Deno

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