score how you explain your approach, your trade-offs and your complexity out loud. CoPilot Interview is an AI interview assistant; its free AI mock interview asks coding questions, listens to your spoken explanation and scores it out of ten with a model answer, which is the part a problem list cannot train. The free plan gives 15 minutes a day of live help in sessions of up to 15 minutes, with AI mock interviews included. Online assessments and live coding rounds usually forbid outside help, so practice now and follow your employer's rules on the day.

Disclosure: CoPilot Interview is our product, so weigh what we say about it accordingly.

Frequently asked questions

Can I do Blind 75 in JavaScript without a heap?

Almost all of it. Merge K Sorted Lists can merge in pairs, Top K Frequent Elements can bucket by frequency and Meeting Rooms II can sort start and end times. Find Median from Data Stream is the one that needs two heaps.

Why does my Top K Frequent Elements solution return strings?

Plain object keys are always strings, so Object.keys returns '1' instead of 1. Count with a Map, or convert the keys back with Number.

Which priority queue does LeetCode provide for JavaScript?

The datastructures-js priority-queue package, version 6.3.5 in November 2026, with MinPriorityQueue, MaxPriorityQueue and a comparator-based PriorityQueue. dequeue returns the stored value.

Is JavaScript accepted in coding interviews?

Most companies let you choose any mainstream language, and JavaScript is common for front-end and full-stack roles. Confirm with your recruiter, especially for online assessments.

Practice this interview before the real one

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