diff --git a/src/promise-fs-opts.js b/src/promise-fs-opts.js
new file mode 100644
index 0000000..7f82ba0
--- /dev/null
+++ b/src/promise-fs-opts.js
@@ -0,0 +1,46 @@
+import fs from 'fs'
+import { promisify } from 'util'
+
+const hasFS = Boolean(fs)
+
+export { hasFS }
+
+/**
+ * @type {any}
+ */
+let _fsReadFn
+/**
+ * @description This function is needed not to initialize the `fs.read` on load time. To run in cf workers without polyfill.
+ * @param {number} fd
+ * @param {Uint8Array} buffer
+ * @param {number} offset
+ * @param {number} length
+ * @param {number} position
+ * @returns {Promise<{ bytesRead: number, buffer: Uint8Array }>}
+ */
+export function fsread (fd, buffer, offset, length, position) {
+  if (!_fsReadFn) {
+    _fsReadFn = promisify(fs.read)
+  }
+  return _fsReadFn(fd, buffer, offset, length, position)
+}
+
+/**
+ * @type {any}
+ */
+let _fsWriteFn
+/**
+ * @description This function is needed not to initialize the `fs.write` on load time. To run in cf workers without polyfill.
+ * @param {number} fd
+ * @param {Uint8Array} buffer
+ * @param {number} offset
+ * @param {number} length
+ * @param {number} position
+ * @returns {Promise<{ bytesRead: number, buffer: Uint8Array }>}
+ */
+export function fswrite (fd, buffer, offset, length, position) {
+  if (!_fsWriteFn) {
+    _fsWriteFn = promisify(fs.write)
+  }
+  return _fsWriteFn(fd, buffer, offset, length, position)
+}
diff --git a/src/reader.js b/src/reader.js
index 3e36f5b..a78cbac 100644
--- a/src/reader.js
+++ b/src/reader.js
@@ -1,15 +1,13 @@
-import fs from 'fs'
-import { promisify } from 'util'
+import { fsread, hasFS } from './promise-fs-opts.js'
 import { CarReader as BrowserCarReader } from './reader-browser.js'
 
 /**
  * @typedef {import('./api').Block} Block
  * @typedef {import('./api').BlockIndex} BlockIndex
  * @typedef {import('./api').CarReader} CarReaderIface
+ * @typedef {import('fs').promises.FileHandle} FileHandle
  */
 
-const fsread = promisify(fs.read)
-
 /**
  * @class
  * @implements {CarReaderIface}
@@ -27,7 +25,7 @@ export class CarReader extends BrowserCarReader {
    * @async
    * @static
    * @memberof CarReader
-   * @param {fs.promises.FileHandle | number} fd - A file descriptor from the
+   * @param {FileHandle | number} fd - A file descriptor from the
    * Node.js `fs` module. Either an integer, from `fs.open()` or a `FileHandle`
    * from `fs.promises.open()`.
    * @param {BlockIndex} blockIndex - An index pointing to the location of the
@@ -53,4 +51,4 @@ export class CarReader extends BrowserCarReader {
   }
 }
 
-export const __browser = false
+export const __browser = !hasFS
diff --git a/src/writer.js b/src/writer.js
index cee84b4..7aa6aeb 100644
--- a/src/writer.js
+++ b/src/writer.js
@@ -1,15 +1,12 @@
-import fs from 'fs'
-import { promisify } from 'util'
 import { readHeader, chunkReader } from './decoder.js'
 import { createHeader } from './encoder.js'
+import { fsread, fswrite, hasFS } from './promise-fs-opts.js'
 import { CarWriter as BrowserCarWriter } from './writer-browser.js'
 
-const fsread = promisify(fs.read)
-const fswrite = promisify(fs.write)
-
 /**
  * @typedef {import('multiformats/cid').CID} CID
  * @typedef {import('./api').BlockWriter} BlockWriter
+ * @typedef {import('fs').promises.FileHandle} FileHandle
  */
 
 /**
@@ -35,7 +32,7 @@ export class CarWriter extends BrowserCarWriter {
    * @async
    * @static
    * @memberof CarWriter
-   * @param {fs.promises.FileHandle | number} fd - A file descriptor from the
+   * @param {FileHandle | number} fd - A file descriptor from the
    * Node.js `fs` module. Either an integer, from `fs.open()` or a `FileHandle`
    * from `fs.promises.open()`.
    * @param {CID[]} roots - A new list of roots to replace the existing list in
@@ -81,4 +78,4 @@ export class CarWriter extends BrowserCarWriter {
   }
 }
 
-export const __browser = false
+export const __browser = !hasFS