Discuss 'Module Augmentation' and its role in third-party library extensibility.

Module augmentation allows developers to extend the types of existing third-party modules from a separate file[cite: 1].

Many libraries (like Express) allow plugins to attach data to objects. Module augmentation uses declaration merging to 're-open' module interfaces. This lets you add properties to a library's types without modifying its actual source code in node_modules[cite: 1].