No side effects on import

Code that executes at import time bypasses the consumer's control plane. A top-level fetch() fires before the consumer has configured anything; a top-level console.log pollutes test output and server logs; a globalThis mutation can conflict with other packages or the host environment. ISO 25010 compatibility and fault-tolerance both degrade when a package modifies global state the moment it's imported. In server-side rendering, side-effecting imports can crash the render pass. In test environments, they produce flaky behavior depending on import order.

Move all initialization into exported constructors or factory functions. Nothing should run at module evaluation time.

// Before — side effects on import:
console.log('SDK loaded')
globalThis.__SDK = true
const config = await fetch('/api/config') // this fires on every import

// After — no side effects, consumer controls initialization:
export class Client {
  constructor(private config: ClientConfig) {
    // initialization here, when the consumer asks for it
  }
}

Also declare in package.json:

{ "sideEffects": false }

This tells webpack, Rollup, and Vite that all modules are safe to tree-shake. If any files have legitimate side effects (CSS imports, polyfills), list only those files in the sideEffects array.