How to Use --allow-write Flag in Deno for File Write Permissions

Use the --allow-write flag when running a Deno script to grant it permission to write files or directories on your system. For example, run deno run --allow-write script.ts to allow file writing. Without this flag, Deno will block any write operations for security.

Syntax

The --allow-write flag is used with the deno run command to allow your script to write files or directories.

You can specify it in these ways:

• --allow-write: Allows writing anywhere on the file system.
• --allow-write=path: Allows writing only inside the specified path.

bash

deno run --allow-write script.ts
deno run --allow-write=./data script.ts

Example

This example shows a script that writes a text file. Running it with --allow-write lets it create the file. Without the flag, Deno will block the write and show an error.

typescript

const encoder = new TextEncoder();
const data = encoder.encode("Hello from Deno!\n");

await Deno.writeFile("hello.txt", data);
console.log("File written successfully.");

Output

File written successfully.

Common Pitfalls

Common mistakes when using --allow-write include:

• Not adding the flag, causing permission errors when writing files.
• Using --allow-write=path but specifying the wrong or incomplete path, which blocks writes outside that path.
• Assuming --allow-write also grants read permission; you must add --allow-read separately if needed.

bash

/* Wrong: Missing --allow-write flag */
deno run script.ts

/* Right: Adding --allow-write */
deno run --allow-write script.ts

Quick Reference

Flag Usage	Description
--allow-write	Allow writing files anywhere
--allow-write=path	Allow writing only inside specified path
--allow-read	Allow reading files (needed separately)
No flag	No write permission, write operations fail

Key Takeaways

Always use --allow-write to enable file writing in Deno scripts.

Specify a path with --allow-write=path to limit write access for safety.

Without --allow-write, Deno blocks all write operations by default.

Combine --allow-write with --allow-read if your script reads and writes files.

Check paths carefully when restricting write permissions to avoid errors.