NeetCode 150 in JavaScript: Built-ins, Gaps and Interview Idioms

JavaScript is a common choice for front-end and full-stack candidates, but it is missing three data structures the NeetCode 150 leans on: a heap, a real double-ended queue and an ordered map. Its numbers, sorting and bitwise operators also behave differently from other languages. This page covers what JavaScript lacks and how to stand in for it, the pitfalls that produce wrong answers, and a plan through the list's 18 categories. For the list itself, see our NeetCode 150 roadmap; for language questions, our JavaScript interview questions.

What JavaScript gives you and what it lacks

The list has 150 problems: 28 easy, 101 medium and 21 hard, in these categories: arrays and hashing 9, two pointers 5, sliding window 6, stack 6, binary search 7, linked list 11, trees 15, heaps 7, backtracking 10, tries 3, graphs 13, advanced graphs 6, 1-D dynamic programming 12, 2-D dynamic programming 11, greedy 8, intervals 6, math and geometry 8, bit manipulation 7. Counts were read from each list's own site data in November 2026. Lists are edited from time to time, so numbers can drift.

Pitfalls that produce wrong answers

• Default sort is by string: [10, 9, 1].sort() gives [1, 10, 9]. Always pass (a, b) => a - b. The comparator must return a number; returning a > b (a boolean) gives unreliable orders. sort is stable and sorts in place; toSorted returns a copy.
• Bitwise operators work on 32-bit signed integers: 1 << 31 is -2147483648. For Reverse Bits and Number of 1 Bits, finish with result >>> 0 to read the bits as unsigned.
• Numbers are 64-bit floats: integers are exact only up to Number.MAX_SAFE_INTEGER, 2 to the power 53 minus 1. Products in Maximum Product Subarray stay safe at LeetCode's limits, but hashing a pair as r * 1e9 + c can lose precision.
• Integer division: use Math.floor(a / b) for non-negative values and Math.trunc(a / b) when Evaluate Reverse Polish Notation needs truncation toward zero; (a / b) | 0 breaks above 32 bits.
• Negative remainders: -1 % 5 is -1; write ((x % m) + m) % m for a true modulo.
• Shared rows: new Array(n).fill([]) puts the same array in every slot. Build grids with Array.from({ length: m }, () => new Array(n).fill(0)).
• Arrays as keys: a Map or Set compares arrays by reference, so seen.has([r, c]) is always false. Use a string key such as `${r},${c}` or the number r * cols + c.

Category by category

• Arrays and hashing (9): for Group Anagrams, build a key from a 26-slot count array joined with a separator; for Top K Frequent Elements, bucket by frequency to avoid needing a heap.
• Two pointers (5) and sliding window (6): s.charCodeAt(i) - 97 indexes a 26-slot Int32Array, which is faster than a Map.
• Stack (6): plain arrays; for Daily Temperatures, store indexes, not values.
• Binary search (7): lo + ((hi - lo) >> 1) is safe because array lengths fit in 32 bits.
• Linked list (11) and trees (15): LeetCode defines ListNode and TreeNode for you; deep recursion can exhaust the call stack, whose depth depends on frame size, so know the iterative traversals.
• Heaps (7) and advanced graphs (6): on LeetCode, use the provided priority queue; in other editors, ask whether you may assume a heap exists, which interviewers often allow if you state its complexity.
• Backtracking (10): push, recurse, pop on one array and save [...path] copies.
• Graphs (13): Array.from({ length: n }, () => []) for adjacency; a head-index queue for BFS.
• Dynamic programming (23): Infinity works as the initial value inside Math.min; typed arrays such as Int32Array are zero-filled for bottom-up tables.
• Greedy, intervals, math and bits (29): sort intervals with (a, b) => a[0] - b[0]; Sum of Two Integers works naturally with 32-bit ^, & and <<.

What LeetCode's JavaScript environment gives you

LeetCode runs JavaScript on Node.js 26.12.1, includes lodash 4.17.21 by default and lets you use the datastructures-js packages, including priority-queue 6.3.5, deque 1.0.8 and queue 4.3.0, as LeetCode's help page on language environments listed it on November 11, 2026. That makes heap problems short on LeetCode. Interview editors usually have none of these, so practise one hand-written heap with push, pop, sift-up and sift-down until you can write it in under ten minutes. TypeScript is often allowed too and catches some of the type slips above.

A six-week plan

1. Week 1: arrays, hashing, two pointers and sliding window, with Map, Set and count arrays.
2. Week 2: stack, binary search and linked lists.
3. Week 3: trees and tries, recursive and iterative.
4. Week 4: write your own heap, then heaps, backtracking and graphs.
5. Week 5: advanced graphs and dynamic programming.
6. Week 6: greedy, intervals, math and bits, then mixed timed review in a plain editor without LeetCode's libraries.

For comparison, the same plan with Java's collections is in NeetCode 150 in Java, and the shorter Blind 75 guide covers the first half of the list.

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:

1. Time-box every attempt: about 25 minutes for a medium before you read a solution.
2. Keep a log with one line per problem: the pattern, the key idea and the complexity.
3. Redo failed problems after three days, a week and three weeks, without looking at your old code.
4. Explain your approach out loud before and while you code, as you would to an interviewer.
5. 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

Is JavaScript good for NeetCode 150?

Yes, especially for front-end roles. You need workarounds for the missing heap, queue and ordered map, and care with sorting and bitwise operators.

Does JavaScript have a priority queue?

Not built in. LeetCode provides the datastructures-js library; in most interview editors you write a small binary heap or ask whether you may assume one.

Why does sort give the wrong order on numbers?

Without a comparator, sort compares values as strings. Pass (a, b) => a - b for ascending numbers.

Should I use TypeScript instead?

If the interview allows it, TypeScript catches some type mistakes. The algorithms and pitfalls are the same as in JavaScript.