How to Cache Modules in Deno: Simple Guide

In Deno, you can cache modules by running deno cache <file> which downloads and stores dependencies locally. To use cached modules without re-downloading, run with --cached-only. To force re-download and update cache, use --reload.

Syntax

The main commands to manage module caching in Deno are:

• deno cache <file>: Downloads and caches all dependencies of the specified file.
• deno run --cached-only <file>: Runs the file using only cached modules, failing if modules are missing.
• deno run --reload <file>: Forces Deno to re-download all modules and update the cache.

bash

deno cache <file>
deno run --cached-only <file>
deno run --reload <file>

Example

This example shows how to cache modules for a script, run it using cached modules only, and then reload modules to update the cache.

typescript

import { serve } from "https://deno.land/std@0.203.0/http/server.ts";

console.log("Starting server on http://localhost:8000");
serve(() => new Response("Hello from Deno!"), { port: 8000 });

Output

Starting server on http://localhost:8000

Common Pitfalls

1. Running without cache: If you run deno run without prior caching, Deno downloads modules each time which slows startup.

2. Using --cached-only without cache: This causes an error if modules are not cached yet.

3. Forgetting to reload: If a remote module updates but you don't use --reload, your program uses stale code.

bash

/* Wrong: Running with --cached-only before caching modules */
deno run --cached-only server.ts

/* Right: Cache modules first, then run with cached-only */
deno cache server.ts
deno run --cached-only server.ts

Quick Reference

Command	Purpose
deno cache	Download and cache dependencies without running
deno run --cached-only	Run using only cached modules, error if missing
deno run --reload	Force re-download and update cache
deno info	Show cached modules and their locations

Key Takeaways

Use deno cache to download and store modules locally before running.

Run with --cached-only to ensure no network requests and use only cached modules.

Use --reload to refresh cached modules when remote code changes.

Running with --cached-only without prior caching causes errors.

Check cached modules and their paths with deno info.