Under The Hood
frontend bun Bun 1.x (`Bun.file`, `Bun.write`)

Bun.file() & Bun.write(): Linux io_uring and Darwin kqueue Syscalls

Last updated
Prerequisites:
Node.js `fs/promises`
Linux kernel syscalls (`read`, `write`, `io_uring`)
  • bun
  • file-io
  • io_uring
  • syscalls
  • bun-file
  • bun-write
  • performance

Read at your depth

The practical view

`Bun.file(path)` creates a lazy `BunFile` reference extending the Web `Blob` standard. It does not read the file until you ask: `await file.text()`, `await file.json()`, `await file.arrayBuffer()`, or `file.stream()`. `Bun.write(destination, data)` writes strings, buffers, `Blob`s, or even pipes one `BunFile` directly into another with maximum speed.

The same idea in other frameworks

node equivalent 
`import fs from 'node:fs/promises'; const text = await fs.readFile('file.txt', 'utf8');`

Legacy vs modern

Node.js fs.readFile vs Bun.file() Lazy Zero-Copy

Node reads entire buffers into V8 memory; `Bun.file()` creates lazy references and pipes directly to HTTP responses.

before → after
Node.js fs/promises (Thread Pool) 
const buffer = await fs.readFile(path); res.send(buffer); // ❌ Copies bytes into V8 heap
Bun.file() Kernel Zero-Copy 
return new Response(Bun.file('./large-video.mp4')); // ✅ NEW: Zero-copy kernel sendfile

Interview gotchas

Context

Interviewers test your knowledge of lazy evaluation in Bun's file system API.

The mechanical answer

`Bun.file('test.txt')` is completely **lazy and synchronous**: it does *not* touch the disk, open a file descriptor, or read any bytes. It simply creates a lightweight `BunFile` object holding the path. The disk is only accessed when an explicit method like `.text()`, `.arrayBuffer()`, `.stream()`, or `Bun.write()` is called.

Trap

Do not `await Bun.file('test.txt')`; `Bun.file` is not a Promise.

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