Los valores null y undefined suelen dar muchos dolores de cabeza a los programadores. En el artículo Handling null and undefined in JavaScript se explica cómo se deben usar en JavaScript:
One aspect of JavaScript development that many developers struggle with is dealing with optional values. What are the best strategies to minimize errors caused by values that could be null, undefined, or otherwise uninitialized at runtime?
Some languages have built-in affordances for those circumstances. In some statically typed languages, you can say that null and undefined are illegal values, and let your programming language throw a TypeError at compile time, but even in those languages, that can’t prevent null inputs from flowing into the program at runtime.
To get a better handle on this problem, we need to understand where these values can come from. Here are some of the most common sources:
User input
Database/network records
Uninitialized state
Functions which could return nothing
