[pmilot]

The nature of the Javascript language makes it possible to sandbox an entire library, ensure that two versions of the same library can run side-by-side without conflicts (because they are both in a different sandbox).

The way sandboxing works is actually not specified by the package manager (npm) nor the language itself (Javascript); each consumer of npm packages can roll their own sandboxing mechanism (webpack, browserify, nodejs, etc.).

There isn't even a common specification for the way packages should export public symbols. You have a choice of CommonJS, AMD, Ecmascript 2015, etc.