Bun Fs vs Deno Fs

Bun and Deno both ship filesystem APIs, but they take opposite philosophies: Bun chases Node compatibility and raw throughput, Deno chases a clean web-standard async surface behind a permission wall. Here's the decisive pick.

What they actually are

Neither 'Bun Fs' nor 'Deno Fs' is a standalone product — they're the filesystem layers of two competing JavaScript runtimes. Bun's filesystem story has two faces: a near-complete node:fs implementation (readFileSync, promises, streams) plus its own Bun.file() and Bun.write() primitives that lazily reference files and accept almost any input type. Deno's story is Deno.readTextFile, Deno.writeFile, Deno.open, and the @std/fs library, all async-first and Web-standard flavored, gated behind --allow-read and --allow-write permissions. So the real question isn't 'which API' — it's which runtime's design choices you want to live with every time you touch the disk. That framing matters because the API ergonomics are a wash; the runtime baggage is where the decision is actually made.

Performance and ergonomics

Bun wins on speed, and it isn't subtle. Bun.file().text() and Bun.write() ride on Zig-level syscalls and consistently out-throughput Node and Deno on read/write benchmarks — Bun built its identity on this. Bun.write() is also delightfully overloaded: hand it a string path and a Response, a Blob, another BunFile, or a typed array and it just does the right thing. Deno's Deno.writeFile wants Uint8Array or a stream and makes you reach for TextEncoder more often than you'd like. Deno's counter-argument is consistency — everything returns a promise, the sync variants are clearly suffixed, and there's no Node-legacy callback sludge lurking. That's genuinely nicer to read. But 'nicer to read' loses to 'faster and accepts everything' when you're shipping a file-heavy service.

Compatibility and ecosystem

This is where Deno fs pays for its purity. For years Deno told you to learn a brand-new fs surface, and the std/fs helpers — ensureDir, copy, walk — are good, but they're an island. Bun bet the other way: implement node:fs so faithfully that the millions of npm packages doing require('fs') just work. That bet won. If a library reads config off disk, watches files, or globs a directory, it runs on Bun unmodified. Deno has since bolted on node:fs compatibility too, which is an admission that the clean-slate approach was a tax, not a feature. Deno's compat layer still trails — edge cases in fs.watch, permissions intercepting node:fs calls, and the occasional 'not implemented' throw. Bun's filesystem is simply the more frictionless place to run the existing JavaScript world.

Security and the permission wall

Here's Deno's one real, defensible win: Deno.readFile fails closed. No --allow-read, no disk access — a supply-chain attacker in your dependency tree can't quietly exfiltrate ~/.ssh. For untrusted code, CI sandboxes, or plugin systems, that default is worth real money, and Bun has no equivalent; Bun fs trusts everything by default, same as Node. But be honest about the cost: in day-to-day development those prompts are friction you'll --allow-all away by Tuesday, and once you do, the security model evaporates. Deno's sandbox is excellent precisely when you don't control the code running — and most filesystem work is your own code, where the wall is just a speed bump. Great feature, narrow blast radius. It doesn't outweigh Bun's speed-plus-compatibility for the common case.

Quick Comparison

Factor	Bun Fs	Deno Fs
Raw read/write throughput	Zig-backed Bun.file()/Bun.write(), benchmark leader	Solid but trails Bun on most fs benchmarks
Node ecosystem compatibility	Near-complete node:fs, npm packages run unmodified	node:fs compat added later, still has edge-case gaps
API ergonomics	Bun.write() accepts almost any input type	Clean async-first Deno.* + @std/fs, but stricter input types
Security by default	Trusts disk access, no sandbox (Node-style)	--allow-read/--allow-write permission wall, fails closed
Standards orientation	Pragmatic, Node-shaped legacy surface	Web-standard, promise-first, tidy std library

The Verdict

Use Bun Fs if: You want maximum throughput, full node:fs compatibility, and ergonomic Bun.file()/Bun.write() helpers without permission ceremony.

Use Deno Fs if: You're already all-in on Deno, want the permission sandbox, and prefer a tidy Deno.* / std/fs async-first API over Node legacy.

Consider: Both implement node:fs now, so the gap narrows yearly. If you're writing portable code targeting both runtimes, lean on the Web Streams / node:fs overlap rather than runtime-specific sugar.

Bun Fs vs Deno Fs: FAQ

Is Bun Fs or Deno Fs better?

Bun Fs is the Nice Pick. Bun's fs is faster, drop-in compatible with the entire Node ecosystem, and gives you Bun.file()/Bun.write() sugar on top. Deno's fs is cleaner on paper but the permission prompts and Node-compat friction tax you on every real project. Speed plus compatibility wins the filesystem layer.

When should you use Bun Fs?

You want maximum throughput, full node:fs compatibility, and ergonomic Bun.file()/Bun.write() helpers without permission ceremony.

When should you use Deno Fs?

You're already all-in on Deno, want the permission sandbox, and prefer a tidy Deno.* / std/fs async-first API over Node legacy.

What's the main difference between Bun Fs and Deno Fs?

Bun and Deno both ship filesystem APIs, but they take opposite philosophies: Bun chases Node compatibility and raw throughput, Deno chases a clean web-standard async surface behind a permission wall. Here's the decisive pick.

How do Bun Fs and Deno Fs compare on raw read/write throughput?

Bun Fs: Zig-backed Bun.file()/Bun.write(), benchmark leader. Deno Fs: Solid but trails Bun on most fs benchmarks. Bun Fs wins here.

Are there alternatives to consider beyond Bun Fs and Deno Fs?

Both implement node:fs now, so the gap narrows yearly. If you're writing portable code targeting both runtimes, lean on the Web Streams / node:fs overlap rather than runtime-specific sugar.

The Bottom Line

Bun Fs wins

Bun's fs is faster, drop-in compatible with the entire Node ecosystem, and gives you Bun.file()/Bun.write() sugar on top. Deno's fs is cleaner on paper but the permission prompts and Node-compat friction tax you on every real project. Speed plus compatibility wins the filesystem layer.

Related Comparisons