Variables

Declarations

Prefer const by default and let when a value changes:

const port = Number(process.env.PORT ?? 8080)
let activeConnections = 0

var is supported for compatibility, but new code should avoid it.

Scope

Block scope works as expected:

if (enabled) {
  const label = 'enabled'
  console.log(label)
}

Avoid relying on subtle hoisting behavior. It makes code harder to check and harder for readers to audit.

Closures

Closures are supported:

function counter() {
  let value = 0
  return () => ++value
}

Use closures for local state, but prefer explicit objects or classes for state shared across modules.

Recommendations

• Keep module-level state small.
• Read environment variables at startup.
• Avoid mutating imported objects from many places.
• Prefer plain data over dynamic property mutation when you can.