name: deno description: Deno 2 development workflow including testing with permissions and JSR package management. Use when working with Deno projects, setting up new Deno applications, or when the user mentions Deno, JSR, or modern JavaScript/TypeScript development.

Deno Development

Version Requirements

• Always use Deno 2

Testing

• Use deno test -P for running tests with permissions granted
• The -P flag allows permissions described in deno.json during testing.
• Define test permissions in the test section of deno.json.

Package Management

• Use JSR (JavaScript Registry) for package dependencies https://jsr.io/
• JSR is the modern package registry for JavaScript and TypeScript

Common Commands

Adding Dependencies

# Add from JSR
deno add jsr:@std/log
deno add jsr:@std/path

# Add from npm (when necessary)
deno add npm:express

Running Tests

# Run all tests with permissions
deno test -P

# Run specific test file
deno test -P test/example.test.ts

Running Applications

# Run with limited permissions - AVOID -A unless absolutely necessary
deno run --allow-read=. main.ts
deno run --allow-net --allow-read=./public server.ts
deno run --allow-env --allow-read=. config.ts

# for `export default { fetch }`
deno serve serve.ts

# Use permission sets from deno.json
deno run -P main.ts

Configuration with deno.json

Permission Sets

Define permission sets in deno.json for consistent security:

{
  "permissions": {
    "default": {
      "read": ["."],
      "env": {
        "allow": ["NODE_ENV", "PORT"],
        "deny": ["SECRET_KEY"],
        "ignore": ["TEMP_*"]
      }
    },
    "server": {
      "read": ["./public"],
      "net": ["localhost:8000", "deno.land"],
      "env": {
        "allow": ["DATABASE_URL", "API_KEY"]
      }
    }
  }
}

Test Permissions

Configure test permissions in deno.json:

{
  "test": {
    "permissions": {
      "read": ["."],
      "net": true
    }
  }
}

Best Practices

• NEVER use -A (all permissions) unless absolutely necessary
• Always use the most restrictive permissions possible
• Use --allow-read=. instead of --allow-read to limit to current directory
• Use permission sets in deno.json for consistent security
• Define test permissions in deno.json under the test section
• Use env.allow, env.deny, and env.ignore for fine-grained environment variable control (read.ignore also available)
• Do not use import statements with full URLs for external dependencies. Instead, use deno add first.
• Leverage Deno's built-in security features
• Use JSR for package discovery and management