Why It's Dangerous (With One Exception)

JavaScript allows adding methods to built-in prototypes like Array.prototype or String.prototype. This is almost always a bad idea.

// BAD PRACTICE:
String.prototype.shout = function() {
  return this.toUpperCase() + '!!!';
};
'hello'.shout(); // 'HELLO!!!'

Why This Is Dangerous

1. Naming Conflicts: Two libraries adding the same method name will silently overwrite each other.
2. Future Standards: If JavaScript officially adds the same method name later, your code breaks.
3. Global Pollution: Every object of that type in your entire app gets the method unexpectedly.

The One Acceptable Exception: Polyfills

Implementing a newer standard feature for older browsers that don't support it natively.