Best AI Coding Assistant for JavaScript in 2026

Six assistants, one 12,400-line plain JavaScript repository, no TypeScript anywhere. We counted the failures that only happen when there is no compiler to catch them.

Cursor 3.7 Pro is the best AI coding assistant for plain JavaScript in 2026. It closed all six of our tasks, converted nine CommonJS files to ESM without a wrong import, and never wrote a type annotation into a .js file. GitHub Copilot Pro at $10 wrote three, and two of those files stopped parsing. TypeScript catches a wrong suggestion with a red underline; a .js project finds out in production.

From October 12 to 16, 2026, six assistants worked the same 12,400-line repository: a Node 22 Express API and a React 19 client, 268 Vitest tests, no TypeScript anywhere, and JSDoc checked by tsc --noEmit --checkJs. Six tasks, identical wording, a fresh checkout for each tool.

Cursor went green on 5 of 6 tasks at the first attempt and 6 of 6 after one follow-up. Copilot finished 5 of 6 and left the module migration at 241 of 268 tests. The gap is not model quality. It is whether the tool runs the suite before it says the work is done.

The Short Answer

Why JavaScript Is the Harder Test

Types are a safety net the assistant never gets credit for. In a TypeScript file, a model that invents a field or forgets a null check earns a red underline and you fix it in three seconds. Remove the types and the same mistake parses, lints clean, and ships.

JavaScript also offers more ways to be plausibly wrong. An await inside .forEach looks like it waits and does not. A method handed to a callback loses this. 0.1 + 0.2 is 0.30000000000000004. No compiler catches any of that, so the suite is the only thing between a confident answer and a bug.

How We Ran It

One repository, one developer, five working days. The API is Express 5 on Node 22 with module type set, better-sqlite3 and Zod 4; the client is React 19 with TanStack Query 5. There is no .ts file in the tree and grep -r "interface " src/ returns nothing.

Each tool started from the same commit with no instructions file on day one. One retry per task was allowed and logged as a follow-up, not a first pass. A task counted only when the whole suite passed and a hidden check written beforehand also passed: importing every public export after the migration, and comparing 5,000 invoice lines against a decimal library for the rounding bug.

The Ranking: Six AI Coding Assistants for JavaScript

1. Cursor 3.7 Pro: best overall ($20)

Cursor indexed the repo, noticed the jsconfig.json and the flat ESLint config, and ran npm test twice without being asked. On the migration it swapped __dirname for import.meta.dirname in the two files that needed it, a detail every other tool missed at least once. Its one follow-up was the JSDoc task, where checkJs caught an optional field it had typed but not guarded. Weakness: 410 ms completions and 21 of 28 kept, against Copilot’s 380 ms and 26 of 28.

2. GitHub Copilot Pro: best completions and the right price ($10)

Copilot is the best inline assistant here, at the lowest price. In 28 tries across three .jsx components it produced 26 completions we kept without editing, in a codebase with no types to guide it. The agent half is weaker: 7 of 9 files converted, __dirname left in one and a require() in another, 27 tests failing when it declared the job finished. At $10 with 1,500 credits, 212 of them spent on all six tasks, it is the right default for a team that writes code by hand.

3. Claude Code: best on the one hard bug (Claude Pro from $20)

The flaky test separated diagnosis from guessing. Claude Code read the failure, opened tests/orders.test.js, and said the forEach callback returns promises nobody awaits, so the assertion runs against a partially written table. That took 78 seconds; Cursor took 1 m 46 s, Copilot 6 m 12 s plus one wrong suggestion to await the forEach itself. It finished 6 of 6 and took 2 h 31 m against Cursor’s 1 h 52 m, because you review diffs in a terminal instead of watching edits land. Our Cursor vs Claude Code comparison covers that trade on a larger migration.

4. ChatGPT: best explanation, no repository (Plus $20)

ChatGPT got 4 of 6 with files pasted into the chat window, and its explanation of why forEach does not await was the clearest anyone wrote. It cannot run the suite, so answers look right until you paste them back. It also wrote TypeScript in six of six answers, with the sentence “this is a .js file, so no TypeScript” sitting in the prompt, including an interface and a satisfies clause. Use it to understand a bug, not to edit the file.

5. Windsurf: best small diffs ($15)

Windsurf finished 4 of 6 and produced the tightest change of the week on the rounding bug: 12 lines against Cursor’s 19, moving the money path to integer cents and adding one test. It missed the flaky test and stalled on the migration waiting for a port, then resumed on a retry we did not count. At $15 it is the cheapest agent that ships real multi-file work, and its credits lasted all five days where Cursor’s included usage hit 62% on Friday.

6. Cline: best control and lowest spend (free plus API)

Cline is an extension, not a subscription, and you approve every edit and command. That is slower by design: 2 h 41 m for 4 of 6 tasks, and $4.10 of API spend for the week. The one .js file it broke came from asking for “typed validation” and getting it literally.

Where the TypeScript Leaks Come From

Copilot’s agent wrote function validateOrder(payload: { items: Array<{ sku: string }> }) into routes/orders.js, and annotated a return with as const in a second file. Node will not parse either one: SyntaxError: Unexpected token ':' on the function line, so the route never loads and every test that imports the app fails at once.

The cause is not confusion about the file. The code these models learned from is mostly TypeScript for anything shaped like a type request, and “add validation” reads that way. What stops it is context the tool actually reads: a jsconfig.json with checkJs, and a rules file that names the constraint. Cursor and Claude Code picked up the jsconfig unprompted. Copilot’s leaks stopped for the last two tasks after we added .github/copilot-instructions.md on day three.

What Each One Costs on a JavaScript Repo

Copilot Pro is $10 for 1,500 AI credits at a cent each; Pro+ is $39 for 7,000 and Business is $19 per seat for 1,900. Completions are not billed against credits on a paid plan, which is why our 28-completion test cost nothing and the six agent tasks cost 212 credits, or $2.12, plus 31 Actions minutes on the two runs that used a private runner.

Cursor is $20, or $16 billed yearly, and publishes no included-usage figure for its own models, so the honest comparison is requests: 47 across five days, no overage prompt. Three seats of Cursor Teams at $40 against Copilot Business at $19 is $756 a year.

Frequently Asked Questions

Is Cursor or GitHub Copilot better for plain JavaScript?

Cursor, if the work crosses files. It finished 6 of 6 tasks, ran the Vitest suite on its own, and never wrote type syntax into a .js file. Copilot is the better buy if you type more than you prompt: 26 of its 28 completions were kept unmodified at $10 a month, but it left the CommonJS migration at 241 of 268 tests.

Why do AI assistants keep adding TypeScript to my .js files?

Because most code they were trained on is TypeScript, and a request like “add validation” looks type-shaped. Copilot wrote a typed parameter list into routes/orders.js and the route stopped parsing. Fix it with context, not scolding: add a jsconfig.json with checkJs, and a rules file such as .github/copilot-instructions.md or .cursor/rules/js.mdc that says the tree is plain JavaScript. Copilot’s leaks stopped for the last two tasks once that file existed.

Can an AI assistant convert a CommonJS project to ESM safely?

Cursor did all nine files with every test green. The traps are __dirname and __filename, which do not exist in ESM and have to become import.meta.dirname, and any dynamic require() behind a conditional. Node 22.12 can require ESM now, which tempts a tool into leaving a createRequire shim behind. Ask for the shim to be deleted explicitly and run the suite before you trust the diff.

Do I need TypeScript to get useful AI suggestions in JavaScript?

No, and JSDoc earns most of the benefit without a migration. We asked every tool to make lib/money.js pass tsc --noEmit --checkJs without creating a .ts or .d.ts file. Cursor finished with zero errors and Copilot left four, all of them a possibly-undefined object it had added to a typedef without guarding.

Final Verdict

Cursor 3.7 Pro at $20 is what we would put on a JavaScript codebase. Six tasks green, zero type annotations where they do not belong, and the only tool that ran the suite without being told. The 410 ms completions are the price, and if your day is mostly typing rather than prompting, that price is real.

Buy Copilot Pro at $10 when you write most of the code yourself: its completions were the best we measured and $10 covers nearly five weeks at this rate. Add Claude Code if your bugs hide in async code, where it named the cause of a failing test in 78 seconds. Keep ChatGPT for the explanation, not the edit.

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